Phosphorus Dynamics in Soil-Water-Sediment Environment

Abstract

Phosphorus, a crucial element for plant growth, is relatively scarce in the Earth’s crust. Its availability in surface soils ranges from 100 to 50 mg/kg. Plants can only absorb phosphorus in the form of orthophosphates, with H2PO4- being most absorbable at low pH levels. The average phosphorus concentration in soil solution is about 0.05 mg/L, but plant-satisfying levels range from 0.003 to 0.3 mg/L, underscoring the need for judicious phosphorus fertilization. Storage and stabilization reactions in soil, mainly facilitated by iron and aluminum oxides, play a key role. Compounds in most soils use hydroxyl exchange mechanisms for H2PO4- adsorption. Under alkaline conditions, minerals like calcium carbonate can absorb H2PO4-/ HPO42−, leading to precipitation. To ensure plant health, phosphorus fertilizers, especially calcium orthophosphates like triple superphosphate, are commonly used. These fertilizers offer essential phosphorus for plant growth and development, supporting vital processes like respiration and photosynthesis. Excess phosphorus in aquatic ecosystems, known as eutrophication, poses environmental risks, often originating from concentrated agricultural and livestock operations. Proper management of phosphorus inputs is crucial for balancing plant growth support with environmental preservation. Understanding phosphorus dynamics in soil, water, and sediments is vital for sustainable agriculture and conservation efforts. Adsorption isotherms provide insights into phosphorus absorption mechanisms in sediments, impacting water quality in surface and subsurface systems.

1. Introduction

Phosphorus is a non-metallic element in the 5^th group of the periodic table with an atomic weight of 30.97, a density of 1.82 g/cm³, and an atomic number of 15. It was discovered in 1669 by a German chemist named Hennig Brand. The melting point and boiling point of this element are 44.1 and 280°C, respectively, and its atomic radius is 1.28 angstroms. Phosphorus is never found in nature in its free state; instead, it’s found combined with other elements in rocks, phosphate minerals, and all living cells. It appears as a waxy white solid with an unpleasant smell. Phosphorus is highly reactive and emits a small amount of light when it reacts with oxygen. It burns rapidly in air, transforming into phosphorus pentoxide. This non-metal does not dissolve in water, but it is soluble in carbon disulfide [1]. Phosphorus is an essential component of biological molecules such as ATP, ADP, and DNA, and it’s also found in bones and teeth. It is present in trace amounts in all the foods we eat. There are various isotopes of phosphorus; only one, 31P, is stable. The other isotopes of phosphorus are radioactive and have short half-lives ranging from a few nanoseconds to a few seconds, except for two isotopes, 32P and 33P, which have longer half-lives of 14 and 25 days, respectively. These longer half-lives make these isotopes useful for analyzing and labeling DNA.

2. Phosphorus in soil

Phosphorus is an essential element for plant growth and survival. However, the amount of phosphorus available in the Earth’s crust is relatively low compared to other essential plant nutrients, ranging from 1100 to 1200 mg/kg. The average concentration of phosphorus in surface soils is estimated to be around 1100–50 mg/kg. Plants can only absorb phosphorus in the form of orthophosphates, with the highest concentration being in the form of H₂PO₄⁻ at low pH levels. This form of orthophosphate is more readily absorbed by plants compared to HPO₄²⁻, which increases in concentration with increasing pH levels. The average concentration of phosphorus in the soil solution is approximately 0.05 mg/L, although this value can vary depending on the soil type. Typically, the concentration of phosphorus in the soil solution that can satisfy plant needs ranges from 0.003 to 0.3 mg/L, as reported by [2, 3]. These findings suggest that while phosphorus is essential for plant growth and survival, it is often limited in availability in the soil. Therefore, it is crucial to carefully manage phosphorus fertilization practices to ensure that plants have access to the necessary amounts of this nutrient for optimal growth and development.

Phosphorus in soil exists in both organic and mineral forms. Typically, 20–80% of total soil phosphorus is in the organic form, while the remainder is in mineral form [4, 5]. However, these forms are not strictly distinct from one another and can transform into each other (Figure 1). The specific types of organic phosphorus are not well understood, but mineral phosphorus is typically associated with iron, aluminum, and calcium. These minerals each have varying solubilities in the soil. The mobility and speciation of mineral phosphorus in soil and aquatic ecosystems are regulated by reactions that occur on the surfaces of colloids [6, 7, 8]. Additionally, Lindsay noted that other reactions, such as simultaneous precipitation by calcium, iron, and aluminum, can also influence the phosphorus cycle in soil [9, 10].

Figure 1.

Phosphoric and mineral cycle in soil.

The mechanisms responsible for converting soluble phosphorus into less soluble states are related to phosphorus storage and stabilization reactions. Nearly all soils contain iron and aluminum oxides and hydroxides, which can exist as separate mineral particles or as coatings on other particles, such as clay. Amorphous aluminum hydroxides may also be present between the expandable layers of aluminosilicates. Many H₂PO₄⁻ adsorbents are present in the soil solution, and most of these adsorbents, such as iron or aluminum, use the hydroxyl (OH⁻) exchange mechanism to absorb H₂PO₄⁻. In alkaline conditions, minerals like calcium carbonate can absorb H₂PO₄⁻/HPO₄²⁻ and cause precipitation by exchanging water, bicarbonate, and hydroxyl ions on their surfaces. Notably, each of these reactions occurs in response to the soil’s pH level. At pH levels lower than 5, hydroxyl metal phosphates (Al (OH)₂H₂PO₄) can form due to the presence of aluminum, iron, or active manganese. In alkaline conditions, dicalcium phosphate deposits without water molecules can form due to the presence of active calcium [6, 11, 12].

As previously mentioned, the natural concentration of phosphorus in the soil is very low, and this small amount of phosphorus is not easily available to the plant. The plant has to compete with the reactions of surface absorption and phosphorus deposition in the soil to obtain the phosphorus it needs. Therefore, to achieve acceptable performance, phosphorus fertilizers should be added to the soil to aid the growth and nutrition of the plant. The most common phosphorus fertilizers are calcium orthophosphate fertilizers, including triple superphosphate and simple superphosphate. Triple superphosphate, with the formula Ca(H₂PO₄)₂, has 44–53% phosphorus oxide [2, 13, 14].

2.1 Sources of phosphorous input

2.1.1 Phosphorous fertilizers

The growth and health of plants depend on several factors, including nutrient supply, soil quality, and exposure to sunlight. Among these, phosphorus is a crucial element that plays a significant role in promoting plant growth and health. Despite its importance in supporting respiration, photosynthesis, cell division and enlargement, and energy storage and transfer, the amount of phosphorus in soil is typically much lower than that of other essential elements like potassium, nitrogen, and calcium. As a result, various types of phosphorus-based fertilizers are produced today to supplement the soil’s phosphorus content and help improve plant growth.

In the nineteenth century, researchers began conducting experiments to examine the effects of phosphorus fertilizers on plant yield. Britain (1843) and Germany (1878) were among the pioneering countries in this field, with subsequent experiments conducted at the Illinois Agricultural Research Station in 1888. Further experiments were carried out in Ontario, Canada, in 1916 [15]. After World War II, the use of animal manure to maintain soil fertility and provide food for the growing population began to expand rapidly [16].

2.1.2 Sources of phosphorus fertilizers

2.1.2.1 Phosphate rocks

These are a valuable source of phosphorus for plants, with the most reactive rocks containing Francolite, an Apatite mineral containing iron and carbonate. When crushed, phosphate rock can provide sufficient plant-available phosphorus in low pH soils. It is commonly used to cultivate plants such as Kathira, oil palm, and coffee in very acidic soils, particularly in warm weather conditions, moist soils, and long growing seasons.

2.1.2.2 Phosphoric acid

Phosphoric acid (H₃PO₄) is another important source of phosphorus for agriculture, containing 17–24% phosphorus (55–39% P₂O₅). It is produced through the reaction of phosphate rock with H₂SO₄ and can be either green acid or acid obtained from a wet process. Phosphoric acid is used to acidify phosphate rock and create calcium and ammonium phosphates, which are important fertilizers. It can also be injected into the soil or irrigation water, particularly in alkaline and calcareous soils.

2.1.2.3 Calcium phosphates

In the past, calcium phosphate fertilizers such as simple superphosphate, triple superphosphate, and enriched superphosphate were among the primary sources of phosphorus. However, unlike phosphoric acid and ammonium phosphates, superphosphates do not have a significant impact on soil pH.

2.1.2.4 Simple superphosphate

Also known as single superphosphate, it contains 7–9–5% phosphorus (P₂O₅ 16–22%). It is a valuable source of both sulfur and phosphorus.

2.1.2.5 Triple superphosphate

This contains 17–23% phosphorus (P₂O₅ 44–52%). Triple Super Phosphate is an excellent source of phosphorus, and its high concentration of phosphorus is advantageous because transportation, storage, and handling constitute a significant part of the total cost of fertilizer consumption.

2.1.2.6 Ammonium phosphate

Ammonium phosphate is produced by reacting phosphoric acid with NH₃. Monoammonium phosphate (MAP) contains 11–13% nitrogen and 21–24% phosphorus (P₂O₅ 48–55%), while diammonium phosphate (DAP) contains 18–21% nitrogen and 20–23% phosphorus (P₂O₅ 53–46%). Both MAP and DAP are granular and water-soluble, making them suitable as starter fertilizers. Their high solubility allows for efficient uptake by plants, resulting in improved growth and yield.

2.1.2.7 Ammonium polyphosphate

Ammonium polyphosphate is produced by reacting pyrophosphoric acid with ammonia. Liquid APP is a cost-effective source of phosphorus that can be used alone or in combination with other liquid fertilizers. Typically, UAN and APP fertilizers are combined and applied in bands below the soil surface.

2.1.2.8 Potassium phosphate

Potassium phosphate products, such as KH₂PO₄ and K₂HPO₄, are highly soluble in water and are commonly used in the horticulture industry. Their high levels of both potassium and phosphorus make them a popular choice for crops such as potatoes, tomatoes, and many leafy vegetables that are sensitive to high concentrations of chloride found in KCl. Additionally, their low salt index makes them well-suited for application near seeds with minimal damage to emerging sprouts.

3. Phosphorous in water

In the 1960s, it was discovered that aquatic ecosystems were experiencing high levels of phosphorus. Further scientific investigations revealed that this phosphorus was originating from livestock farms and concentrated agriculture located kilometers away from these water ecosystems. These areas, which are the source of phosphorus input to aquatic ecosystems, are known as concentrated polluting points. During that decade, due to advancements in technology and the creation of laws limiting the risk of pollution from these points, identifying and controlling these sources became easier. However, today, the transfer of phosphorus from unknown agricultural sources is a serious and worrying threat to the environment, as it is practically impossible to control these sources. The phosphorus eventually reaches water reservoirs, causing many problems. This results in exorbitant fees for using these waters [17, 18].

Phosphorus is lost from watersheds due to soil erosion and washing and is transferred to aquatic ecosystems through surface and subsurface runoff. When the runoff reaches surface water, the washed (dissolved) elements along with the eroded materials are emptied into these waters, creating certain problems. Sediments fill the reservoirs of dams, corrode water treatment facilities, and reduce the quality of drinking water. The elements present in these waters cause the phenomenon of enrichment, which cannot be ignored because it can cause many problems. Water enrichment causes toxic substances to be released from aquatic organisms into the water, leading to diseases in humans, with neurological problems being the most common in recent decades. The transfer of phosphorus to surface waters, deposition, and release of phosphorus from sediments into the water causes the phenomenon of surface water enrichment [19, 20, 21].

Phosphorus losses are typically observed in the following three ways in the field [22]:

1. Dissolved phosphorus and/or phosphorus bound to solid soil particles-suspended in water-created by erosion and accompanied by runoff flows, which are transferred to surface waters such as rivers and lakes.

2. Dissolved phosphorus and/or phosphorus bound to solid soil particles reach the surface of underground water sources due to infiltration runoff in the soil profile (Leaching).

3. Seepage or lateral infiltration of phosphorus transferred to underground water to lakes, rivers, and other surface streams.

On the other hand, the transfer of phosphorus from topsoil to surface and subsurface waters can be summarized in three mechanisms:

3.1 Sources of phosphorus input

Water pollution refers to the contamination of lakes, rivers, oceans, and underground waters by chemical and microbial waste products. When harmful substances are directly or indirectly discharged into water sources, and if these substances limit the widespread use of water, the water is considered polluted. The effects of water pollution are felt by plants and living organisms that inhabit these waters. In many cases, the effects of pollution destroy natural biological groups as well as individual and collective species [23].

Non-point pollution can cause physical damage to waterways by increasing sedimentation. This occurs when erosion deposits fine soil particles in the water, making it murky. The extent of sediment accumulation is influenced by factors such as the speed of the flow, shear stress, and soil grain stability. The type and quantity of sediment that originates from agricultural areas differ from that of forested areas [24]. The consequences of rising sediment levels are serious, including:

• Silt particles in the water can wear out the water pump and other facilities related to water transfer.

• Using murky water in agriculture can also cause problems. For instance, utilizing muddy water in irrigation systems leads to wear and tear of pumps, distribution systems, and other irrigation devices. Consequently, this can result in complications with the irrigation system.

• The introduction of suspended substances into streams, lakes, and reservoirs threatens aquatic life. Studies have shown that the greater the amount of suspended matter in the water, the more detrimental the effects on aquatic life.

• Sediments can cloud the water, preventing sufficient light from reaching aquatic depths, thereby disrupting the photosynthesis process.

• The contamination of waters with mud reduces the esthetic appeal of recreational centers.

3.1.1 Point and non-point sources

Water pollutants are divided into two categories based on the emission source: point pollutant (point or concentrated pollution) and non-point pollutant source (non-point pollution).

A point source of pollution refers to a specific and identifiable location that releases pollutants into the receiving environment. Examples of such sources include wastewater discharged by industries, power plants, and urban sewage treatment facilities. The Clean Water Act (CWA) in the United States provides a regulatory definition for point sources. The CWA’s definition of a point source was updated in 1987 to encompass municipal sewage systems, industrial wastewater, and construction wastewater.

A non-point pollutant source refers to a source of pollution that does not have a specific and identifiable entry point into the receiving environment. Typically, non-point sources include water and runoff from agricultural lands, mines, construction sites, roads, and urban areas. Additionally, air pollution settling on water sources also constitutes non-point pollution. Unlike point source pollution, which originates from a single source, non-point pollution is the result of the accumulation of pollution amounts collected from a large basin. Non-point sources of pollution pose the biggest threat to surface and subsurface drinking water sources worldwide. Erosion is also a significant source of non-point pollution due to the large amounts of chemical fertilizers, insecticides, pesticides, and other substances used in agricultural lands, which have an average erosion rate higher than other land uses. Consequently, agricultural lands are one of the primary sources of this type of pollution.

3.1.2 Soil

Phosphorus is a crucial element for modern agriculture, widely used in chemical fertilizers to enhance crop yields. In recent years, phosphorus consumption has increased due to the depletion of soil phosphorus caused by extensive crop harvesting. The significance of phosphorus in the crop production system is evident from the doubling of its use in fertilizers since 1960, while its annual global production has remained less than 2 million tons over the past decade. As depicted in Figure 2, phosphorus primarily leaves the land through runoff in solution and attached to particles, while a small fraction infiltrates the groundwater through preferential flow. The potential for phosphorus to reach groundwater is relatively low compared to nitrogen, owing to its low solubility. However, soil particles have a high capacity to adsorb insoluble forms of phosphorus, and hence, soil functions like a filter. Phosphorus loss from agricultural land is classified into three levels:

1. Rapid loss of soluble phosphorus in a short period of time after the application of animal and chemical fertilizers.

2. Slow loss of soluble phosphorus after fertilization.

3. Loss due to erosion.

Figure 2.

Potential routes of phosphorus exit from agricultural lands.

Animal and chemical fertilizers contain a significant amount of soluble phosphorus, which can enter runoff due to rainfall after application and increase the concentration of this element in the runoff up to 100 times its normal level [25]. The amount of phosphorus that can be stored or adsorbed on the surface of clay depends on the type and quantity of clay, iron oxide, and organic matter. The critical concentration of phosphorus varies in different surface waters, as shown in Table 1 [26].

Phosphorous concentration (μg/L)	Explanation	Reference
10	Average dissolved phosphorus in runoff from agricultural fields	Vollenweider (1968)
100	Critical concentration of dissolved phosphorus in lakes	USEPA (1986)
50	Critical concentration of total phosphorus in current	USEPA (1986)
1000	Critical concentration of total phosphorus in lakes	USEPA (1986)

Table 1.

Critical concentration of phosphorus in surface waters.

United States Environmental Protection Agency

Phosphorus is primarily released from agricultural lands in association with sediments. Proper management of plowing can reduce erosion and phosphorus loss. Therefore, in agricultural fields with a steep slope, it is advisable to use protective plows or perform contour cultivation [27]. Studies have demonstrated that total phosphorus losses are higher in fields with Chisel plowing compared to minimum plowing, whereas soluble phosphorus losses are higher in fields with minimum plowing [28]. The retention or release of phosphorus in water depends on the physical, chemical, and biological conditions of the compounds in the water system. The intensity of phosphorus discharge varies from one river to another, and even within different parts of a river, changing with seasonal variations [29]. Phosphorus storage in water is influenced by several factors, such as the biomass of macrophytes, the absorption of this element by aquatic plants, the expansion of the river margin, and nutrient concentration [30].

3.1.2.1 Phosphorus leaching

Some researchers suggest that phosphorus can be absorbed into the subsoil through the vertical movement of water in the soil profile, as the subsoil layers have a high absorption capacity for phosphorus [31]. Thus, the transfer of phosphorus from these soils to underground water and drains is expected to be very low or negligible [24]. However, studies have shown that despite the high absorption capacity of the underlying soils, significant concentrations of phosphorus can still be found in the drains of these fields [11]. These studies indicate the presence of preferential flow, which is a flow that occurs through permanent gaps in the field, and may develop more during dry seasons [32]. The creation of preferential flow does not require certain conditions such as the intensity and duration of rainfall or the permeability and saturation conditions needed to create surface runoff [33]. It should also be noted that preferential flow reduces the contact time between the soil and the infiltrating water, further contributing to the transfer of phosphorus to the drains. Even though subsurface soils have a high absorption capacity, preferential flow can still result in the transfer of phosphorus from these soils to the drains.

3.1.2.2 Erosion and runoff

Soil erosion and its impact on surface water is a complex process that depends on various factors such as soil characteristics, hydrological, and climatic conditions. Farm sediment erosion, which occurs in furrows and inter-furrows, is influenced by raindrop impact and surface currents. Raindrops typically cause soil particles to detach from each other, while surface currents cause both particle detachment and transport down the slope. The effectiveness of these processes is greatly affected by surface soil type, rainfall duration, and the amount of plant cover and residues. Soil surface coverage and erosion have an inverse relationship, so implementing soil conservation practices can reduce erosion [34, 35]. Runoff usually occurs in fields through two types of flow: surface and subsurface flow. Surface flow can sometimes penetrate into the soil along the slope and move laterally below the surface before reappearing on the soil surface. Losses of phosphorus in agricultural land occur through surface runoff, which can be in the form of dissolved or sediment-bound phosphorus. Phosphorus bound to sediments includes phosphorus that is bound with soil mineral particles and organic matter. During surface runoff in cultivated fields, 80% of phosphorus losses occur in the form of sediment-bound phosphorus [36]. In contrast, runoff from grasslands, meadows, uncultivated soils, and forests transports fewer sediments, so most of the phosphorus that is transported downstream is in the form of dissolved phosphorus in water [24, 37]. During runoff movement along sloping surfaces, phosphorus from the soil and plant residues can dissolve and enter the runoff (Figure 3). In summary, [39] have concluded in their research that the total phosphorus loss can be determined based on the sources of phosphorus, the chemical and biological changes and transformations of phosphorus, and the mobility of phosphorus in the soil.

Figure 3.

Phosphorus movement paths in the field (adapted from [38]).

The interaction between the surface soil (1–2 inches) and water (rain or irrigation water) creates the possibility of releasing phosphorus in a soluble form from the soil and plant residues. In most watersheds, the amount of phosphorus transferred through surface runoff is greater than subsurface runoff, as phosphorus is typically low in deep soil layers. When runoff penetrates into the soil, water-soluble phosphorus is absorbed into the subsurface soil, reducing the concentration of phosphorus in the subsurface runoff compared to surface runoff. However, sandy soils, peaty soils, and soils with preferential flow can exclude this case [40].

The subsurface transfer of phosphorus has been studied very little, and it was previously assumed that the amount of transfer through subsurface runoff is very small. However, some researchers believe that a significant portion of phosphorus (more than 0.1 mg/L) is transported in the form of orthophosphate in subsurface runoff [41]. Although other forms of phosphorus may also be found in subsurface runoff [33], studies have reported a correlation between the amount of dissolved phosphorus in the runoff and the amount of phosphorus in the soil. If the amount of phosphorus in the soil exceeds 60 mg/kg, the concentration of phosphorus in the runoff will significantly increase [32]. The type of vegetation and management practices can also have a significant impact on this phenomenon.

When the concentration of phosphorus exceeds 60 mg/kg, the concentration of total phosphorus in the runoff increases sharply. The reason for this is that when phosphorus exceeds this limit, it is absorbed into sites with less absorption and storage energy and is easily released into water [42]. However, using arsenic phosphorus to predict the concentration of dissolved phosphorus in runoff has many problems, including the depth of sampling to determine arsenic phosphorus, which is usually taken from the depth of the plow. The depth of the interaction between the runoff and the soil is estimated to be between 1 and 2 inches [24]. To overcome such problems, two Dutch scientists, [43], used the soil phosphorus saturation percentage to assess the risk of phosphorus leaching into underground water. This percentage is calculated by dividing the amount of phosphorus available to the plant by the maximum amount of phosphorus fixation in the soil. During their research, a critical limit of 25% for phosphorus saturation was established (for soils in the Netherlands). If the phosphorus saturation exceeds this amount, phosphorus will be released into the water. Every soil has the potential to absorb and store phosphorus, and this potential depends on the physical-chemical characteristics of the soil. The importance of iron and aluminum oxides and other soil minerals in controlling the solubility and absorption of phosphate in the soil has been identified [2].

The rate of weathering and release of phosphorus from phosphate rocks is very low and is estimated to be about 0.01−5 km/ha.year [44]. The rate of weathering and release of phosphorus from phosphate rocks depends on effective factors such as the type of phosphorus rock, their size, temperature, and water quality. A positive correlation exists between the amount of chemical weathering and runoff generated in a watershed. When the amount of runoff is high, the amount of weathering is almost 10 times more than when the amount of runoff is low [45].

3.1.3 Sediment

Sediment moves downstream along slopes through waterways. Larger particles are deposited first due to the reduction in runoff transmission power, while finer particles, such as organic materials and clays, along with silt-sized particles, enter water ecosystems along with water-soluble phosphorus. Eventually, these sediments reach rivers and water reservoirs, such as natural lakes, ponds, and reservoirs behind dams, and settle. Some of the eroded materials are so small that they settle much more slowly and can be resuspended with even slight turbulence in the water. Dynamic water conditions or changes in the redox potential can resuspend solid particles and release their adsorbed elements, such as phosphorus and nitrogen, into the water [46, 47, 48]. Therefore, the relationship between suspended particles and dissolved phosphorus in lake water is of great chemical importance.

Four main mechanisms have been identified for the release of phosphorus from sediments [49, 50, 51]. These mechanisms can cause changes in the mineral structure of sediments, which can, in turn, lead to the release of phosphorus into the water column above the sediment [46].

The four mechanisms are briefly discussed hereunder.

Absorption: Phosphorus desorption can occur in the following three different situations:

1. Phosphorus bound to calcium and magnesium minerals may be released into the water due to a reduction in pH [52].

2. An increase in temperature can also affect pH. In this case, phosphorus may be separated from iron, aluminum, or manganese [53].

3. The increase in the concentration of sulfate and chlorate ions in water may compete with phosphate ions for absorption on iron oxides [54].

Dissolution: The dissolution of minerals containing phosphorus may cause the release of this element under the following conditions:

1. Microbial activities can produce organic or mineral acids that can dissolve phosphorus bound to iron, aluminum, or manganese [55]. These activities may also release chelates such as gluconate and 2-ketogluconate. Other factors to consider include the revival of H₂S and iron. Additionally, a decrease in pH can result from the absorption of ammonium ions by fungi, leading to dissolution [56].

2. Nitrate increases the redox potential [56]. Other cases include the recovery of sulfate to pyrite and the recovery of methane.

3. Stumm and Morgan [54] pointed out that an increase in salinity and a decrease in pH decrease phosphorus absorption on iron oxides.

Ligand exchange: As a result of increasing pH, the concentration of OH⁻ increases and ligand exchange takes place with PO₄⁻³ located on iron hydroxides [57].

Enzymatic hydrolyses: Enzymatic hydrolyses in aquatic ecosystems are critical processes driven by microbial activities.

3.1.3.1 Microbial activities

Enzymes present in the structure of microbes cause mineralization of organic phosphorus and accelerate its release [14, 58]. An increase in temperature due to the activity of microbes intensifies microbial activities [53].

3.1.3.2 Decomposition

During decomposition, a recovery state occurs. In this case, trivalent iron is regenerated to ferric iron, and phosphorus bound to iron is released into the sediments. Manganese also undergoes regeneration under these conditions [59].

3.2 Eutrophication

The term “eutrophic” originates from the Greek word “Eutrophos,” meaning “good formation” and “proper union.” Similarly, the term “eutrophication” is derived from the Latin words “affectus,” meaning “good,” and “nutrimens,” meaning “food.” In Greek, “trophi” means food, and this term is usually accompanied by prefixes such as “oligo,” “meso,” “eu,” and “hyper,” which mean “rare, little,” “moderate,” “abundant,” and “severe, high,” respectively. Therefore, the words “oligotrophic,” “mesotrophic,” “eutrophic,” and “hypertrophic” are used to express the different states of nutrient levels in lakes or freshwater environments.

There are several definitions of eutrophication, and some of these definitions have fundamental differences from others. One of the differences is related to whether this phenomenon is simply a mechanism for increasing nutrients or if it should also include issues related to enrichment [60, 61, 62].

1. In South Africa, this phenomenon is generally defined as “eutrophication is the natural aging mechanism of lakes” [60].

2. The natural mechanism of increasing biological production due to the increase (enrichment) of nutrients [63].

3. Excessive growth of algae due to the enrichment of water with nutrients, especially nitrogenous and phosphorous compounds, and the reduction of water quality due to the disruption of the existing balance between microorganisms [62].

3.2.1 Eutrophication factors

In natural lakes, there is a distinction between natural and artificial (created by humans) eutrophication. Natural eutrophication depends only on the geology of the area and the natural characteristics of the watershed. If eutrophication is related to human activity (adding a large amount of nutrients to the aquatic ecosystem), which exacerbates this natural phenomenon, these nutrients originate from point and non-point sources of pollution, such as phosphorus in sediments [60]. The change in land use alters the amount of nutrients in the runoff. According to studies conducted, phosphorus removed from agricultural lands is at least five times greater, and from urban areas, 10 times greater than from forest lands. Enrichment of water by nutrients exists in nature, but human activities intensify it, and this process occurs almost everywhere in the world. Three important human-related sources of nutrient input are:

1. Runoff and erosion.

2. Leaching of substances in the chemical fertilizers from agricultural fields.

3. Wastewater from urban areas and sewage from factories.

The most significant cause of eutrophication is the introduction of a large amount of nutrients into the water, which causes an imbalance in the nutrient cycle and greatly increases the amount of phytoplankton biomass, ultimately leading to the growth of algae. The direct result of the increase in biomass is the high consumption of oxygen underwater [62].

3.2.2 Trophic states

Various indicators are used to measure the degree of this phenomenon, and one of the best indicators is the use of Nurenberg’s classification [64]. This classification is based on the concentration of total phosphorus and nitrogen and chlorophyll a (Table 2) [65, 66].

Trophy status	Total phosphorus concentration	Total nitrogen concentration	Chlorophyll a concentration
Oligotrophic	>0.01	>0.35	>0.0035
Mesotrophic	0.03–0.01	0.65–0.35	0.009–0.0035
Eutrophic	0.1–0.03	1.2–0.65	0.025–0.009
Hypertrophic	0.1<	1.2<	>0.025

Table 2.

Proposed classification system for the status of the Nuremberg trophy.

All units are based on mg/l.

The concept of trophic state is complex and lacks a precise definition. In the past, the trophic state was mainly related to the amount of nutrients in a lake. However, it now also takes into account biological factors that can cause changes in the lake’s morphology. The term “eutrophic state” is not only applicable to lakes with high nutrient levels, but also to shallow wetlands and places where aquatic plants grow, even if nutrient levels are not high. Lakes are classified into four groups based on their trophic state: “oligotrophic,” “mesotrophic,” “eutrophic,” and “hypertrophic.”

3.2.2.1 Oligotrophy

If the water is in this state, it contains few nutrients and there is minimal biological activity. The level of photosynthesis is very low and the water is very clear, allowing sunlight to penetrate deeply. Usually, the dissolved oxygen content in the water is high.

3.2.2.2 Eutrophy

In eutrophic conditions, the situation becomes more complex. In this case, the water contains an abundance of nutrients, which fuels high levels of biological activity. As a result, light is absorbed on the surface of the water and does not reach the depths, resulting in cloudy water. Oxygen levels in the water are very low, and deposited organic materials decompose through anaerobic processes, further contributing to eutrophication. The lack of oxygen in these waters also makes them uninhabitable for fish, which often die off. Nutrient enrichment is the main factor responsible for the degradation of surface water quality, causing intense growth of algae, which can reduce activities related to fisheries, recreation, and industry.

Algae grow rapidly in eutrophic waters and form a layer on the surface that can lead to a reduction in dissolved oxygen levels, increased suspended solids, and a decrease in the diversity of aquatic species. Strong growth of cyanobacteria, in particular, can occur in these waters, causing health problems for humans and livestock if these waters are used. The growth of algae can also release toxic and volatile substances, causing nerve damage and raising concerns about the phenomenon of nutrient enrichment. While nitrogen and carbon are also essential for the growth of aquatic organisms, controlling their levels is difficult due to the atmospheric and water cycles of these elements. However, phosphorus has been identified as the most limiting element for the growth of aquatic organisms, as evidenced by numerous studies (Figure 4).

Figure 4.

The role of phosphorus in eutrophication of water.

3.2.3 Consequences of eutrophication

The primary consequence of eutrophication is a decrease in the amount of dissolved oxygen in water. During photosynthesis, plants release oxygen into the water using sunlight. Conversely, in dark conditions, organisms, plants, and aerobic microorganisms consume dissolved oxygen during respiration. The balance between photosynthesis and respiration depends on the growth and population of the biomass. As a result of the high accumulation of biomass and the presence of an oxidation mechanism, underwater sediments form from the living mass, leading to a depletion of dissolved oxygen. Additionally, some decomposing bacteria consume oxygen from sulfate, resulting in the release of sulfur and the trapping of oxygen in the upper layers of the water. This reduction in oxygen levels endangers aquatic life. In the absence of oxygen, certain types of food can become toxic compounds, such as nitrate turning into ammonium, sulfate turning into hydrogen sulfide, and carbon dioxide turning into methane. These compounds are harmful to aquatic organisms.

1. Increasing the cost and problems associated with purifying water to make it drinkable.

2. Decreasing the quality of edible fish and replacing them with fast-growing macrophytes and algae.

3. Accumulation of sediment on the seabed, which causes a decrease in depth, an increase in vegetation, and disturbance to navigation.

4. Decomposition of algae products and surface scum that cause unpleasant odors (hydrogen sulfide, methane, etc.).

5. Increasing the population of pathogenic insects and mosquitoes.

3.3 Evidences on the importance of phosphorus in aquatic ecosystems

Experiments have shown that in lake systems, an increase in phosphorus relative to nitrogen leads to a significant increase in the number of cells of certain algae. This suggests that phosphorus is more limiting than nitrogen in this type of lake [67]. Additionally, evidence has shown that in cases of phosphorus deficiency in aquatic ecosystems, there is a direct correlation between the growth rate of algae (the rate of cell division) and the amount of phosphorus available within each cell [68, 69].

In 1972, Powers et al., conducted an experiment in which they placed 320 liters of water from Lake Organ and Minnesota in a closed environment and enriched it with various nutrients. By adding phosphorus, they observed a positive response from the system and concluded that phosphorus is the primary limiting factor. An experiment conducted by Schindler between 1974 and 1977 in the Ontario experimental lake clearly demonstrated the limitation of phosphorus. In this experiment, the lake water was saturated with phosphorus for several years, and the lake used atmospheric carbon and nitrogen to grow algae. The result of this work was an increase in primary nutrients, creating a eutrophic state in the lake water. An excess of phosphorus in the water acted as the trigger for the excessive bloom of cyanobacteria. When phosphorus, carbon, and nitrogen were added in deficiency, the effects were minimal [70].

In 1976, Vollenweider Weider created a model that could predict the eutrophic state in lakes and water reservoirs, with the only input element being phosphorus. This model was used worldwide and predicted the creation of the eutrophic state with very high accuracy. This model is in the form of Eq. 1:

$${\mathit{Cl}}_a=\frac{\left(L_p/Q_s\right)}{\left[1+\sqrt{\frac{z}{Q_s}}\right]}$$E1

These are: Cl_a is the algal biomass in units (mg/m³), L_p is the input amount of phosphorus (g/m².d¹) and Q_s is the output amount of the lake per unit of lake area (m/Acre).

A group of researchers has discovered a sigmoidal relationship between the logarithm of the total phosphorus concentration in the summer and the logarithm of chlorophyll levels. As the total phosphorus concentration increases, the amount of chlorophyll reaches a constant state, beyond which the amount of chlorophyll remains almost constant with further increases in phosphorus. This finding contrasts with the previously assumed linear relationship. In the case where the phosphorus concentration reaches its maximum value, adding more phosphorus to the water does not increase the chlorophyll concentration, while adding nitrogen intensifies the enrichment phenomenon [71]. It should be noted that as we move from freshwaters to coastal waters and oceans, the limiting element changes from phosphorus to nitrogen [72]. However, some researchers have challenged this idea [73]. Due to the high persistence of phosphorus in lakes, it is still considered the most limiting factor [70].

3.4 Different parts of phosphorus in water

There are three main terms used to describe phosphorus in water: soluble reactive phosphorus (DRP), total phosphorus (TP), and phosphorus bound to suspended solids. DRP refers to the portion of water phosphorus that can pass through a filter with a pore size of less than or equal to 0.45 μm and is analyzed using a colorimetric method. This method only measures the readily available portion of phosphorus, known as soluble orthophosphate, which directly contributes to water enrichment for aquatic plants. However, it should be noted that the filter with a pore size of less than 0.45 micrometers sometimes fails to effectively separate phosphorus.

To obtain the total phosphorus content of water, unfiltered water is digested with a strong acid, and the phosphorus content is measured using a colorimetric method. During the digestion stage, polyphosphates and phosphates attached to organic materials, which cannot be measured using the colorimetric method, are converted to orthophosphate and measured. Subtracting DRP from TP yields the phosphorus bound to suspended solids in the water. It is important to note that suspended solids play a crucial role in the phosphorus cycle in water. Therefore, water analysis measures these three parts of phosphorus: DRP, phosphorus bound to suspended solids, and TP (which is the sum of the previous two parts).

3.5 Phosphorus cycle in water and the role of suspended particles and sediments in this cycle

In aquatic ecosystems such as lakes and rivers, phosphorus is typically present in the pentavalent form. This includes various types of compounds such as orthophosphates, pyrophosphates, long-chain polyphosphates, organophosphate esters, and phosphodiesters, as well as organic phosphates. When phosphorus enters surface water, it can become attached to solid particles in the form of phosphates and organophosphates. Through chemical or enzymatic processes, this bound phosphorus may then be released into the water as orthophosphates, which are the only form of phosphorus that can be absorbed by plants, algae, and bacteria through hydrolysis. Phosphorus is highly dynamic in water environments and is biologically active. Once phosphorus reaches surface water, it may become attached to solid particles and settle at the bottom of lakes. Microbial communities can then gradually consume the organic parts of these particles, releasing the phosphorus in the form of orthophosphate back into the water, as shown in Figure 5. It is important to note that phosphorus bound to solid particles and dissolved organic phosphorus are not inert in water conditions, as they can convert into soluble orthophosphate under certain conditions.

Figure 5.

Schematic diagram of the phosphorus cycle between water and bottom sediments [70].

Phosphorus that enters lakes, reservoirs, and river estuaries is absorbed by various biological structures, sediments, and living organisms and deposited in the depths (as shown in Figure 5). This process helps prevent the widespread distribution of phosphorus in the water, but it also renders aquatic ecosystems highly sensitive to excessive phosphorus. In oligotrophic conditions, the deposited phosphorus may remain in sediments for years. However, in eutrophic conditions, the deep water is often in an anaerobic state, which can also occur in shallow waters on warm, windless nights. In this eutrophic state, most of the phosphorus deposited in sediments decomposes and spreads into the water.

Some researchers suggest that phosphorus attached to solid particles entering surface waters does not have much effect on the growth of aquatic algae [74]. However, several reports have indicated that dissolved orthophosphates in surface waters are linked to their dynamic relationship with particulate phosphorus and sediments at the lake bottom [75]. The dynamic balance between phosphorus bound to suspended particles and dissolved phosphorus is known as a phosphate buffering mechanism in water [76]. From a kinetic perspective, dissolved phosphorus reacts with suspended particles in water during two stages. The first stage, or fast stage, occurs in less than a few seconds, while the second stage, or slow stage, takes several days. During the fast phase, phosphorus attaches to the surfaces of suspended particles, but during the slow phase, phosphorus penetrates into the structure of the particles. When runoff with suspended particles is discharged into surface waters, exchanges occur between phosphorus attached to these particles and phosphorus dissolved in the water, creating a new balance. If the concentration of soluble phosphorus in the lake water is low, phosphorus is released from the suspended sediments into the water, and vice versa.

As suspended particles settle on the lake bottom, complex conditions arise. Bacterial activities cause gradual mineralization of organic phosphorus, releasing it into the water within the pores of lake bottom sediments. In the next step, dissolved phosphorus may spread into the lake water or be absorbed by the surface of the particles (sediments) before it can spread into the water. Phosphorus bonding with aluminum hydroxides and ferric iron on the surface of sediments can be very strong. If the water in the sediments’ pores undergoes regeneration (due to biological activities), ferric iron is converted to ferrous iron, and the bond between iron and phosphorus becomes very weak, facilitating phosphorus to enter the water column [50, 77, 78]. Therefore, the amount of phosphorus exchange between the water column and bottom sediments may change seasonally.

3.6 Range of phosphorus soluble in water

To understand the extent of eutrophication caused by phosphorus in water, it is crucial to measure the total phosphorus concentration instead of just orthophosphate. This is because orthophosphate concentrations can rapidly change within minutes under enriched conditions. Laboratory studies indicate that the concentration of phosphate required for balanced algae growth ranges from 0.003 to 0.8 μg/L. However, in Lake Michigan, it was observed that a concentration of 15 μg/L significantly increased carbon fixation and chlorophyll concentration. While a few micrograms per liter of phosphorus can enhance algae growth in most water systems, there is no agreement among researchers on the exact concentration that causes eutrophication. Some researchers suggest that a concentration between 0.01 and 0.03 mg/L of phosphorus can lead to excessive growth of harmful algae, while others propose a threshold of 0.02 mg/L for accelerated eutrophication. It is essential to note that the appropriate threshold for phosphorus concentration depends on the specific area being studied.

3.7 Methodologies for addressing insights into phosphorus pollution in aquatic environments

Various methodologies are employed to study and address phosphorus pollution in aquatic environments.

These methods allow researchers and environmental professionals to assess the extent of pollution, identify sources, and develop effective mitigation strategies.

Here are the brief details of some common methodologies used.

3.7.1 Water sampling and analysis

Water samples are collected from different points within a water body and analyzed for phosphorus content. Techniques like colorimetry, spectrophotometry, and inductively coupled plasma (ICP) analysis are used to quantify total phosphorus and different forms (e.g., dissolved, particulate). This helps in understanding the distribution and concentration of phosphorus in the water column. Here’s how water sampling and analysis are applied to the study of phosphorus pollution in aquatic systems:

3.7.1.1 Sample collection

Water samples are collected from different locations, depths, and time points within the aquatic environment. Sampling may target areas influenced by pollution sources, such as agricultural runoff or wastewater discharges.

3.7.1.2 Phosphorus forms

Different forms of phosphorus are analyzed, including dissolved reactive phosphorus (DRP), particulate phosphorus, and total phosphorus. These forms provide insights into the various pathways and sources of phosphorus pollution.

3.7.1.3 Sampling depth profile

Collecting samples at different depths allows researchers to understand vertical variations in phosphorus concentrations. This is particularly important in stratified water bodies.

3.7.1.4 Sampling frequency

Frequent sampling over time provides data on temporal variations in phosphorus levels, capturing seasonal changes and short-term fluctuations.

3.7.1.5 In-situ measurement

In addition to collecting samples for laboratory analysis, in situ sensors and probes can measure real-time water quality parameters, including phosphorus levels.

3.7.1.6 Laboratory analysis

Laboratory techniques such as colorimetry, spectrophotometry, and ICP analysis are used to quantify phosphorus concentrations in water samples. These methods provide accurate and quantitative data.

3.7.1.7 Quality control

Proper sample handling, preservation, and storage are essential to maintain the integrity of collected samples. Quality control measures ensure that results are reliable.

3.7.1.8 Nutrient ratios

Besides phosphorus concentrations, nutrient ratios such as the nitrogen-to-phosphorus ratio (N:P ratio) are often assessed to understand nutrient limitations and potential impacts on ecosystem dynamics.

3.7.1.9 Source identification

Isotopic analysis of phosphorus can help identify pollution sources. Different sources have distinctive isotopic signatures that can be traced in water bodies.

3.7.1.10 Spatial mapping

Results from water sampling can be spatially mapped using Geographic Information Systems (GIS) to visualize the distribution of phosphorus pollution within aquatic systems.

3.7.1.11 Long-term monitoring

Continuous or periodic water sampling and analysis provide data for long-term monitoring programs, allowing researchers to track changes over years or decades.

3.7.1.12 Baseline assessment

Water sampling establishes baseline data on phosphorus levels, aiding in evaluating the effectiveness of pollution control measures and management strategies.

3.7.1.13 Regulatory compliance

Water sampling is often conducted to assess compliance with water quality standards and regulations related to phosphorus pollution.

3.7.2 Sediment analysis

Sediments can act as reservoirs of phosphorus, contributing to its release into the water. Sediment core sampling and analysis provide information about historical phosphorus deposition and storage. Sequential extraction methods are used to determine different forms of phosphorus bound to sediments.

3.7.3 Nutrient budgets

Nutrient budgets involve accounting for inputs and outputs of phosphorus within a water body. This includes quantifying point sources (e.g., wastewater treatment plants) and non-point sources (e.g., agriculture runoff). By understanding the sources and pathways, effective management strategies can be devised.

3.7.4 Modeling

Mathematical models, such as hydrodynamic and water quality models, simulate phosphorus transport and distribution in aquatic systems. These models consider factors like flow rates, sedimentation, and nutrient interactions to predict phosphorus behavior under different scenarios. Modeling and mathematical approaches help synthesize complex interactions within aquatic systems and provide insights into how phosphorus pollution affects water quality, ecosystem health, and the efficacy of management strategies. They are valuable tools for designing effective interventions to mitigate phosphorus pollution and its environmental consequences.

3.7.4.1 Hydrodynamic models

These models simulate water movement, flow patterns, and circulation within aquatic systems. They help understand how phosphorus is transported through water bodies and how it interacts with sediment and other components.

3.7.4.2 Water quality models

Water quality models integrate hydrodynamics and biochemical processes to simulate nutrient dynamics, including phosphorus concentrations. These models consider factors like nutrient uptake, sediment interactions, and biological processes that influence phosphorus levels.

3.7.4.3 Eutrophication models

Eutrophication models specifically focus on nutrient enrichment, including phosphorus, and its effects on aquatic ecosystems. They help predict the development of algal blooms, oxygen depletion, and other consequences of nutrient pollution.

3.7.4.4 Mass balance models

Mass balance models quantify phosphorus inputs, outputs, and internal cycling within a water body. They consider sources such as point and non-point pollution, as well as sinks like sedimentation and nutrient uptake by organisms.

3.7.4.5 Reaction kinetics models

These models describe the rates at which chemical reactions involving phosphorus occur in aquatic systems. They help predict how phosphorus transformations and interactions with other elements change over time.

3.7.4.6 Sediment transport models

These models focus on sediment dynamics and how they transport phosphorus. They help understand erosion, sedimentation, and the release of phosphorus from sediment into the water column.

3.7.4.7 Nutrient loading models

Nutrient loading models estimate the amount of phosphorus entering aquatic systems from various sources, such as agricultural runoff, urban stormwater, and wastewater discharge.

3.7.4.8 Scenario analysis

Modeling allows researchers to explore different scenarios and assess the potential outcomes of management strategies. For example, they can predict the effects of reducing phosphorus inputs from specific sources.

3.7.4.9 Validation and calibration

Models are often calibrated and validated using real-world data to ensure accuracy. This involves adjusting model parameters to match observed conditions and using historical data to test the model’s predictive capabilities.

3.7.4.10 Policy and decision support

Mathematical models provide a scientific basis for making informed decisions about phosphorus management. They help evaluate the potential impact of different policies and management actions.

3.7.4.11 Data integration

Models often require input data such as nutrient concentrations, flow rates, and bathymetry. Remote sensing, water sampling, and other data collection methods provide essential information for model inputs and validation.

3.8 Phosphorus estimation methodologies

3.8.1 Estimation of different forms of phosphorus

Phosphorus exists in various forms, including dissolved reactive phosphorus (DRP), particulate phosphorus (PP), and total phosphorus (TP). Laboratory methods are crucial for measuring different forms of phosphorus in water. There are a number of methods widely used for measuring different forms of phosphorus in water samples. However, it is important to note that the choice of method depends on factors such as the specific form of phosphorus being measured, the sensitivity required, and the laboratory’s equipment and capabilities. Proper sample collection, handling, and preservation are essential to ensure accurate results. Additionally, quality control measures, including calibration with standards and replicates, help ensure the reliability of measurements.

Here are common laboratory methods used to measure these different forms:

3.8.1.1 Dissolved reactive phosphorus (DRP)

3.8.1.1.1 Molybdate blue method

This colorimetric method involves reacting phosphate ions with molybdate reagents to form a blue complex. The intensity of the blue color is proportional to the concentration of DRP and can be measured spectrophotometrically.

Major procedure involved include:

1. Sample collection and preparation

   • Collect a representative water sample in a clean and acid-washed container to prevent contamination.

   • If necessary, filter the sample to remove any particles that could interfere with the colorimetric reaction.

2. Reagent preparation

   • Prepare the molybdate reagent by mixing ammonium molybdate and a reducing agent (ascorbic acid) in an acidic solution.

   • The reagents are usually available as a kit or can be prepared according to standard protocols.

3. Color development

   • In a cuvette or test tube, add a measured volume of the sample and an appropriate volume of the molybdate reagent.

   • Allow the mixture to react for a specific period to allow the formation of the molybdenum blue complex.

4. Measurement

   • Using a spectrophotometer, measure the absorbance of the blue-colored solution at a specific wavelength, typically around 880 nm.

   • The absorbance value is proportional to the concentration of DRP in the sample.

5. Calibration and calculation

   • Create a calibration curve using known concentrations of phosphate standards treated with the same reagents and procedure.

   • Use the calibration curve to determine the concentration of DRP in the sample based on its absorbance value.

6. Quality control

   • Include blank samples (water without phosphate) and replicate measurements to ensure accuracy.

   • Regularly calibrate the spectrophotometer using standard solutions.

7. Reporting results

   • Express the results in units of phosphorus concentration (e.g., mg/L or μg/L).

3.8.1.1.2 Ascorbic acid method

Phosphate reacts with ascorbic acid in the presence of ammonium molybdate to form a blue color. The color intensity is measured using a spectrophotometer.

Major procedure involved include:

1. Sample collection and preparation

   • Collect a representative water sample and ensure it is properly preserved and stored to prevent changes in phosphorus concentrations.

   • If needed, filter the sample to remove any particulate matter.

2. Reagent preparation

   • Prepare the molybdate reagent by dissolving ammonium molybdate in an acidic solution.

   • Prepare a solution of ascorbic acid in deionized water.

3. Color development

   • In a cuvette or test tube, add a measured volume of the sample, the molybdate reagent, and the ascorbic acid solution.

   • Mix the contents thoroughly and allow the reaction to occur for a specified period.

4. Measurement

   • Use a spectrophotometer to measure the absorbance of the blue-colored solution at a specific wavelength, usually around 880 nm.

5. Calibration and calculation

   • Create a calibration curve using known concentrations of phosphate standards treated with the same reagents and procedure.

   • Use the calibration curve to determine the concentration of DRP in the sample based on its absorbance value.

6. Quality control

   • Include blank samples (water without phosphate) and perform replicate measurements to ensure accuracy.

   • Regularly calibrate the spectrophotometer using standard solutions.

7. Reporting results

   • Express the results in units of phosphorus concentration, such as mg/L or μg/L.

3.8.1.2 Particulate phosphorus (PP)

• Filtration and Gravimetric Method: Water samples are filtered to separate suspended particles. The filter is dried and weighed before and after filtration. The difference in weight represents the mass of particulate phosphorus.

3.8.1.3 Total phosphorus (TP)

1. Persulfate Digestion Method: Water samples are digested with persulfate to oxidize all forms of phosphorus to orthophosphate. The resulting orthophosphate is then measured using colorimetric methods like those mentioned for DRP.

   Major procedure involved include:

2. Sample collection and preservation

   • Collect a representative water sample in a clean and acid-washed container. For accurate results, ensure proper preservation and storage.

3. Reagent preparation

   • Prepare a reagent solution by dissolving ammonium persulfate and potassium persulfate in deionized water.

   • Optionally, prepare a blank solution using deionized water to account for any background contamination.

4. Digestion

   • In a digestion tube or vessel, add a measured volume of the water sample and an appropriate volume of the persulfate reagent.

   • Heat the mixture in a digestion block or other appropriate heating apparatus to a high temperature (usually around 120–160°C) for a specified period (often about 1–2 hours).

   • The high temperature breaks down the organic and inorganic phosphorus compounds, converting them to orthophosphate.

5. Cooling and dilution

   • Allow the digested sample to cool to room temperature after the digestion period.

   • Dilute the digested sample with deionized water to bring it within the working range of the colorimetric method.

6. Orthophosphate measurement

   • Use a colorimetric method, such as the Molybdate Blue Method or the Ascorbic Acid Method, to measure the concentration of orthophosphate in the digested sample.

   • Prepare standard solutions of known phosphate concentration to create a calibration curve for accurate quantification.

7. Quality control

   • Perform replicate measurements and include a blank solution to ensure accuracy.

   • Regularly calibrate the colorimetric instrument using standard solutions.

8. Calculation and reporting

   • Calculate the total phosphorus concentration in the sample based on the measured orthophosphate concentration and any dilution factors used.

3.9 Monitoring employing remote sensing

Satellite and aerial imagery can be used to detect changes in water quality associated with phosphorus pollution. Remote sensing allows for the monitoring of algal blooms, turbidity, and other indicators of nutrient enrichment. Remote sensing plays a crucial role in assessing and monitoring phosphorus pollution in aquatic environments. It provides valuable information about water quality, algal blooms, sediment dynamics, and other indicators of nutrient enrichment.

Here are some key applications of remote sensing in the context of phosphorus pollution in aquatic systems:

3.9.1 Algal bloom detection and monitoring

Remote sensing can identify and track algal blooms, which often result from excessive phosphorus levels. Sensors on satellites and aircraft can detect the unique spectral signatures of chlorophyll-a, a pigment in algae. Monitoring changes in chlorophyll-a concentrations helps in early detection and management of harmful algal blooms.

3.9.2 Water transparency and turbidity

Phosphorus pollution can lead to reduced water transparency due to suspended particles. Remote sensing measures water turbidity, which is indicative of sediment and nutrient loads. Monitoring turbidity helps understand how phosphorus affects water clarity.

3.9.3 Total phosphorus concentrations

Remote sensing algorithms can estimate total phosphorus concentrations in water bodies by analyzing spectral data. Correlations between spectral reflectance and nutrient concentrations allow researchers to infer phosphorus levels.

3.9.4 Erosion and sediment transport

Sediment transport is closely linked to phosphorus pollution. Remote sensing helps monitor changes in shoreline morphology and sediment plumes, providing insights into erosion patterns and sediment dynamics.

3.9.5 Land use and land cover changes

Remote sensing helps monitor changes in land use and land cover, including urban expansion and agricultural activities. These changes often contribute to phosphorus pollution through runoff and erosion.

3.9.6 Reservoir management

Remote sensing assists in managing reservoirs by tracking sediment deposition and nutrient accumulation. This information helps make informed decisions about water quality and ecosystem health.

3.9.7 Spatial analysis and mapping

Remote sensing data, when combined with Geographic Information Systems (GIS), enables the creation of spatial distribution maps of phosphorus concentrations and pollution sources. These maps guide management efforts.

3.9.8 Temporal trend analysis

Remote sensing allows for long-term monitoring of phosphorus-related parameters. Analyzing trends over time helps understand the effects of phosphorus pollution and assess the effectiveness of mitigation strategies.

3.9.9 Early warning systems

Remote sensing can be integrated into early warning systems for algal blooms. Timely detection of blooms allows water managers to take proactive measures to protect public health and ecosystem integrity.

3.9.10 Data integration

Remote sensing data can be combined with other datasets, such as water quality measurements and meteorological data, to develop comprehensive models for predicting phosphorus pollution dynamics.

3.10 Monitoring employing biological monitoring

Aquatic organisms respond to changes in phosphorus levels. Monitoring the health and composition of aquatic communities can provide insights into the effects of phosphorus pollution. Biotic indices, like the Trophic State Index, are used to assess water quality based on the presence of indicator species.

3.11 Mitigation approaches

3.11.1 Best management practices (BMPs)

BMPs involve implementing strategies to reduce phosphorus inputs. This includes practices such as buffer zones, cover crops, and optimized fertilizer application in agriculture, as well as improved wastewater treatment methods. BMPs play a significant role in mitigating phosphorus pollution in aquatic environments. These practices are a set of guidelines, strategies, and techniques designed to minimize the release of phosphorus into water bodies. They address various pollution sources and promote sustainable land and water management.

Here are some key BMPs used to tackle phosphorus pollution in aquatic systems.

3.11.1.1 Agricultural BMPs

• Nutrient management plans: Implementing precise fertilizer application based on soil nutrient levels to reduce excess phosphorus runoff from agricultural fields.

• Cover crops: Planting cover crops during fallow periods to reduce soil erosion and nutrient runoff.

• Buffer zones: Creating vegetated buffers along water bodies to trap sediment and nutrients before they enter aquatic environments.

• Conservation tillage: Reducing soil disturbance during planting to minimize erosion and nutrient loss.

• Rotational grazing: Managing livestock grazing to prevent overgrazing, soil compaction, and nutrient runoff.

3.11.1.2 Urban BMPs

• Storm water management: Implementing green infrastructure, such as permeable pavements, rain gardens, and detention basins, to capture and treat stormwater runoff before it enters water bodies.

• Erosion control: Using erosion control measures on construction sites to prevent sediment and nutrient runoff into nearby water bodies.

• Reduced lawn fertilization: Encouraging responsible lawn care practices, including proper fertilization and avoiding excessive phosphorus use on lawns.

3.11.1.3 Wastewater treatment BMPs

• Advanced treatment systems: Upgrading wastewater treatment plants with advanced nutrient removal technologies to reduce phosphorus discharge into receiving waters.

• Phosphorus removal facilities: Adding specific treatment units to remove phosphorus from wastewater before discharge.

• Septic system maintenance: Properly maintaining septic systems to prevent nutrient leaching into groundwater and nearby water bodies.

3.11.1.4 Riparian zone restoration

• Vegetation planting: Restoring native vegetation along shorelines and riverbanks to stabilize soil, prevent erosion, and filter runoff.

• Natural shoreline design: Creating gentle, natural shorelines instead of hardened structures to promote sediment and nutrient retention.

3.11.1.5 Educational and outreach programs

• Public awareness: Educating residents, farmers, and businesses about the impacts of phosphorus pollution and the importance of BMPs.

• Training workshops: Providing training and workshops on implementing effective BMPs to different stakeholders.

3.11.1.6 Land use planning and zoning

• Zoning regulations: Incorporating phosphorus pollution considerations into land use planning and zoning regulations to prevent new pollution sources.

• Smart growth: Encouraging compact and sustainable development to reduce urban sprawl and its associated runoff.

3.11.1.7 Research and monitoring

• Monitoring programs: Establishing long-term water quality monitoring programs to track phosphorus levels and assess the effectiveness of BMPs.

• Adaptive management: Using collected data to adjust and refine BMP implementation based on real-world results.

3.11.2 Riparian restoration

Restoring riparian vegetation along water bodies helps in reducing phosphorus runoff and erosion. Plant roots stabilize soil, preventing sediment and phosphorus from entering waterways.

3.11.3 Green infrastructure

Incorporating green infrastructure like constructed wetlands and vegetated swales can capture and filter phosphorus from storm water runoff before it reaches water bodies.

3.11.4 Public awareness and education

Increasing public understanding of the sources and consequences of phosphorus pollution can lead to better practices, such as proper waste disposal and reduced fertilizer use.

4. Phosphorus in sediments

The role of sediment as a chemical pollutant depends on various factors, such as the size of the particles that make up the sediment, and the amount of organic matter and nutrients present in it. Nutrients can be released through surface runoff, which is influenced by soil type, vegetation, rainfall, and land use practices. The use of phosphate and other fertilizers in agricultural lands, as well as landfills, can contribute to the transfer of persistent pollutants to the soil and, ultimately, to runoff through leaching [79].

Since erosion tends to selectively transport the smallest soil particles, which are often rich in nutrients and organic substances, the high specific surface area of these particles can lead to the absorption of nutrients, toxins, and pests, resulting in their accumulation in sediments. Suspended sediments are the primary vehicle for transporting nutrients, toxic substances, and chemical elements in aquatic environments. Therefore, sediments from eroded soils can have different environmental impacts depending on the materials and elements they contain.

Concerns about the increase in nitrogen, phosphorus, and pesticide compounds in surface and underground waters have been growing in Europe since the 1970s. Agriculture is identified as the main source of phosphorus compounds and sediments in a recent comparison between domestic, industrial, and agricultural sources of pollution in the Mediterranean basin. The European community has responded with the directive EEC 16/6/1991, which aims to protect waters from pollution caused by nitrates from agriculture. In France, this issue has led to the formation of an advisory committee under the supervision of the Ministry of Agriculture and Environment to reduce nitrogen and phosphorus pollution in agriculture.

An analysis by the United States Environmental Protection Agency in 1994 identified agriculture as the primary land use that degrades wetlands and other wetland areas. As nutrients are removed from the soil during the erosion process and transferred to rivers, water reservoirs, and other water sources, eutrophication and oxygen depletion can occur, leading to a decline in water quality [80, 81, 82].

The physical-chemical properties of sediments, including amorphous iron and aluminum content, the amount of clay, and other characteristics, play a critical role in the absorption of phosphorus. Once absorbed, phosphorus can be released back into the water due to various biological and chemical activities, leading to water enrichment in certain cases. As stated by Sposito [83], the absorption and removal of phosphorus from surfaces are more complex than for other elements in soil.

4.1 Adsorption of phosphorus on sediments

Adsorption of phosphorus onto sediments is subject to complex conditions, as the physical, chemical, and biological characteristics of sediments play decisive roles in the absorption and retention of phosphorus. Under different environmental conditions, each of these features plays a different role. Iron and aluminum oxides and hydroxides are among the important chemical properties known to play a role in phosphorus absorption and storage [17]. The high ability of metal oxides and hydroxides to absorb phosphorus is probably due to their high specific levels [84]. Research has shown that the bond between phosphorus and amorphous aluminum is stronger than that with iron, as the bond with aluminum involves both electrostatic and hydrogen interactions, whereas with iron, it is primarily electrostatic [85]. Conducted research on sediment drains and concluded that over 88% of the phosphorus absorption capacity is related to the iron extracted with ammonium oxalate [86].

The amount of silt and clay in sediments is another parameter known to be important in phosphorus absorption. Sanyal and De Datta [87] have highlighted the significance of the relationship between phosphorus absorption and the amount of clay, which may be due to the high specific levels of clay. Some researchers have shown that reducing the particle size exponentially increases the amount of phosphorus absorption [88, 89]. It should also be noted that smaller particles are more easily disturbed by factors such as water waves and become suspended, providing more opportunities to react with dissolved phosphorus in water compared to larger particles [90]. Phosphorus is absorbed in the form of an inner-sphere complex on colloid surfaces, meaning that phosphorus absorption takes place in specific positions on colloid surfaces, and no water molecule is present between the surface and phosphate ion. The phosphate ion is directly absorbed by the functional groups on the surface of colloids. Phosphorus absorption occurs in two stages: surface absorption and sedimentation in a sequential form [85].

Organic carbon is one of the most important parameters in absorption, and studies have shown that the absorption capacity of phosphorus is dependent on the amount of organic matter in the sediments [91, 92]. There is a significant relationship between the absorption capacity and the amount of organic matter [20]. Ngoyan and Sukias [86] reported a very high correlation between iron and aluminum extracted with oxalate and organic carbon. This study showed that iron and aluminum may bond with organic carbon and play a much more effective role in phosphorus absorption. Under calcareous conditions, simultaneous absorption or precipitation of phosphorus with calcium carbonate has been reported. Solid phase reactions of CaCO₃ control phosphorus reactions. The pH level of sediments also plays a key role in phosphorus absorption. With a change in pH, iron and aluminum precipitation occurs. When the pH is higher than 8, phosphorus precipitates by forming a bond with calcium [93].

4.2 Phosphorus adsorption isotherms

Previous studies on soil have shown that the process of phosphorus absorption is non-linear due to varying energy levels in different binding sites on particles. The high-energy sites are usually occupied and filled first. The exchange of phosphorus between sediments that are resuspended in water and the surrounding water follows a two-stage dynamic, with rapid exchange occurring at the surface of particles and much slower exchange occurring internally [94]. Short-term equilibrium in the range of several days and long-term equilibrium over several months to several years are assumed to model these exchanges in soils. During periods of rapid transport, the absorption of phosphorus into soil materials decreases due to short contact times, low water-to-soil ratios in the flow path, and possibly facilitated colloidal transport. As a result, heavy rainfall and high concentrations of phosphorus may have strong impacts on the dynamics of phosphorus in agricultural basins with low drainage, and high concentrations of phosphorus may occur during periods of high flow. However, true equilibrium is rarely achieved.

Isotherm equations are used to study the mechanisms of phosphorus absorption in sediments and soil [84]. These equations allow for the determination of important absorption parameters, such as the phosphorus concentration at the equilibrium point (EPC₀), the degree of phosphorus saturation, and absorption energy [86, 95, 96]. If the EPC₀ and degree of phosphorus saturation in the soil are low and absorption energy is high, the sediments will have a high capacity to absorb phosphorus. Comparing the EPC₀ and the concentration of reactive phosphorus in the water solution (DRP) can determine whether the sediments act as a phosphorus reservoir or a source of phosphorus.

In general, there are three main reasons for using absorption isotherms: (1) identification of compounds that play a role in the absorption and release of phosphorus, (2) forecasting the amount of fertilizer needed to maximize production, and (3) studying the nature of absorption to understand the mechanism of these processes [87].

To study phosphorus absorption in sediments, a surface adsorption isotherm is generated by shaking the sediment sample with increasing concentrations of the ion solution. The amount of ion absorbed in each sample is calculated by measuring the difference between the initial concentration and the equilibrium concentration. Laboratory methods, such as those developed by Nelson and Logan in 1983, are commonly used to measure the characteristics of phosphorus absorption in sediments. The results obtained from these methods are then fitted with standard absorption isotherm models, such as Langmuir, Freundlich, and Temkin, as described in the studies by Rhue and Harris [97] and Graetz and Nair [98]. These equations are suitable for methods that reach equilibrium in a short time, but their range of success is limited to a certain concentration of phosphorus, as reported by [99].

Various equations have been employed to elucidate the relationship between the amount of phosphorus absorbed per unit weight of the adsorbent and the concentration of phosphorus in the solution.

4.2.1 Langmuir equation

The Langmuir equation is widely used in soil science. It was first used by Fried and Shapiro [100] and later by Olsen and Watanabe [99] to explain phosphate absorption in soil. The Langmuir equation is based on three main assumptions [101]:

1. The surface absorption energy is constant and independent of the amount of surface coverage, which means the surface is homogeneous.

2. Surface absorption occurs at specific sites without any interaction between the absorbing molecules.

3. The maximum surface absorption corresponds to a complete molecular layer covering the entire active surface of the absorber.

The general form of the Langmuir equation is as follows (Eq. 2):

$$S=\frac{\mathit{bK}C}{1+\mathit{KC}}$$E2

The surface adsorption isotherm can be expressed using the Langmuir equation, where S represents the amount of absorption, b (also known as S_max) is the maximum absorption of a single layer that occurs as the equilibrium concentration, C, increases, and K is a parameter that reflects the surface’s absorption capacity. The Langmuir equation has one main advantage: it reveals the maximum absorption capacity of a surface (S_max). Although other equations, such as Redlich-Peterson and Fowler-Guggenheim, can predict the maximum absorption, they are more complex and challenging to use. In comparison to other isotherm equations, the Langmuir equation offers more comprehensive information about phosphorus absorption in soil and sediment [102]. Haines et al. [103] has utilized mathematical methods to further explore the features of these equations and showed that the slope of the Langmuir equation at a concentration close to zero is equal to:

$$\underset{\mathrm{c}\to 0}{lim}\frac{\mathrm{ds}}{\mathrm{dc}}=\mathrm{bK}$$E3

She also showed that we will achieve maximum absorption on surfaces when the equilibrium concentration is large enough:

$$\underset{\mathrm{c}\to \infty }{lim}\mathrm{s}={lim}_{\mathrm{c}\to \infty }\frac{\mathit{bK}C}{1+KC}=\frac{\mathit{bK}}{K}=b$$E4

In this equation, it is assumed that the absorbing sites do not participate in the absorption after absorbing phosphorus. In simpler terms, phosphorus absorption on absorbent surfaces occurs as a single layer [97, 102]. The graphic form of this equation is in Figure 6.

Figure 6.

Schematic display of Longmuir isotherm.

Sₒ: It is known as the amount of primary phosphorus in the absorption phase. EPCₒ is also known as the concentration of phosphorus at the equilibrium point. At this concentration, the absorption and excretion of phosphorus in sediments reach equilibrium [93, 104]. Between So and EPCₒ, phosphorus is released into the water. The slope of the isotherm line, referred to as K or absorption energy as mentioned before, represents the rate of change. In this graph, it is observed that after the initial rapid absorption, the amount of absorption gradually increases with the equilibrium concentration until reaching the maximum absorption (S_max). After reaching S_max, the slope of the line remains constant.

4.2.2 Freundlich equation

The Freundlich equation is commonly used for describing phosphorus absorption in soils and has the ability to fit absorption data quite well in most soil types [101]. Freundlich discovered a certain relationship that describes absorption from dilute solutions. This equation is primarily derived from experimental observations, but it can also be obtained theoretically by assuming that the bond energy decreases exponentially with the surface coverage, which is likely closer to the actual absorption conditions [105].

The Freundlich equation is considered superior to the Langmuir equation because it is simple and based on more realistic assumptions. It can account for non-ideal absorption on heterogeneous surfaces and the absorption of multiple layers. Assuming that the reduction in absorption energy with increasing surface coverage is due to the uniformity of the absorbing surfaces justifies the Freundlich equation [101]. The Freundlich equation is commonly defined as Eq. 5:

$$S=K_f{C}^{\alpha }$$E5

In this equation, α (alpha) and K_f are called adjustable positive values, where α is usually between zero and one, but in some cases it is seen that it is more than one. By using logarithms on equal sides, this equation becomes a straight line. Although the equation itself is sometimes used unchanged for simplicity. This equation with all its ability is not able to predict the maximum amount of absorption, that is, if the equilibrium concentration moves toward its maximum possible value, the amount of absorption also moves toward its maximum value, but it does not reach a constant value. (Opposite to the Langmuir equation), that is:

$$\underset{c\to \infty }{lim}S=\underset{c\to \infty }{lim}{K}_{f}C=K_f\left(\infty \right)=\infty$$E6

Therefore, this equation suggests that as the equilibrium concentration increases, the absorption on the surfaces also increases, and this increase in the amount of absorption is exponential according to the equation. According to some researchers, this equation is better at low equilibrium concentrations [2].

4.2.3 Tamkin’s equation

This equation has found a special place in soil science due to its simpler formula compared to other equations.

$$S=K_1\mathit{ln}c+K_2$$E7

In this equation, K₁ and K₂ are constant values. The weak point of this equation is the inability to predict the maximum amount of absorption on the surface [103].

4.2.4 Risk assessment indicators

In order to identify the points of the sediments that have the greatest potential to release phosphorus into the water, two important parameters, the equilibrium concentration of phosphorus at the zero point (EPCₒ) and the degree of phosphorus saturation (DPS) are used.

4.2.4.1 Phosphorus concentration at the equilibrium point

The most important result obtained from isotherm studies is the equilibrium phosphorus concentration (EPCₒ), which represents the equilibrium between absorption and desorption processes in pollution studies [36, 95]. By comparing the EPCₒ value to the concentration of dissolved reactive phosphorus (DRP) in the water solution, it is possible to determine whether sediments act as a source or sink of phosphorus. If EPCₒ is higher than the DRP concentration (EPCₒ > DRP), phosphorus will be released from the sediments into the water column. Conversely, if EPCₒ is lower than the DRP concentration (EPCₒ < DRP), sediments are absorbing phosphorus from the water column [84]. In simpler terms, sediments can either release or absorb phosphorus depending on the EPCₒ value, with higher EPCₒ values indicating a higher likelihood of phosphorus being released from sediments into the water column. The EPCₒ value is graphically represented by the point where the isotherm curve intersects the x-axis (equilibrium concentration axis).

4.2.4.2 Degree of phosphorus saturation in sediments

The investigation of the degree of phosphorus saturation can be traced back to the research of Breeuwsma and Silva [43], who aimed to establish a correlation between soil phosphorus content and its transfer to groundwater in Dutch soil. Since then, many researchers have used and modified their approach [86, 106, 107] to determine the critical level of phosphorus saturation. Points with a degree of saturation higher than this critical limit are more likely to release phosphorus into water compared to those with lower saturation levels. Breeuwsma and Silva [43] determined the critical level of phosphorus saturation in Dutch soils to be 25%. Sallade and Sims [40] raised this limit to 40% for sediments in American drains. Ngoyan and Sukias [86] found that although the degree of phosphorus saturation in some sediments located in drains was between 64 and 68%, the dissolved phosphorus concentration in the water of these drains was still low, indicating that the level was not sufficient to cause enrichment. On average, the degree of phosphorus saturation increased to more than 65%. The degree of phosphorus saturation reflects the actual state of phosphorus in the soil [108]. It indicates the amount of phosphorus accumulation in sediments relative to the maximum absorption capacity of phosphorus [86], as expressed in Eq. 8:

$$DPS\left(\%\right)=\frac{TP}{PSC}\times 100$$E8

That DPS is the degree of phosphorus saturation in percentage and PSC indicates the maximum absorption capacity of phosphorus at the desired depth from the soil [17]. TP is the amount of total phosphorus (which is obtained by digesting the sediment sample). The higher the degree of saturation in sediments, the greater the risk of releasing phosphorus into water [86]. Various methods have been used to measure the degree of saturation. Sometimes formula 9 is used in acid soils:

$$DPS\left(\%\right)=\frac{P_{ox}}{{\mathit{Fe}}_{ox}+{\mathit{Al}}_{ox}}\times 100$$E9

P_ox in this formula is phosphorus extractable by ammonium oxalate and Fe_ox + Al_ox is the sum of iron and aluminum extractable by ammonium oxalate [109, 110]. Sometimes Olsen or Mehlich phosphorus together with single point absorption index is used to determine the degree of saturation [102, 111]. These methods are referred to as indirect methods. From the direct method to measure the degree of phosphorus saturation, we can refer to the isotherm equations. Which is obtained by determining the amount of absorbed phosphorus and total phosphorus in sediments.

Methods such as the first two mentioned can be used to determine the degree of phosphorus saturation in specific soils, but the isotherm method is also time-consuming [108]. To address these challenges, Pöthig et al. [108] proposed a simpler method. They demonstrated that this method is applicable to all soil types and is independent of land use.

Water-soluble phosphorus (WSP) is determined based on the ratio of phosphorus absorption to total phosphorus (SP/TP). Each soil has its own level of phosphorus absorption and total phosphorus, but dividing these values cancels out their mutual effects:

$$DPS\left(\%\right)=\frac{TP}{PSC}\times 100$$E10

$$PSC=TP+SP$$E11

where SP is known as absorbed phosphorus and is calculated by isotherm experiments [108]. From the combination of the above two equations, the following relationship is obtained:

$$DPS\left(\%\right)=\frac{1}{1+\frac{SP}{TP}}\times 100$$E12

Therefore, WSP can be used instead of SP/TP ratio:

$$DPS\left(\%\right)=\frac{1}{1+f\left(WSP\right)}\times 100$$E13

After examining more than 400 different soil samples (sandy, loamy, peaty and calcareous soils), the following relationship was proposed:

$$DPS\left(\%\right)=\frac{1}{1+1.25\times WS{P}^{-0.75}}\times 100$$E14

In this method, when the concentration of phosphorus soluble in water exceeds 5 mg/kg, the degree of phosphorus saturation approaches 70–80%, which indicates a high risk of phosphorus transfer from soil to water. Due to the ease of determining the degree of phosphorus saturation, this method can determine the dangerous points easily and in the shortest time [108].

5. Conclusion

Phosphorus is a chemical element with the symbol “P” and atomic number 15. It is a fundamental element for life, playing a vital role in biological processes, energy transfer, and genetic information storage. While it is crucial for ecosystems, managing its levels in the soil-water-sediment environment is essential to prevent pollution therein and maintain their health. Notably, the transfer of phosphorus from soil to water is a complex and significant process that plays a crucial role in shaping the nutrient dynamics and ecological health of soil-water-sediment environment. The aspects discussed in this chapter may help in understanding the major insights into this transfer, which in turn may help in the effective management for preventing phosphorus pollution and maintaining soil-water-sediment health and quality. Further, a sound knowledge of the nutrient enrichment processes; pathways of phosphorus entry; sediment transport; ecological consequences; major ecosystem disturbances; mitigation strategies; regulatory actions; research and monitoring; and collaborative efforts is required for getting more insights into the implications and importance of phosphorus transfer processes in the soil-water-sediment environment.