Phosphate Solubilization Improvement for Plant Uptake from Phosphate Rock and Phosphate Solubilizing Microbes Consortium: Impact on Food Security

Abstract

Poverty alleviation in Sub-Saharan Africa is proportionally dependent on soil management. Low crop productivity has been linked to hunger and poverty as soil degradation is undeniably the cause. This chapter gives a general overview from major findings on how microbes could improve phosphate (P) levels in soils by enhancing its solubility. A cross-sectional study was under taken to highlight the role played by phosphate-solubilizing microbes—arbuscular mycorhizal fungi (AMF) and phosphate-solubilizing bacteria (PSB) in improving phosphate solubility. About 30–50% of phosphorus is organic which the plants could readily assimilate, while 50–70% is inorganic and inaccessible to plants. There are several mechanisms the plants utilize to optimize nutrient uptake from the root hairs to various parts of the plant to maximize crop production. The utilization of readily available minerals such as phosphate rock is known to play vital role in plant ecology and evolution, in checking drought stress, heavy metal toxicity, nutritional imbalances, plant pathogens, and salinity. Therefore, soil improvement using rock phosphate could potentially act in synergy with the phosphate-solubilizing microbes to boost phosphate levels in the soil. This could be a welcome development in low-income economies in the sub-Saharan Africa (SSA) to boost yield for profit maximization.

1. Introduction

Food production is crucial for our existence and every other life on earth. The rate at which phosphorus (P), a critical ingredient in growing food, is declining is alarming. It was estimated that existing rock phosphate reserves could be exhausted in the next 50–100 years [1, 2, 3]. Phosphorus is the second most likely nutrient deficient in the soils after nitrogen [3]. It is one of the most important nutrients for crop production. P availability is usually low in soils around the globe. Moreover, the low efficacy of P fertilizers in acidic and calcareous soils restricts P availability [4]. Recently, it has become increasingly difficult for small-scale farmers in developing countries to purchase chemical P fertilizers.

Rock phosphate is a sedimentary rock that contains high amounts of phosphorus. The rock is mined and in it contains clay and limestone [5]. Rock phosphate had a long history and had been used as organic fertilizer for gardens. It’s known for keeping plants healthy and encouraging new growth [5]. Rock phosphate can be used as crude phosphatic fertilizer by direct application to field soil. It was established that double application of RP and phosphate-solubilizing fungi/bacteria improve P content in soil [6]. Moreover, the farmers in sub-Saharan Africa (SSA) are facing high prices of phosphate fertilizers because of the low solubility of the African RPs. Therefore, an affordable P fertilizer supply from local low-grade phosphate rocks would strengthen crop production in SSA and improve farmers’ income.

Fungi play fundamental roles in regulating key ecosystem processes such as decomposition of organic matter and plant–soil relationship [7]. Arbuscular mycorrhizal fungi (AMF) on the other hand are widespread obligate plant symbionts that can colonize the roots of most land plants. They also assist in obtaining nutrients and protection against environmental stresses [8]. It was similarly reported that AMF symbiosis improves plant stress resistance and soil stability, making it a promising addition to sustainable agricultural practices [9]. It’s crucial to develop a method to enhance P solubility in African degraded soils. There have been significant positive reports on application of partially acidulated RPs on crop cultivation including pearl millet, sorghum, cowpea and maize. It’s well known that calcination of RP with Na carbonate increases its solubility, but this method is a classical option, which has been employed for ages in solubilizing low-graded RPs. It’s well known that calcination with Na carbonate increases its solubility but proven difficult due to high content of impurities such as silicates [10].

There is need to develop knowledge on natural biological agents that may sustainably improve crop performance and lessen our reliance on technology [11]. AMF symbiosis may be optimized to improve the sustainability of agricultural systems by increasing the ability of crops to absorb soil P, resist pathogens, and tolerate drought stress [12]. AMF are among the microbial groups that could solubilize mineral phosphates and improve plant phosphorus nutrition. AMF inoculation was also reported to induce spectacular stimulations of the plant growth and phosphorus foliar content [13].

2. Contributions of phosphate-solubilizing bacteria (PSB) and arbuscular mycorrhizal fungi (AMF) in improving P availability

A better understanding of how the beneficial microbes (PSB and AMF) and phosphate rock interact is crucial to preserving the soil fertility and improving the economic and environmental sustainability of crop production in P-deficient soils [14]. Previous studies have reported that SiO₂ is not the major constituent of RP but CaO (51.23%) [15] and could increase plant uptake of P. Combining Si and microorganisms application has been proposed to effectively induce and improve plant growth and nutrition [16, 17, 18]. It was earlier observed that AMF and Si (from SiO₂—a constituent of RP) work together to improve plant growth regardless of stress conditions. Similarly, PBS and SiO₂ synergistically help plants better uptake P [19, 20]. Another study indicates that SiO₂ contained in the rock phosphate is a quasi-essential nutrient and is beneficial to plants, especially when under different stresses such as drought, heavy metal toxicity, nutritional imbalance, plant pathogens, and salinity [21]. Si fertilization increases P levels in different crops and improves plant growth by enhancing P availability for plants [22]. PBS generally have the ability to weather silicates, likely because basic metabolic activities such as organic acids production and respiration can cause the weathering of minerals [23].

3. Phosphate solubilization sources and mechanism of action

Phosphate-solubilizing microorganisms technology improves saline-alkaline soil fertility and agricultural use without causing any environmental or deleterious health implication that accompanies the continuous use of synthetic fertilizers [24]. The phenomenon of P desorption by PSM usually occurs along with the drop of pH [25]. The ability of PSM to convert insoluble organic and inorganic phosphorus depends on the nutritional richness of the soil and the physiological and growth status of the organism. PSM isolated from saline-alkaline soils–soils with a high level of nutrient deficiency, or soils from extreme temperature environments, have the tendency to solubilize more phosphate than PSM of soils from moderate conditions [14].

P solubilization has been reported to be secretion of organic acids (e.g., oxalic acid, citric acid, acetic acid, succinic acid, etc.) produced by phosphate-solubilizing microbes to solubilize insoluble P by lowering of pH. The hydroxyl and carboxyl groups present in the organic acids chelating the cations (mainly calcium) bound to phosphate, leading to increased solubility and availability of mineral phosphates, to ensure the microbial growth, organic acid production, and RP solubilization of metabolizable carbon compounds [26, 27, 28, 29, 30].

Mechanism of action of P solubilization could be observed when a phosphate-solubilizing microbe is grown in a specialized highly alkaline medium known as Pikovskaya medium. These phosphate-solubilizing microbes are then incubated at 30°C for six days. There has been a conflicting report on the influence of temperature on phosphorus solubilization by microbes. White et al. [31] found 20–25°C as the optimum temperature for maximum microbial phosphorus solubilization while 28°C was reported by [32, 33]. In addition, others including [34, 35, 36, 37] have recorded 30°C as the best temperature for P solubilization. [36, 38] reported P solubilization at extreme temperature of 45°C in desert soil while [39] reported solubilization at a low temperature of 10°C. The ability of these phosphate-solubilizing microbes to form halo zones when cultured on the PVK medium is a clear indication that the organism can solubilize phosphate. Later, the PSB/AMF could then be centrifuged at 5000 rpm for 10 min [40]. Supernatant of blended cultures was filtered through 0.20-μm syringe filters (cellulose acetate), and the organic acid could then be detected using HPLC (shimadzu SPD-M20A) with PDA detector by using methanol 5% and 0.1% TFA 95% as mobile phase with flow rate of 1 ml per minute with column particle size 5 μm, length 25 cm, and column diameter 4.6 mm (Merck C18 column). The extent of P solubilization by the fungal strain could then be determined by measuring the clear zone (in mm) around the colonies by taking the halo zone formation into consideration [25, 41, 42, 43, 44].

4. Role of rock phosphates and phosphate-solubilizing microbes in food security

Food security is heavily dependent on phosphorus availability—an essential component of the mined phosphate rock [45]. The fertilizer industry is aware of the fact that the quality of RP reserve is declining and the extraction, processing, and shipping of phosphate rock are highly expensive [46, 47, 48]. With the current global population boom, coupled with insecurity challenges in mines posed by armed banditry herdsmen, Boko haram, and the Islamic State in West Africa Province (ISWAP) in this part of the world, optimizing food security is essential in curbing poverty and hunger management.

Emphasis on this chapter is not limited to improving RP solubility alone but also how to ensure food security under diminishing P resources due to injudicious use of P fertilizers [47]. This is made possible by application of phosphate-solubilizing microbes/biofertilzers in addition to the RP. This could be a more promising approach for safer and sustainable agriculture. PSM include bacteria (Pseudomonas sp., Agrobacterium sp., Bacillus sp., Actinobacteria etc), fungi (Achrothcium, Alternaria, Arthrobotrys, Aspergillus, etc.), and Arbuscular mycorrhizae (Funneliformis mosseae, Rhizophagus irregularis, etc). [12, 17, 48].

Therefore, inoculation of soil with PSMs was reported to be a widely accepted environmentally friendly approach for increasing soil soluble P concentrations and agricultural productivity [48]. It was established that bioavailable P content in soil is an important element that improves plant P uptake, which in turn results in higher crop yields [14, 24]; most studies have considered PSM as a promising inoculant/biofertilizer for raising the productivity of agronomic crops in agroecological niches [49, 50, 51]. Soil microorganisms improve plant nutrient acquisition. They are involved in an extensive range of biological processes as well as the conversion of insoluble soil nutrients [34]. A number of these soil microbes are capable of solubilizing and mineralizing insoluble soil phosphorus for the growth of plants. To this end, application of phosphate PSMs as biofertilizers will bring favorable effects on plant growth and reduce the cost of production, which may likely enhance the farmers’ income in sub-Saharan Africa.

5. Conclusion

Based on previous studies, we could conclude that AMF, PBS, and RP consortium could synergistically act to more effectively increase the plant uptake of P, improving the growth of different plant more than when each was solely applied as soil amendment. These in turn boost yield and generate income of poor Sahelian subsistent farmers. In order to avoid a future food-related crisis, phosphorus scarcity needs to be recognized and addressed in contemporary discussions on global environmental change and food security. This review has shown that phosphate rocks and phosphate-solubilizing microbes have great potential as biofertilizers, improving soil inorganic phosphate levels and increasing its bioavailability for plant use. This will no doubt promote sustainable agriculture, improve the fertility of the soil, and hence increase crop productivity.

6. Future prospects

There is great potential in formulation of biofertilizer using RP and phosphate-solubilizing microbe. This could potentially overcome abiotic stress such as drought and salinity, which could improve crop yield. AMF, PSB, and RP interaction to affect P availability is poorly understood [35], thus a better understanding of AMF and PSB and RP would allow growers to rely less on chemical P fertilizers and instead utilize biological processes to maintain fertility and enhance plant growth. AMF and PBS could work together to yield sustainable plant growth in malnourished environments. Combinations of AMF and PSB are commonly used to increase crop yields, improve fruits quality, boost phytoremediation, enhance the fertilizer nutrient use efficiency, lower chemical fertilization application requirements, and increase salinity tolerance [52, 53]. The use of silicon (Si) fertilizer has also been proposed as an environmentally friendly, ecologically compatible method of improving plant growth and the resistance to multiple environmental stresses including nutritional imbalances. Previous studies have reported that Si increases plant uptake of P [21, 54, 55]. Combining Si and microorganism applications has been proposed to efficiently improve plant growth and nourishment. Previous studies have observed that AMF, PBS, R, and Si could work together to improve plant growth regardless of the stress conditions [56, 57, 58, 59, 60, 61].

Conflict of interest

The authors declare that they have no known competing financial or personal relationship that could have appeared to influence the report in this paper.