Gaseous Losses of Nitrogen from Rice Field: Insights into Balancing Climate Change and Sustainable Rice Production

Abstract

The world is confronted with one of the most difficult tasks of the twenty-first century, satisfying society’s expanding food demands while causing agriculture’s environmental impacts. Rice security is the food security for South Asian countries. Rice production requires a large amount of water and fertilizer, especially nitrogenous fertilizer, where urea works as the primary source of nitrogen (N). Different biogeochemical conditions, such as alternate wetting and drying (AWD), intermittent drainage, agroclimatic conditions, oxic-anoxic condition, complete flooded irrigation,. have severe impacts on GHGs emission and nitrogen use efficiency (NUE) from rice fields. For sustainable production, it is a must to mitigate the emissions of GHGs and increase NUE along with cost minimization. But analytically accurate data about these losses are still not quantifiably justified. In this chapter, we will show the proper use of the measured data with suitable results and discussions to recommend the future cultivation system of rice for sustainable production.

1. Introduction

Nearly half (3.5 billion) of the global population relies on rice (Oryza sativa) for sustenance [1]. High-yielding rice varieties (HYV) are widely used in modern agricultural practices to feed the world’s teeming population and are accompanied by an increase in the need for chemical fertilizers, particularly nitrogenous fertilizers. A paddy rice field is the center of nitrogen (N input and output. The average N application at rice fields in Japan is 80 kg ha⁻¹ season⁻¹, in the USA is 140 kg ha⁻¹ season⁻¹, and in Bangladesh is around 100 kg ha⁻¹ season⁻¹ [1, 2, 3] where the global use of different N-fertilizer was 77 Tg N year⁻¹ (Figure 1). Application of N fertilizer in the paddy rice field is equivalent to more than 60% of the use of the total fertilizers, and it is expected to rise from 107 to 115 million tons (MT) over the 2015/23 period, with an average annual growth rate of 2.4% [5]. However, nitrogen use efficiency (NUE) is only about 30–35%, where the fates of the rest of applied N are leaching, denitrification, nitrification, and ammonia (NH₃) volatilization loss. Rice production generates 1.5% of total anthropogenic greenhouse gas (GHG) emissions globally, but it accounts for 32% of agricultural GHG emissions in Bangladesh [6]. Paddy fields and other agricultural practices, cattle farms, landfills, fossil fuel burning, etc., are major sources of GHGs emissions to the atmosphere.

Figure 1.

Globally used different N-fertilizer [4].

For rice production, about 3 cm height of standing water is required throughout the rice growing seasons, and about 3500–4000 L of water is required per kg rice grain. Irrigation designs to water save such as alternate wetting and drying (AWD) and dry direct-seeded rice (DSR) planting systems are becoming increasingly important in several rice-growing countries, as it has been found that AWD practice tends to save 38% of water which was needed to be used in irrigation purpose without any reduction in yield [7]. However, in terms of global climatic concerns, AWD has the harsh effect that it emits more N₂O rather than CH₄. Carbon dioxide (CO₂), CH₄, and N₂O are listed as greenhouse gases, but NH₃ is another common gas derived from rice fields and this contributes indirectly to global warming. About 10–60% of the applied nitrogen in rice fields could be lost by NH₃ volatilization, which is the major pathway of N loss, while 5–10% of the applied nitrogen could be lost through denitrification [8, 9]. The amount of N loss as NH₃ in Asia’s agriculture is expected to rise from 4.6 Tg N year⁻¹ in 1961 to 13.8 Tg N year⁻¹ in 2000 [10, 11], and in the next three decades, it will be 18.9 Tg N year⁻¹. The fierce application of N-fertilizer increases the reactive N species in the atmosphere, changes N-cycle, and is subjected to global warming and climate change.

Climate change is mostly driven by the influence of rising temperatures in the earth’s atmosphere, where greenhouse gases are responsible for the event. Without regard for the planet’s well-being, industrialized and developing countries release GHGs to boost their economy. The greenhouse effect is not only a localized concern as the gases have long-lasting self-life in the atmosphere. The GHGs are not confined to the territory of the producer countries. Beyond the emitting country’s border, people must feel the harsh effect of GHGs globally. All countries are not equally responsible for the GHGs effect, but all are the potential to contribute more or less to cause climate change. The most challenging fact in this era is to balance food security and climate change by reducing NH₃, CH_4, and N₂O emissions from rice fields while maintaining or increasing rice production [12]; thus, it is necessary to develop climate-smart technology and techniques.

This part of the chapter reconnoiters the current rice production scenario with the fierce application of N-fertilizers and their contribution to global warming by emitting GHGs. This chapter elaborates on the contribution of rice cultivation to GHG gas emissions and subsequent global warming. Finally, a climate-smart agriculture section will be discussed, which is an effective strategy to minimize GHG emissions and mitigate global warming.

2. Climate change and rice production

The rise in the earth’s surface temperature was felt and noticed mostly in the last three decades of the nineteenth century. The earth’s surface temperature has been warming at the rate of 1.09°C since the middle of the nineteenth century [13]. Human-induced factors, including GHGs emissions and land use changes, play a vital role in climate change. Where initially, natural factors were also considered for climate change but recently, anthropogenic factors have raised global CO₂ content from 284 ppm (in 1832) to 410 ppm (in 2013) [14], which has now reached 418 ppm and wetland paddy field is a hotspot for CH₄ production [15]. The N₂O concentration increases from 271 ppb (in 1750) to 331 ppb (in 2018) [15]. There has been a 20% increase in N₂O since the industrial era, with an increased rate of 0.95 ppb yearly [15]. These are major GHGs in the atmosphere, responsible for global warming and climate change. Unevenness in climatic conditions is evidence of oscillated temperature changes, precipitation frequency and pattern, and extreme stress events. A global population of about 9 billion people necessitates a 40% increase in rice production by 2030 and a 70% increase in current production by 2050 to feed them. The most densely populated countries in the world, China and India, account for 20% and 28.5% of the total global rice area, respectively [16]. As a result, they contribute significantly to CH₄ emissions [17]. Compared with CO₂, the global warming potential (GWP) of N₂O is 265–298, and the GPW of CH₄ is 34 [15]. Rice at its flowering stage is very susceptible to temperature for pollen viability; hence, global warming is also a concern for food security. Nitrogen fertilizers occupy more than 70% of all the chemical fertilizers in rice fields. However, from winter rice fields, a large portion of applied N is lost to the environment in the form of ammonia (NH₃) through volatilization (about 17%) [2]; this gas has a very short lifespan (a few hours to 12 days) in the atmosphere [15]. Ammonia acts as a potential source of N₂O, chlorofluorocarbon (CFC), and aerosol, and after the break down, it travels as particulate matter [18]. When aerobic rice cultivation is practiced, it will be a great source of N₂O emissions. Nutrient mining, skewed N, P, and K application, poor nutrient use efficiency, and loss of nutrients are key challenges of nutrient use that threaten the economic and environmental sustainability of rice production worldwide [19]. Nutrient management strategy for enhancing rice productivity, especially in an intensive rice-based cropping system, should simultaneously aim to enhance eco-efficiency and sustainability by reducing chemical N fertilizer application. An appropriate rice cultivation practice is needed to increase NUE, preserve natural resources, and make the environment pollution free from GHGs to resist climate change.

3. Why is ammonia a matter of concern for GHGs emissions?

Nitrogen in different forms, for instance, ammonia volatilization, nitrification, and denitrification, all contribute to climate change in varying degrees. Denitrification was discovered to be more significant in Griffith, Australia, whereas volatilization was more significant in Munoz, Philippines [20, 21]. Environmental pollution is also possible due to NH₃ loss as about 17% of applied N lose as NH₃ (Figure 2) at the winter rice growing season is possible [2]. Ammonia is the main component of soluble alkaline gas in the atmosphere, enhancing the production of reactive N species as air pollutants (aerosol, nitrous oxide, etc.) causing global warming and significantly affecting local air quality [22]. It can travel long distances before being converted into fine particles [23]. The NH₃ emission increases with fertilizer application rate with times (Figure 2). Consequently, NH₃ deposition negatively affects living organisms as NH₃ reacts with other air pollutants to create tiny particles that can lodge deep in the lungs, causing asthma attacks, bronchitis, and heart attacks [24]. Compared to other GHGs, ammonia has a short atmospheric lifespan (2–10 days) and no direct greenhouse with acids to form salts and return to earth, similar to N-fertilizer [15]. Significant reductions in N₂O and NO emissions from flooded rice soils are an alarming global concern [19]. Therefore, mitigation of NH₃ and N₂O emissions from the cropland system is an urgent demand for environmental and economic protection.

Figure 2.

Simulated ammonia (NH3) emissions in response to application of synthetic nitrogen (N) fertilizer in: (a) the 1960s, (b) the 1980s, (c) the 1990s, and (d) the 2000s [3].

4. Nitrogen (N) transformation pathways from rice fields

4.1 Nitrogen cycle

Nitrogen cycle is a biogeochemical process through which N is converted into many forms, consecutively passing from the atmosphere to the soil to organism and back into the atmosphere. In rice fields, N is applied mostly in the form of urea [25] or urea-containing fertilizer as the principal form of synthetic N. After fertilization, urea immediately undergoes hydrolysis in the presence of standing water, and ammonium bicarbonate is also used in China as N-fertilizer. In rice fields, the water availability facilitates the urease enzyme activity, and the applied urea dissociates into ammonium, bicarbonates, and hydroxyl ions. Among the plant nutrients, N is mostly mobile in nature; thus, total recommended dose of N-fertilizer is divided into two or three equals and applied in two or three splits in rice field through broadcast onto the standing water. First split will be applied on 10–14 days of transplanting (DAT) the maximum tillering stage, at 30–35 DAT second split will be applied, and finally 10–15 days after second split the third split will be applied. The N cycle includes several biological and non-biological processes in a rice field under aerobic and anaerobic conditions. The biological processes are ammonification, mineralization, nitrification, denitrification, N fixation, N assimilatory reduction and microbial synthesis of ammonium and organic N into microbial cells, plant uptake, and conversion of ammonium and nitrate N into plant proteins (Figure 3). The non-biological processes are ammonia volatilization, leaching of nitrite and nitrate N to groundwater, ammonium fixation into soil clay minerals, and precipitation of nitrate and ammonium N [26].

Figure 3.

Nitrogen cycle in rice field.

4.1.1 Mineralization

Mineralization (or ammonification) of soil N is the term used for the process by which nitrogen in organic compounds is converted by soil microorganisms into ammonium ions (NH₄⁺) as following Eq. (1) [27].

$$\text{Complex organic nitrogen}\to \text{Ammonium}$$E1

4.1.2 Nitrification

Nitrification is the oxidation of ammonium NH₄⁺-N to nitrites and nitrates. The two groups of organisms that are the primary nitrifying bacteria, that is, Nitrosomonas Sp. and Nitrobacter Sp. The general oxidative processes involved can be represented by the following Eqs (2) and (3).

$${\text{2NH}}_4^{+}+{\text{3O}}_2\to {\text{2NO}}_2^{-}+\text{4H}+{\text{2H}}_2\mathrm{O}$$E2

$${\text{2NO}}_2^{-}+{\mathrm{O}}_2\to {\text{2NO}}_3^{-}$$E3

4.1.3 Denitrification

Denitrification (or nitrate reduction) is a more complex and less understood process than nitrification. Denitrification is a redox process involving nitrogen compounds to obtain energy. There are two processes of nitrate reduction: assimilatory and dissimilatory. When the dissolved oxygen level drops to low levels for the aerobic metabolism of facultative organisms, they can turn to nitrate reduction, presented in the Eq. (4) [28].

$${\text{2NO}}_3^{-}\to {\text{2NO}}_2^{-}\to \text{2NO}\to {\text{2N}}_2\mathrm{O}\to {\mathrm{N}}_2$$E4

4.1.4 Leaching

Leaching of nitrite or nitrate refers to the removal of nitrite or nitrate from the plant root zone by the movement of water through the soil body. Since nitrite (NO_2⁻) and nitrate (NO_3⁻) are negatively charged, they are found to move freely with the water unless soils have a significant anion exchange capacity. It was estimated that 55 Tg of nitrate is leached from agricultural soils every year [29]. Leaching of nitrogen from soil reduces the bioavailability of plants and impacts the environmental quality.

4.1.5 Ammonia volatilization

It is a non-biological process that occurs at the soil surface when ammonia from urea or ammonium-containing fertilizer (urea) is converted to ammonia (NH₃) gas at high pH. Ammonia volatilization occurs when ammonium ions are present in a neutral or in alkaline medium [30]. Due to continuous flooding, the soil pH of rice fields remains 6.5–7 or a little more; thus, ammonia is formed continuously in flooded rice fields through mineralization, which under certain favorable conditions can be lost to the atmosphere as NH₃, presented in Eq. (5). Ammonia can also be produced from waste products of wild animals, cow dung, or compost.

$${\mathrm{NH}}_4^{+}+{\mathrm{OH}}^{-}\to {\mathrm{NH}}_3\uparrow +{\mathrm{H}}_2\mathrm{O}$$E5

4.1.6 Anammox

Anammox stands for anaerobic ammonium oxidation, a recent discovery in N-cycle. In the anammox, NH₄⁺ is oxidized to nitrite as an electron acceptor and then elemental N₂ by a series of metabolic processes under anaerobic circumstances [8]. This process is carried out by a group of bacteria, that is, annamox, and they are abundant in marine ecosystem so the anammox is a critical issue in the marine ecosystem to understand N-cycle. The anammox bacteria are also found in terrestrial ecosystem. The idea of the N cycle in paddy fields has changed because the paddy field acts as a niche of anammox bacteria by providing favorable conditions (water logging (anaerobic condition) and high NH_4⁺ and NO₃⁻ content) for their living and activity [8]. According to reports, the anammox mechanism is responsible for 4–37% of the nitrogen loss in agricultural soils [31]. The organic content, NOx concentration, environmental stability, salinity, and temperature have all been identified as key influencing elements in the ecological dispersion of anammox bacteria and their contribution to N loss in natural environments [32]. The significant loss of N due to anammox occurring in paddy fields is similar to that for NH₃ volatilization (up to 40%), leaching (9–15%), runoff (5–7%), and denitrification (up to 40%) (Table 1) [8, 33, 34].

Country/year	2016	2017	2018	2019	2020
Indonesia	5.0973	5.0690	5.2031	5.1137	5.1279
China	6.8563	6.9122	7.0280	7.0562	7.0402
India	3.7902	3.8493	3.9568	4.0577	3.9623
Bangladesh	4.5863	4.6619	4.7257	4.8088	4.7402
Thailand	2.9678	3.0690	3.0380	2.9164	2.9064
Cambodia	3.4443	3.5388	3.5875	3.6721	3.7568
Myanmar	3.8181	3.8218	3.8568	3.7957	3.7711
Pakistan	3.7716	3.8526	2.5629	2.4436	2.5244
Philippines	3.8690	4.0061	3.9718	4.0449	4.0888
Vietnam	5.5738	5.5476	5.8180	5.8371	5.9201

Table 1.

Paddy production (metric tons/ha) of major rice growing countries; (2016−2020); (FAOSTAT).

5. Gaseous loss of nitrogen from paddy rice fields

5.1 Nitrous oxide (N₂O) emission

With its current atmospheric concentration of 350 ppb, nitrous oxide (N₂O) is one of the major greenhouse gases, contributing about 5% of the overall greenhouse effect [35]. With a relatively large global warming potential of about 298 times that of carbon dioxide in a 100-year horizon [36], N₂O is one of the main greenhouse gases that cause global warming and ozone depletion [37]. Agricultural activities are responsible for two-thirds of the total anthropogenic nitrous oxide (N₂O) emissions worldwide [38]. Nitrous oxide is emitted into the atmosphere from both natural (about 60%) and anthropogenic sources (approximately 40%), including oceans, soil, biomass burning, fertilizer use, and various industrial processes [39]. Agriculture is a major source of anthropogenic N₂O emissions [40]. During rice production, puddling is operated which normally shuts the water transmission pores resulting in very low water percolation and gaseous exchange between water and air surface. Nitrous oxides are produced from rice fields through nitrification and denitrification processes (Figure 4). Emissions of nitric and nitrous oxides are the result of microbial nitrification and denitrification in soils, controlled principally by soil water and mineral N contents, labile organic carbon, and temperature. Nitric oxide is a direct intermediate of both nitrification and denitrification [4]. In submerged soils, nitrification occurs in aerobic sites at the floodwater-soil and root-soil interfaces. Denitrification occurs upon diffusion of the NO_3⁻ to the anaerobic bulk soil (Figure 3). Denitrification is favored over dissimilatory reduction to (NH₄⁺ → NO₃⁻ → NO₂⁻ → NH₄⁺) because of the large ratio of available carbon to electron acceptors in submerged soils. Denitrification is likely to proceed completely to N₂ with little accumulation of N₂O because of the very large sink and therefore steep concentration gradient of O₂, and because carbon is less likely to be limiting. However, this will not be the case when submerged soil is drained and air enters, leading to gradients of oxidation from the surfaces of soil cracks toward the anaerobic interiors of soil clods. Now conditions are favorable for the production of nitrous and nitric oxides [4, 41, 42].

Figure 4.

Principle N transformation pathways leading to the emission of N₂O in soils.

The ammonia and nitrate are likely to be converted to N₂O through nitrification and denitrification, but it varies in different cultural practices. Although N₂O emissions from rice fields are substantially smaller than methane emissions in flooded conditions, they have been a long-standing reason for concern [43]. Several researchers have done experiments on the emissions of nitrous oxides from paddy fields. Major rice-producing countries viz. China, India, Indonesia, Japan, Philippines, USA contribute to global warming by emitting greenhouse gases like N₂O. Tables 2 and 3 show the account of how much these countries emit N₂O from their paddy fields and compare different management practices as well.

Complete flooding condition
Country	Location	Type of organic amendmenta	Amount of organic amendment, kg ha^−1	Type of nitrogen fertilizerb	Tested mitigation optionc	Amount of chemical N, kg ha^−1	N2O emission, g ha^−1	Reference
China	Yancheng, Jiangsu	no	0	U		225	2150	Xu et al. [45]
China	Shenyang	no	0	no		0	40	Chen et al. [46]
China	Shenyang	no	0	U		374	60	Chen et al. [46]
China	Shenyang	FYM	37,500	U		374	60	Chen et al. [46]
China	Shenyang	FYM, Azolla	37,500	U		374	60	Chen et al. [46]
India	New Delhi	no	0	no		0	38	Ghosh et al. [47]
India	New Delhi	no	0	U		120	168	Ghosh et al. [47]
India	New Delhi	no	0	U	DCD	120	80	Ghosh et al. [47]
India	New Delhi	no	0	AS	DCD	120	82	Ghosh et al. [47]
India	New Delhi	FYM	6000	U		60	593	Pathak et al. [48]
Indonesia	Bogor, West Java	no	0	no		0	315	Suratno et al. [49]
Indonesia	Bogor, West Java	no	0	U		86	649	Suratno et al. [49]
Japan	Ryugasaki, Ibaraki	Straw	5000	${\mathrm{NH}}_4^{+}$		90	65
Philippines	Los Banos	no	0	AS		120	97	Abao et al. [50]
USA	Louisiana	no	0	U		0	74	Smith et al. [51]
Mid-season drainage or AWDd
China	Fenqiu	SM	5000	ABC, U		364.5	4416	Cai et al. [52]
China	Yingtan, Jangxi	vetch	33,750	U		276	2810	Xiong et al. [53]
China	Beijing	no	0	ABC		125	2029	Khalil et al. [54]
China	jiangsu	no	0	ABC		191	2389	Zheng et al. [55]
India	New Delhi	no	0	U	DCD	140	142	Kumar et al. [56]
India	New Delhi	no	0	U	TS	140	142	Kumar et al. [56]
India	New Delhi	no	0	no		140	142	Majumdar et al. [57]
India	New Delhi	no	0	no	NEU	140	57	Majumdar et al. [57]
India	New Delhi	FYM	0	no		60	714	Pathak et al. [48]

Table 2.

N₂O emissions from paddy fields during the cropping season [44].

^a

Type of organic amendment: FYM, farmyard manure

^b

Fertilizer type: AS = ammonium sulfate; U = urea; ${\mathrm{NH}}_4^{+}$ = complex fertilizer including ${\mathrm{NH}}_4^{+}$.

^c

Mitigation options: DCD = dicyandiamide. NEU = Nitrogen use efficiency, neem-coated urea; TS = thiosulfate.

^d

AWD: Alternate wetting and drying.

The table shows that N₂O emissions from rice fields are higher in AWD practice than continuous flooding condition. The paddy field can also emit N₂O if remains fallow. The following table shows N₂O emissions from paddy fields during the fallow Period.

Country	Location	N2O Flux, μg m^−2 h^−1	N2O Emission, g N ha^−1	Measurement Period in Fallow Season, Days	Reference
China	Shenyang	55.0	3050	231	Chen et al. [46]
China	Shenyang	31.9	888	116	Hou et al. [58]
China	Jiangsu	57.2	2895	229	Zheng et al. [55]
Japan	Tsukuba, Ibaraki	10.6	577	226	Nishimura et al. [59]
Japan	Ryuugasaki, Ibaraki	9.9	606	254	Tsuruta et al. [60]
China	Yingtan, Jangxi	11.0	430	165	Xiong et al. [53]
Philippines	Los Baños	138.7	1198	36	Abao et al. [50]
Philippines	Los Baños	32.4	560	72	Abao et al. [50]

Table 3.

N₂O emissions from Paddy fields during the fallow period [44].

5.2 Ammonia emissions

In the rice field, N-fertilizers are applied in two or three splits. During the first split, the plants are very small and unable to use the applied N fully, resulting in the maximum N loss through volatilization in this period; the standing water in the rice fields favors this process (Figure 5) [2]. In the second and third applications, the plants’ canopy and root systems will be established, and the N loss through volatilization is less, but the N adsorption by crop increases (Figure 6) [2, 4]. Several methods (enclosure method, continuous airflow enclosure method, micrometeorological method, simple low-cost chamber method, wind tunnel method venting method, etc.) are used for ammonia volatilization loss measurements from rice fields (Table 4). Often the loss caused at an early stage of plant is about 30–40% loss of applied N and sometimes more than 60% of the applied N, whereas, at later stages, the loss will be half of the earlier one [20, 33]. In rice fields, the loss is responsive to the crop demand and applied N rate. The NH_4⁺ ions have a positive relation with ammonia volatilization loss (Figure 7b) and N-fertilizer application [65]. In rice field, the NH_4⁺ concentration varies from 0 to 1.72 mg/l [66], 0.2–4.5 mg/l [67] and 2–9.2 mg/l [65, 68] due to variation in soil properties (clay particle), soil pH (Figure 7a), agriculture operation, climatic condition, hydraulic properties, irrigation, and nitrogen management practices. It becomes more likely that the equilibrium will shift from non-volatile NH_4⁺ to volatile NH₃ gas as the pH of the rice field water rises. The ionized ammonia (NH_4⁺) releases 1 mole H⁺ during subsequent volatilization of non-ionized ammonia (NH₃). The NH_4⁺ is primarily formed in the soil because urease activities are much higher than in the floodwater, so urea moves downward through mass flow and diffusion. The produced ions move between soil and floodwater, and ammonium ion converted into ammonia gets lost in the atmosphere in the volatilization process. Nitrogen loss through NH₃ volatilization can be 20–30 kg ha⁻¹ [69] and NH₃ loss can be 46% in rangeland and if the N fertilizer increased 100%, then the volatilization loss will go up to 31% during the rice growing season [70]. The NH_4⁺ concentration in surface water and the fertilization timing influences the NH₃ loss. A significant amount of loss in the first 1–7 days of fertilization then gradually the rate flattened [23]. The urea and NH_4⁺ enter 1–2 cm depth of soil surface within a week; thus, the broadcast urea should match the crop demand for maximum N recovery, and crops with superficial root systems benefit from absorbing NH_4⁺ [71]. Ammonia volatilization loss occurs after ammonium bicarbonate and urea application to flooded rice during transplanting, with losses of 39% and 30% of applied N, respectively [72].

Figure 5.

Role of ammonium in greenhouse effect.

Figure 6.

Ammonia loss at different plant growth stages in presence of water stress [61].

Region	Year	Method	NH3 emission	References
Global	2000	DLEM-Bi-NH3	13.6 ± 0.5	Xu et al. [3]
	1995		12.4 ± 0.3	Xu et al. [3]
	2000	IPCC Tier 1 guideline	7.7	IPCC [62]
	2000	Process-based model	12.0	Riddick et al. [63]
	1995	Constant EF	9.0	Bouwman et al. [64]

Table 4.

Estimates of global NH₃ emissions (expressed in Tg N year⁻¹) based on different approaches [3].

Figure 7.

Relationship between (a) NH₃ flux and soil pH (b) NH₃ flux and NH₄⁺.

The use of N-fertilizer in paddy water encourages algal growth [73]. Algae photosynthesize, removing CO₂ from the water and reducing the formation of carbonic acid. While daylight hours are ideal for this process, the paddy field water pH can rise as high as 9.0. These pH levels are the ideal conditions for NH_4⁺ compounds to release NH₃ into the air, and soil pH has a strong relation with NH₃ loss (Figure 7a). The crop’s recovery or nitrogen use efficiency varies between 30% and 40%, and this mostly depends on the plant root’s capability to drag N from the downward-moving pool of N [4]. The initial distribution of urea in the soil, hydrolysis rate, and N absorption rate by rice roots influences cumulative NH₃ volatilization. Adding organic matter in rice fields may increase CO₂ and reduce soil pH, but this will not work to reduce volatilization loss as the produced CO₂ may interact with the diurnal floodwater pH; however, the daily average soil pH remains unaffected.

It is defined that high soil pH (pH < 9) or alkaline soil conditions favor the volatilization rate. In contrast, volatilization loss is minimum in acidic soils but also can occur, especially volatilization loss occurs in calcareous soils when ammonium-containing fertilizers are applied on the soil surface. Basic soils containing Ca(OH)₂ may react with (NH₄)₂CO₃ to NH₄OH, easily decomposing to NH₃ and H₂O.

$$\mathrm{Ca}{\left(\mathrm{OH}\right)}_2+{\left({{\mathrm{NH}}_4}^{+}\right)}_2{\mathrm{CO}}_3\to {\mathrm{NH}}_4\mathrm{OH}+{\text{CaCO}}_3\left(\mathrm{ppt}\right)$$E6

$${\mathrm{NH}}_4\mathrm{OH}\leftrightarrow {\mathrm{NH}}_{3\left(\mathrm{g}\right)}+{\mathrm{H}}_2\mathrm{O}$$E7

From the equations, it can be concluded that (1) at higher pH values, NH₃ volatilization is more pronounced, or (2) amending the solution with NH₃ gas-producing amendments will cause the reaction to move leftward, resulting in a rise in soil pH. Acidification is not caused by removing fertilizer N from the soil by crops as NH_4⁺. After fertilization, low floodwater pH values in acidic soils resulted in (8−18%) volatilization loss, which is less than the denitrification loss (40−50% of applied N), where the total nitrogen loss ranges from 48% to 60% of applied N. Low solar radiation and poor algal growth at a rice paddy field in acid soils biosphere also facilitate low volatilization loss, in comparison with floodplain or calcareous soils [72]. It can be noted that only when the water entering the rice field differs from the concentration of acid or base from the water leaving the field does a permanent change in soil pH [4].

Application of 71.4 Tg N year⁻¹ as chemical fertilizer raised NH₃ emission from 2.8 ± 1.5 to 12.0 ± 0.8 Tg N year⁻¹ [74]. The highest global mean NH₃ emission was estimated at 16.7 ± 0.5 Tg N year⁻¹ in 2010. They also identified four major crops for NH₃ emission determination due to N fertilization, where in 2000 the largest NH₃ emitter was rice field (23.5%), followed by wheat (22.8%) then corn (21.9%), and the lowest emission was from soybean (<10%) field (Figure 8). The high emission from rice fields was due to increased N fertilizer application with increased cultivable area.

Figure 8.

Crop-specific NH₃ emissions from synthetic N fertilizer application [3].

6. Gaseous losses of nitrogen under different water management techniques in rice field

6.1 Alternate wetting and drying (AWD)

Due to the continuous ponding of water in the rice field, the N transformation and transport processes are unique in rice ecosystem [75]. In rice field about 3600 l water Kg⁻¹ grain is kept where 50–80% of the water loses through deep percolation [68], causing water and nitrogen loss from the field [76]. In conventional paddy cultivation, water requirement is high though the water is not fully used for the crop rather it facilitates nutrient loss. So, to save water resources, several water management techniques for rice fields are invented but among them, widely accepted technique is the alternate wetting and drying (AWD) irrigation practice [77]. Rice production is expected to shift from continuous flooding irrigation to AWD irrigation practice. Alternate wetting and drying is water-saving technique where the soil is not constantly flooded instead after the ponded water has receded, it is left to drain for one or more days before being re-flooded. The aerobic (dry) and anaerobic (wet) alternate conditions in the field exert complexity in N transformation processes. This increased yield and biomass over continuous flooding practice (Figure 9). In the 1980s and 1990s, research into AWD as a water-saving strategy began in China and India. The practice of AWD was originally tested as a water-saving strategy in the Philippines in 2002, and then in Bangladesh at the Bangladesh Rice Research Institute (BRRI) in 2005 [7]. But it is evident that this practice reduces CH₄ emission in the field by 38% [79] but there are also subsequent evidences that AWD increases N₂O emission [6, 80]. Conventional tillage practice reduces global warming potential (GWP) by 10–16% in comparison with new modern practices [81]. However, the decrease in CH₄ emissions brought about by AWD substantially balances the increase in N₂O emissions created.

Figure 9.

Comparison of alternate wetting and drying (AWD) and continuous flooding (CF) practices on (a) root-straw biomass and grain yield (b) harvest index of yield and total N [78].

Transformational modifications (anaerobic rice systems into aerobic) in rice cultivation practices sustain yield (Figure 6a) but at the cost of higher N loss [17] mostly N₂O. In comparison with conventional rice cultivation practice with water stress condition of −15 to −30 k Pa at 15–20 cm below the soil surface the grain yield is reduced by 11–32% [82, 83]. In Arkansas, it was stated that their less aggressive AWD treatment resulted in a 48% drop in methane emissions and that overall greenhouse gas (GHG) emissions were reduced by 45% when the increase in nitrous oxide was taken into consideration [79]. In AWD, the N₂O is prompted by enhanced denitrification of NO_3⁻ during the rewetting of dry soils and the following increased nitrification of NH₄ during the dry phase [6]. During the drying phase of AWD irrigation practice, the soil aeration is good and this may improve the land quality, grain yield, utility of N, and water productivity along with reduction in N leaching, while N loss through ammonia volatilization and denitrification, and nitrification increased in AWD practice due to aerobic condition [84]. In comparison with conventional tillage, ammonia volatilization increases by 14% in medium moisture stress conditions and 17% in severe moisture stress conditions whereas denitrification increases by 7% in both medium and severe moisture stress conditions [61, 68]. Table 5 shows a typical comparison:

Treatments	Annual emission of N2O (kg N2O-N ha^−1)
Water seeded with conventional continuous flood irrigation	0.102
Water seeded with alternate wetting and drying (AWD)	0.142
Drill seeded with alternate wetting and drying (AWD)	0.616

Table 5.

Comparison between conventional irrigation and AWD based on N₂O emission in water seeded and drill seeded planting system [85].

By managing water table levels through controlled drainage and controlled irrigation.

Their effect on nitrification and volatilization can be determined. As the water table control levels increased, irrigation water volumes in the controlled irrigation paddy fields decreased. Seasonal ammonia volatilization losses reduce with the successive increase in controlled water table and the range varies from 53 kg N ha⁻¹ in near to surface to 59 kg N ha⁻¹ in below surface level [61]. The application of controlled drainage by raising water table to a suitable level could effectively reduce irrigation water volumes and ammonia volatilization losses from paddy fields [61]. The combination of controlled irrigation and controlled drainage is a feasible water management method of reducing ammonia volatilization losses from paddy fields where NH_4⁺ undergoes an oxidation process in dry spell that converts it to NO_2⁻ and finally to nitrate NO_3⁻, where it is reduced to N₂ and nitrous oxides, which are also released into the atmosphere as a product of denitrification [86]. In AWD irrigation practice, the alternate aerobic and anaerobic conditions accelerate denitrification and volatilization process of N loss, whereas nitrification process is more favorable in anaerobic conditions. The redox potential under ponding condition is lower and this facilitates the N₂O formation via denitrification [81]. The NH₄⁺ ions are higher in topsoil layers than in lower layers (10–70 cm) of soils throughout the rice growing season and this may occur due to (1) the presence of 3–5 cm thick plow pans below 15–20 cm of the soil surface which restricts the NH₄₊ ion movement from the top layers to the lower layers (Figure 4; [87]) and (2) the negatively charged soil colloid have high affinity to make bond with positively charged ions and restrict the ions from being lost. The AWD irrigation practice increases soil temperature. Temperature and soil moisture are the major factors for N transformation pathways where the high temperature is potential influencer for higher NO_3⁻ concentration than NH₄⁺ concentration due to denitrification and nitrification processes under soil moisture stress conditions in AWD [88]. In AWD irrigation practice, the ammonia volatilization and nitrification processes are increased compared to conventional rice cultivation practice [65]. Denitrification can turn ammonium into nitrite, nitric oxide, or nitrous oxide, which can contaminate groundwater or the atmosphere if certain conditions are met during nitrification. Furthermore, ammonium can react with nitrite in the soil to produce dinitrogen gas as a result of nitrite accumulation.

The N mineralization can go up to 75.5–80 kg ha⁻¹ under no moisture stress condition where at 20–35% of the maximum water holding capacity of soil N mineralized by 55 to 64 kg ha⁻¹ from 135 kg N ha⁻¹ [65]. In dry conditions, OM decomposition leads to high ammonification of N, which is followed by NH3 loss during flooding. Aridity causes ammonification followed by nitrification-denitrification, resulting in the production of nitrogen dioxide and nitrous oxides again. In AWD irrigation practice, a large portion of N loss will occur in volatilization process accounting to 21% of the applied N and continuous flooding cause 13% N loss of the applied N. The NH₃/NH_4⁺ ratio was largely determined by soil and floodwater pH, which influenced ammonia volatilization. Higher irrigation water levels can reduce ammonia losses, because of an NH_4⁺ dilution effect [89]. Nitrification-denitrification losses of fertilizer-N are six times greater under AWD than continuous flooding [78].

6.2 Dry direct seeded rice (DSR) planting system

In contrast to transplanting seedlings from the nursery, direct seeding of rice refers to the practice of starting the rice crop from seeds which are put directly in the field [18]. The three main direct seeding techniques for rice (DSR) are water seeding (seeds sown into standing water), wet seeding (sowing pregerminated seeds on wet puddle soils), and dry seeding (Sowing dry seeds into dry soil [18]). Dry seeding has been practiced as a principal method of rice establishment in developed countries since 1950s [90]. Puddling limits soil permeability and produces a hard pan below the plow-zone in the classic transplanting system (TPR). Due to percolation, surface evaporation, and puddling, there are significant water losses as a result. Contrary to puddle transplanting, dry seeding enables the production of dry season (Boro) rice with less than 50% of the irrigation water needed [18]. But there is a significant matter to consider here. Since the field is supposed to be dry for a long period of time while practicing DSR technique, there is a chance that it would emit N₂O (Table 6). It is quite an established fact that dry farming increases N₂O production. It is found that DSR technique causes N₂O emissions, and it cannot be neglected.

District	Emission of N2O (t CO2 eq. ha^−1) in conventional puddled transplanted rice	Emission of N2O (t CO2 eq. ha^−1) in dry direct seeded rice planting system
Amritsar	0.4	0.6
Barnala	0.4	0.6
Bathinda	0.3	0.5
Faridcot	0.5	0.6
Fatehgarh Sahib	0.5	0.6
Ferozepur	0.5	0.6
Gurdaspur	0.4	0.5

Table 6.

Emission of nitrous oxide (N₂O) under conventional and direct-seeded rice in different districts of Punjab, India [91].

7. Strategies to reduce GHGs emission from rice fields

Agriculture is the major source and worse victim of climate change effects. Paddy field emits CH₄ and N₂O which are major GHGs due to their high GWP and it was estimated that agriculture contributes 52% CH₄ and 84% N₂O anthropogenic GHG emission. The agriculture is also subject to 20–40% soil organic matter loss due to cultivation [15]. As consequence, the nutrient holding capacity of soil is also reduced. The wise use of agricultural technologies and practices are potential source to reduce GHG emission from cropland and other agricultural sectors. If the modern strategies are successfully applied, they are more likely to boost crop and animal production than to diminish it (Table 7). A list of probable practices has given below:

7.1 Ways of reducing ammonia volatilization

7.1.1 Salts of potassium

When urea is added to soil, it converts to ammonium carbonate, which is prone to NH₃ volatilization loss. When calcium or magnesium nitrate or chloride salts are combined with urea, they minimize volatilization by generating ammonium chloride or nitrate [92]. Potassium (K) indirectly reduces NH₃ volatilization loss by enhancing calcium carbonate precipitation in high Ca soil by replacing Ca from the exchange complex. Potassium nitrate (KNO₃) or potassium chloride (KCl) might be utilized to reduce NH₃ volatilization losses [93]. When Mg, Ca, or K were combined with urea, the ammonia loss from urea decreased with increasing cation/N ratio [94].

7.1.2 Inhibitors

When urea is added to the soil, the urease enzyme hydrolyzes it and converts it to ammonium carbonate. The use of urease inhibitors reduces urease activity at the soil surface, allowing urea to migrate deeper into the soil before hydrolysis. The urease inhibitors phenyl phosphor diamidate (PPD) and N-(n-butyl) thio phosphoric triamide (NBPT) were successful in lowering ammonia volatilization loss in laboratory and greenhouse trials [95, 96]. Freney et al. [97] discovered that using an algal inhibitor (copper sulfate terbutryn) reduced ammonia volatilization loss, resulting in a 0.3−0.6 t ha improvement in rice output. Rawluk et al. [98] reported that the use of NBPT with granular urea reduced ammonia volatilization loss by 28−88% [93].

7.2 Denitrification minimization

Through the nitrification process, the ammonium ion remaining in the soil-water system is easily transformed to nitrite, then to nitrate. The nitrate ion is lost due to denitrification and leaching. The nitrification process is followed by denitrification. Denitrification loss will be decreased if ammonium nitrification into nitrate is delayed or reduced. As a result, nitrification inhibitors, such as dicyandiamide (DCD), iron pyrite, nitrapyrin, phenylacetylene, encapsulated calcium carbide, terrazole, and others, can be used to reduce denitrification losses [93, 99]. One of the key three greenhouse gases is nitrous oxide, which is emitted from agricultural soils owing to denitrification loss (methane, carbon dioxide, and nitrous oxide). The use of plant residues with high polyphenol content and protein binding capacity may help to minimize nitrous oxide emissions [93, 100].

7.3 Biochar addition

When tested on 14 different agricultural soils, biochar was shown to reduce denitrification and N₂O emissions by 10−90%, with a consistent reduction of the N₂O/(N₂ + N₂O) ratio, indicating that biochar reduces N₂O emissions by facilitating the last step of the denitrification process and producing more N₂ rather than N₂O [101]. However, biochar application in some soils can accelerate nitrification and increase N₂O emissions; hence, the effect of biochar application on N²O emissions is connected to the primary N₂O production pathway that runs in a soil [102]. Uzoma et al. [103] said the increase in anion exchange capacity of biochar reduces leaching of anionic (NO₃⁻) nutrients while the cation exchange capacity increases the adsorption of cation (NH₄⁺) nutrients. Therefore, this implies that application of inorganic fertilizer N alongside biochar improves retention and uptake of both NO₃⁻ and NH₄⁺.

7.4 Integrated nutrient management

Enhancing NUE can be accomplished by integrated nutrient management, which includes the use of organic manures, green manures, legumes, agricultural wastes, and biofertilizers. Organic manures are an additional source of nutrients and improve the effectiveness of fertilizers. Combining the use of organic manure and nitrogen fertilizer helps provide a steady supply of nitrogen, reduces loss, and improves the application of nitrogen [104]. Rice cv. Pusa Basmati-1 recovered more nitrogen from the fertilizer when N was applied, half as urea and half as FYM on a sandy loam soil [105]. The partial factor productivity of nitrogen (PFPN) values for the rice-rice system ranged from 26 to 52 kg grain kg⁻¹ N under recommended NPK, but they increased to 33–77 kg grain kg⁻¹ N with the substitution of 25% N through FYM in kharif rice and the reduction of 25% N in succeeding rabi rice (Table 8). Due to the fixation of atmospheric N, green manuring with legumes enhances soil N. GM enhances the physical, chemical, and biological characteristics of soil in addition to minimizing leaching and gaseous losses of N. Sunnhemp (Crotolaria juncea) and dhaincha (Sesbania aculeata) are the most common GM crops. Incorporating legumes could provide an average of 50–60 kg N ha⁻¹, according to an evaluation of a variety of leguminous crops for satisfying the N demand of a succeeding nonlegume crop [104].

Country	Location	Water regimea	Nitrogen fertilizerb	Tested mitigation optionc	Amount of chemical N, kg N ha^−1	Emission of tested mitigation option plot,d %	Reference
India	New Delhi	MSD	U	DCD	140	84e	Kumar et al. [56]
India	New Delhi	MSD	AC	DCD	140	68e	Kumar et al. [56]
India	New Delhi	MSD	U	NUE	140	89	Majumdar et al. [57]
India	New Delhi	CF	PN	DCD	120	87e	Ghosh et al. [47]
India	New Delhi	CF	U	DCD	120	39e	Pathak et al. [48]
Philippines	Los Banos	RF	U	Slow-U	90	3e^,f	Abao et al. [50]

Table 7.

Available data on possible mitigation options [44].

^a

Water regime: MSD, midseason drainage; CF, continuous flooding; RF, rain-fed, wet-season.

^b

Fertilizer type: AS, ammonium sulfate; PN, potassium nitrate; U, urea.

^c

Mitigation options: DCD, dicyandiamide; NEU, neem-coated urea; TS, thiosulfate; slow-U, slow-release urea.

^d

Fertilizer-induced N₂O-N emission of the tested mitigation option plot compared with that of the conventional fertilizer plot.

^e

Significantly different from conventional fertilizer plot at P < 0.05 by Duncan’s multiple range test. Statistical test results are from the original papers.

^f

Fertilizer-induced N₂O emission could not be calculated because no zero-N control plot was available. Thus, the percent of N₂O-N emission (including background emission) from the tested mitigation option plot is compared with that from conventional fertilizer plot is shown.

Crop	N rate (kg ha^−1)		Grain yield (tn ha^−1)		AEN
	FNP	INM	FNP	INM	FNP	INM
Rice	167	135	5.90	6.50	9	16
Wheat	325	130	5.76	6.02	3	11
Maize	263	158	8.45	8.90	5	11

Table 8.

Grain yield and nitrogen recovery of different cereals as influenced by integrated nitrogen management [106].

7.5 Slow-release fertilizer

Nitrate-containing fertilizers are subjected to leaching while ammonium and amide-containing nitrogenous fertilizer are more susceptible to volatilization loss. Slow-release fertilizers can reduce the nitrogen loss by delaying nitrogen release and enhancing the synchronization between crop demand and soil nitrogen supply [107]. These compounds are produced by treating highly soluble urea fertilizer with substances that prevent or slow down the fertilizer’s hydrolysis to ammonium.

For example, urea–formaldehyde, isobutylidene diurea (IBDU), resin-coated fertilizers (e.g., Osmocote^®), polymer and sulfur-coated urea. (brady) The nitrogen uptake for the treatments of BU (Bare Urea), CRU (Controlled release urea), MBC (50% BU + 50% CRU) and MBCB (50% BU + 50% CRU + biochar) significantly increased by 28.3%, 73.0%, 80.0% and 91.1% over that of the CK, respectively [108]. Nitrogen recovery and yield of basmati rice under different slow-release nitrogenous fertilizers are in Table 9.

N source	N rate (kg ha^−1)	Grain yield (q ha^−1)	N uptake (kg ha^−1)	AEN
PU	120	32.9	70.2	3.47
PNGU	120	36.2	82.7	6.25
KEU	120	32.5	71.9	3.13

Table 9.

Nitrogen recovery and yield of basmati rice under different slow-release nitrogenous fertilizers [109].

N Sources: PU—Prilled urea, PNGU—Pusa Neem Golden Urea, KEU—Karanj Coated Urea.

Slow-release urea enhanced single rice yield by 6.0−31.2% and NUE by 20.3−96.5% compared to the same amount of conventional urea or slow-release urea applied alone when slow-release N comprised 30−70% of the total N [110].

The maximum possibility of N fertilizer contributing to the environmental effect (MPEI) would be lowered by 67% when NUE were to increase by 50% and the N fertilization rate were to decrease by 34% [111].

7.6 Nitrification inhibitors

Inhibitors of nitrification are substances that prevent the Nitrosomonas bacteria from converting NH_4⁺ to NO_2⁻ in the first phase of nitrification and that will raise NUE and crop output by ensuring a larger concentration of ammoniacal nitrogen in the soil medium [109]. Nitrification inhibitors including dicyandiamide (DCD), nitrapyrin (N-Serve^®), 3, 4-Dimethylpyrazole phosphate (DMPP), Ca-carbide, and etridiazol (Dwell^®) have been developed by chemical companies. These substances can temporarily stop the generation of NO₃ when combined with nitrogen fertilizers. Temporarily is the essential word here, as the inhibition typically only lasts a few weeks (less if soil temperatures are over 20°C) when nitrification conditions are good [73].

8. Conclusion

Adaptation of modern agricultural practices, such as DSR and AWD, somehow reduce NH₃ and CH₄ emission but increase N₂O emissions from rice fields. To off-set gaseous losses of N from rice field with environmental benefits IPNS-based strategies are highly required and deep placement of urea is also a possible method. Soil is a heterogenous body with complex ecosystem where the interactions among soil properties (pH, SOC, TN, and mineral N) are associated with the mitigation of the GWP from N₂O and CH₄ emissions for sustainable agriculture.