Greenhouse Gas Emissions from Salt-Affected Soils: Mechanistic Understanding of Interplay Factors and Reclamation Approaches

Abstract

Salt-affected soils contain high levels of soluble salts (saline soil) and exchangeable sodium (alkali soil). Globally, about 932 million ha (Mha), including 831 Mha of agricultural land, is salt-affected. Salinity and sodicity adversely affect soil microbial diversity and enzymatic activities, and thereby carbon and nitrogen dynamics and greenhouse gas (GHG) emissions from soils. In this review article, we synthesize published information to understand the impact of salinity and sodicity on GHG production and emissions from salt-affected soils, and how various reclamation amendments (gypsum, phosphogypsum, organic manure, biochar, etc.) affect GHG emissions from reclaimed soils. Nitrous oxide (N₂O) and methane (CH₄) emissions are of greater concern due to their 298 and 28 times higher global warming potential, respectively, compared to carbon dioxide (CO₂), on a 100-year time scale. Therefore, CO₂ emissions are given negligible/smaller significance compared to the other two. Generally, nitrous oxide (N₂O) emissions are higher at lower salinity and reduced at higher salinity mainly due to: (a) higher ammonification and lower nitrification resulting in a reduced substrate for denitrification; (b) reduced diversity of denitrifying bacteria lowered down microbial-mediated denitrification process; and (c) dissimilatory nitrate reduction to ammonium (DNRA), and denitrification processes compete with each other for common substrate/nitrate. Overall, methane (CH₄) emissions from normal soils are higher than those of salt-affected soils. High salinity suppresses the activity of both methanogens (CH₄ production) and methanotrophs (CH₄ consumption). However, it imposes more inhibitory effects on methanogens than methanotrophs, resulting in lower CH₄ production and subsequent emissions from these soils. Therefore, reclamation of these soils may enhance N₂O and CH₄ emissions. However, gypsum is the best reclamation agent, which significantly mitigates CH₄ emissions from paddy cultivation in both sodic and non-sodic soils, and mitigation is higher at the higher rate of its application. Gypsum amendment increases sulfate ion concentrations and reduces CH₄ emissions mainly due to the inhibition of the methanogenesis by the sulfate reductase bacteria and the enhancement of soil redox potential. Biochar is also good among the organic amendments mitigating both CH₄ and N₂O emission from salt-affected soils. The application of fresh organic matter and FYM enhance GHG emissions for these soils. This review suggests the need for systematic investigations for studying the impacts of various amendments and reclamation technologies on GHG emissions in order to develop low carbon emission technologies for salt-affected soil reclamation that can enhance the carbon sequestration potential of these soils.

1. Introduction

Soil salinization and sodification are serious causes of land degradation particularly in arid and semi-arid regions worldwide. Soils with high levels of soluble salts (saline soil) and exchangeable sodium (alkali soil) are considered salt-affected soils [1]. Globally, 831 million ha (Mha) of the land of which ~20% is agricultural and ~33% is irrigated, is distributed among 120 countries and is salt-affected [2,3]. The expansion of these soils is expected to increase due to climate change, intrusion of sea water in coastal regions, and poor irrigation management in canal command areas [4]. Enhanced intensity and frequency of extreme events, particularly storms/cyclones in coastal areas, have been observed during the last 50 years [5]. Additionally, unjustifiable use of groundwater, excess use of synthetic fertilizers, and poor soil management are the causes of salt-induced soil degradation [6]. The saline (with excess salt) and alkali (with high residual alkalinity) groundwater are generally associated with the development of salt-affected lands [7]. Globally, 2400 Mha of land area (16% of total land) is underlain with the saline/alkali groundwater at the shallow/intermediate depth and the maximum area (14% of total saline/alkali water area) is found in the Basin of West and Central Asia [7]. The changing environmental scenario would further reduce the availability of good quality waters for irrigations [5], which will further enhance the utilization of marginal quality waters for irrigation and consequently enhance the expansion of area under salt-affected soil [8].

The enhanced greenhouse gas (GHG) emissions are the real cause of the greenhouse effect and agriculture contributes significantly [9] towards emissions through key processes/managements such as methane (CH₄) from enteric fermentation and rice cultivation, nitrous oxide (N₂O) from the application of synthetic fertilizers, and carbon dioxide (CO₂) from tillage operations [10,11]. The emissions of N₂O and CH₄ are of greater concern due to 298 and 28 times higher global warming potential (GWP) than that of CO₂ [10] on a 100-year time scale, respectively [5]. Global GHG emissions from agricultural activities were 9.3 Giga tonnes (Gt) of CO₂ equivalents (CO₂ eq) in 2018 [2]. The contribution of CH₄ and N₂O emissions from crops and livestock was 5.3 Gt CO₂ eq with agricultural soils and enteric fermentation being major sources contributing 39.5 and 39.2%, respectively [2]. The emissions of CO₂, CH₄, and N₂O occur from the agricultural soils through microbial-mediated processes/pathways. CO₂ flux from agricultural soils can be due to (i) soil respiration (root and microbial respiration), (ii) ecosystem respiration, and (iii) net ecosystem exchange (NEE), i.e., the difference between plant photosynthesis and ecosystem respiration (heterotrophic, as well as autotrophic) [12]. Under anoxic conditions, CH₄ is produced by methanogens and consumed by methanotrophs under oxic and anoxic conditions [10,13,14,15]. N₂O is produced mainly through the denitrification process in anaerobic environments and the nitrification (hydroxylamine oxidation and nitrifier denitrification) process in aerobic environments [16,17,18].

Irrigation with saline/sodic waters induces changes in soil structure and adversely affects the microbe-mediated soil processes [19]. Generally, the excess salt in soils restricts the microbial population and their activity through osmotic stress [20]. High salt concentration in soil inhibits the soil organic matter decomposition through alteration of microbial activities leading to either decrease or increase in mineralization of carbon (C) and nitrogen (N) [21,22]. However, inhibition of N mineralization is temporary and recovers at later stages [23]. GHG emissions from soils are governed by microbial activities involved in organic matter decomposition, nitrification, denitrification, methanogenesis, and CH₄ oxidation processes, and salinity and sodicity have significant effects on these processes [24]. Usually, GHG emissions decrease with increased soil salinity and sodicity. A decrease in N₂O [25,26], CH₄ [27,28], and CO₂ [29,30] emissions with increased salinity and sodicity has been reported in several studies. However, reports on increased N₂O [30], CH₄ [24,31], and CO₂ [31] emissions are also available. To our best knowledge, the review article concerning the N₂O, CH₄, and CO₂ emissions from the salt-affected soils, various factors affecting their emissions, and the impacts of reclamation approaches of salt-affected soils on GHG emission has not yet been published. Therefore, the present manuscript has been organized to understand the conditions, interplaying factors along with the impact of reclamation processes on the GHG emissions from the salt-affected soils to refine the reclamation practices and ecosystem sustainability along with the climate change mitigation in these affected agro-ecosystems.

2. Salt-Affected Soils, Global Extent, and Distribution

Salt-affected soils have a high concentration of soluble salts in such a quantity that negatively affect normal growth and productivity [32]. These problematic salts are mainly carbonates (CO₃²⁻), bicarbonates (HCO₃⁻), chlorides (Cl⁻), and sulfates (SO₄²⁻) of sodium (Na⁺), calcium (Ca²⁺), and magnesium (Mg²⁺) [33]. Salt-affected soils are classified into three categories, i.e., (i) saline, (ii) alkali/sodic, and (iii) saline-alkali/saline-sodic soils based on the electrical conductivity of saturated paste extract (EC_e), pH of saturated paste (pH_s), exchangeable sodium percentage (ESP), and sodium adsorption ratio in saturated paste extract (SAR_e) [33]. Saline soils contain high concentration of neutral salts mainly chlorides and sulfate of sodium, calcium and magnesium, and have EC_e > 4 dS m⁻¹, pH_s < 8.5, ESP < 15, and SAR_e < 13 [34]. Alkali/sodic soils possess excess contents of carbonates, bicarbonate and silicate salts of sodium, and characterized by EC_e < 4 dS m⁻¹, pH_s > 8.5, ESP > 15, and SAR_e > 13. While saline-alkali soils have high levels of both soluble salts and ESP, and these show EC_e > 4 dS m⁻¹, SAR_e > 13, ESP > 15, and variable pH_s due to collective effects of both salinity and sodicity [35]. Detailed characteristics of salt-affected soils are given in

Table 1. Types/classes of salt-affected soils and their chemical characteristics.

Salinity Class	ECe (dS m−1)	ESP	(pHs)	(SARe)	Type of Dominant Salts	Problems Associated
Saline soils	>4	<15	<8.5	<13	High levels of soluble salts of chlorides (Cl−) and sulfate (SO42−) of sodium (Na+), calcium (Ca2+), and magnesium (Mg2+)	Hinder water absorption by plants due to high osmotic effects. Possible toxicity and antagonism of dominant ions at higher electrical conductivity
Alkali/sodic Soils	<4	>15	>8.5	>13	High concentration of carbonate (CO32−) and bicarbonate (HCO3−) salts of sodium (Na+) in soil solution, and Na+ on exchange sites	Sodium, Carbonate, and bicarbonate toxicity to plants. Dispersion of soil structure due to high sodium content. Slaking, swelling, and hard setting of soil surface. Seasonal waterlogging
Saline-alkali/sodic soils	>4	>15	Variable	>13	The combined effect of excess salts and high exchangeable sodium percentage	Hinder water and nutrients uptake due to high osmotic effects. Sodium, carbonate, and bicarbonate toxicity to plants. Dispersion of soil structure due to high sodium content. Slaking, swelling, and hard setting of soil surface

EC_e = electrical conductivity of saturated paste extract; pH_s = pH of saturated paste; ESP = exchangeable sodium percentage; SAR_e = sodium adsorption ratio in saturated paste extract.

. Globally, about 20% of agricultural land (831 Mha) is salt-affected [3]. Out of the total salt-affected soils, 47.8% (397 Mha) is saline and 52.2% (434 million ha) is sodic in nature (Figure 1). The area of these salt-affected soils is distributed in 120 countries and represents 10, 20, and 33% of the global, agricultural, and irrigated lands, respectively [33,36]. Globally, the Amu-Darya and Syr-Darya River Basins (Aral Sea Basin) in Central Asia, the Indo-Gangetic Basin in India, the Indus Basin in Pakistan, the Yellow River Basin in China, the Euphrates Basin in Syria and Iraq, the Murray-Darling Basin in Australia, and the San Joaquin Valley in the United States are the well-known regions where salinization is extensively reported [37]. The highest (~340 Mha) salt-affected area, i.e., 50% of total global sodic soils, is found in Australia, followed by Central and South Asia (~212 Mha) [38,39]. Kazakhstan (~60 Mha) and Uzbekistan (~28 Mha) are the main salinity-affecting countries in the Central Asian region [39,40]. In India, the problem extends over an area of about 6.73 Mha (2.96 Mha saline soil, and 3.77 Mha sodic soil) land, which is about 2% of India’s total geographic area (TGA) [41,42].

3. Microbial Response to Salinity and Sodicity

Soil microbial communities perform an essential role in the organic matter decomposition, nutrient cycling, and GHG production/consumption and both Soil salinity/sodicity adversely affect the microbial biomass [44] and play a decisive role in the structuring, and distribution of microbial communities [45]. Siddikee et al. [46] reported the adverse impact of soil salinity/sodicity on microbial activities and biogeochemical processes that are essential in the mineralization of nutrients. The salinity/sodicity has an extensive impact on GHG production and emissions from agricultural soils mainly due to their influences on the growth and activities of nitrifying, denitrifying, methanogen and methanotrophs. The mechanisms which may interpret the relationship between salinity/sodicity and these microorganisms are described and depicted in Figure 2. A high concentration of soluble salts inhibits microbial growth due to the adverse impact of increased osmotic stress in general and specific ion toxicity in particular [19]. Soil salinity reduces microbial activities by altering the soil’s physicochemical properties [47]. Increased sodicity decreases the O₂ diffusion in the soil due to the blocking of soil pores and consequently decreases soil respiration [48]. At a high level of soil salinity, the toxicity of Cl⁻ and hydrosulfide (HS⁻) ions caused adverse impacts on microbial growth and thereby on N₂O and CH₄ emissions [49].

High salinity levels inhibit the nitrification rate and decrease the availability of nitrate (NO₃⁻) which further limit the denitrification process and thereby reduces the N₂O emissions [50]. Rysgaard et al. [51] reported that an increase in salinity decreases the ammonia adsorption by soil sediments which potentially enhances the availability of ammonia and thus stimulates nitrification [51]. Higher salinity limits the availability of soil organic matter/organic substrates for heterotrophic bacteria and alters the abundance and activities of microbes [52]. It also limits the biodegradation of complex organic substrates into simpler ones (H₂, formate, acetate, alcohol, and other compounds) which are used by methanogens for CH₄ production and thereby reduces CH₄ emissions [53]. The decrease in microbial enzymes and gene activities involved in organic matter decomposition, nitrification, denitrification, methanogenesis, and CH₄ oxidation has also been reported [54]. Increasing salinity enhance the ability of dissimilatory nitrate reduction to ammonium (DNRA) to overcome the denitrification [55]. Because increasing salinity enhances the population and activity of sulfate reducers [56] and consequently may increase HS⁻ ions. These HS⁻ ions have a more inhibitory effect on denitrifying bacteria than DNRA [57].

Figure 2. Response of nitrifying, denitrifying, methanogens, and methylotrophs/methanotrophs involved in GHG production and consumption to soil salinity and sodicity. Orange (nitrification, denitrification, methanogenesis, and methylotrophy), blue (nitrification/denitrification), and green (methanogenesis/methylotrophy) colors represent the GHG production process/pathways affected by salinity and sodicity.

Figure 2. Response of nitrifying, denitrifying, methanogens, and methylotrophs/methanotrophs involved in GHG production and consumption to soil salinity and sodicity. Orange (nitrification, denitrification, methanogenesis, and methylotrophy), blue (nitrification/denitrification), and green (methanogenesis/methylotrophy) colors represent the GHG production process/pathways affected by salinity and sodicity.

3.1. Effect of Salinity/Sodicity on Nitrification/Denitrification

3.1.1. Nitrification

Nitrification is a two-step process involving (a) ammonia oxidation regulated by ammonia-oxidizing archaea (AOA) and (b) nitrite oxidation process regulated by ammonia-oxidizing bacteria (AOB) [18]. The response of AOA and AOB to salinity level is controversial. Some studies advocated that nitrification is predominated by AOA at salinity levels up to 10–20 parts per thousand (ppt) while activities of AOB decreased at increased salinity [58]. Other investigations advocate that the activities of AOB were more than AOA at increased salinity levels [59]. These contradictory observations indicate that besides the salinity/sodicity levels other factors also affect the community structure and predominance of AOA and AOB. Slightly acidic to neutral pH and availability of ammonia favor the growth of AOB as compared to AOA [60,61]. Guo et al. [62] reported that saline water irrigation increased soil salinity and ammonical N and lower AOA/AOB ratios. Kaushik and Sethi [63] observed a significant reduction in the growth of both ammonium oxidizers and nitrite oxidizers in rice rhizosphere under increased salinity, and ammonium oxidizers were found more susceptible than nitrite oxidizers to salt stress. Magalhães et al. [64] observed a higher nitrification rate with increasing salinity from 0 to 15 ppt in the Douro River estuary. Zhou et al. [36] found that the growth and activities of nitrifiers were optimum at 5–10 ppt and excess levels of salinity (>10 ppt) showed the inhibitory effect. Overall, moderate salinity enhanced the nitrification rate while excess salinity decreases. Soil moisture is a key factor influencing nitrification and denitrification rates at higher salinity levels [65]. Denitrification had a significantly positive relationship with soil moisture and it increased with an increase in soil moisture when the soil water content was less than 27.03% and decreased with an increase in soil moisture when the soil water content was more than 27.03% [66].

3.1.2. Denitrification

Denitrification is a microbial-mediated multi-step anaerobic process in which nitrate (NO₃⁻) is sequentially reduced into nitrite (NO₂⁻), nitric oxide (NO), nitrous oxide (N₂O), and finally into atmospheric nitrogen (N₂) [67]. Denitrification is facilitated by a combination of four independent enzymes, i.e., nitrate reductase (NAR/NAP), nitrite reductase (NIR), NO reductase (NOR), and N₂O reductase, which have been encoded by narGH/napA, nirK/nirS, nonB, and nosZ genes, respectively [68]. Denitrifying bacteria perform differentially under saline and sodic environments. Several studies advocate that soil salinization consistently reduces the denitrification [36,69]. However, Franklin et al. [52] observed an abundance of denitrifiers at low salinity (5 ppt). The negative association of denitrifiers with sediment salinity (0–36 ppt) in estuaries has been explored by Giblin et al. [50] and Santoro et al. [49]. Zhang et al. [70] envisage that increased salinity increases nitrification and denitrification below the threshold value of salinity (EC = 1.13 dS m⁻¹) and decreases nitrification and denitrification above the threshold salinity.

Santoro et al. [49] found that the diversity of two genes nirS and nirK was negatively associated with salinity in coastal aquifers and the relative abundance of nirS was higher than nirK. This indicated the environmental effect of salinity on the metabolic performance of the microbial populations. Wang et al. [69] advocated that salinity significantly reduced the abundance of nirK, nirS, and nosZ genes and decreased the population of denitrifying bacteria. Shao et al. [71] found increased N₂O efflux under >0.5% salinity conditions. Fiedler et al. [72] reported a 42 times higher abundance of nitrite reductase nirS in salt–affected soil than in controlled soil which suggests that the saline soils have a higher potential for denitrification. N₂O reductase (nosZ) is more sensitive to soil salinity and depressed significantly under salinity conditions hence N₂O is not reduced into N₂ resulting in more N₂O accumulation and effluxes from the denitrification [73]. Decoupling of nitrification or denitrification processes under saline conditions might be a possible mechanism for increasing N₂O effluxes under salinity [71]. However, some other investigations also concluded that N₂O emissions are negatively associated with soil salinity [74].

3.2. Effect of Salinity/Sodicity on Methanogenesis/Methanotrophs

Methanogenesis is the microbial-mediated process of CH₄ production from the complex organic material by bacteria and archaea. Bacteria hydrolyze the complex organic material into simpler substrates (H₂, formate, certain alcohols, acetate, etc.) which are further consumed by methanogens as food and produced CH₄ [75]. Limited literature is available about the abundance and community structure of methanogens and methylotrophs in the hypersaline environment. Ollivier et al. [76] reported generation of CH₄ from H₂ + CO₂ at salinities up to 240 ppt but not from acetate even at salinities > 60 ppt. Scholten et al. [77] investigated the abundance and distribution of methanogen-specific functional genes (mcrA) in hypersaline environments. Investigations by Bebout et al. [78] and Smith et al. [79] did not report any significant contribution of methanogenesis in carbon remineralization under a hypersaline environment. This might be due to the predominance of sulfate reducers or high oxygen concentration in a photic zone that precludes methanogenic activity from such environments under field conditions [78,79].

Heyer et al. [80] reported that the culture of methanotrophic bacteria is capable of growing to a 15% NaCl level. Nguyen et al. [81] reported very low methane emissions from paddy cultivated in salt-affected but cow manure addition enhanced CH₄ emissions significantly due to improved relative abundance of methanogens by enhancing soil properties and nutrient availability. Shao et al. [71] and Xiao et al. [82] concluded that salinity imposes more inhibitory effects on methanogens than methanotrophs or methylotrophs. Low salinity favors the growth of methanogens because Na⁺ ions are required by methanogens for growth, amino acid transportation, methanogenesis, and internal pH regulation [83]. Weston et al. [84] investigated that acetoclastic methanogens were significantly inhibited by the in-situ addition of saline solution in the Delaware Wetland of New Jersey. However, hydrogenotrophic methanogens did not show any significant change in methanogenesis rate under the same investigation.

4. GHG Emissions from Salt-Affected Soils

4.1. Nitrous Oxide Emissions

N₂O is produced through nitrification (conversion of NH₄⁺ to NO₂⁻, and NO₂⁻ to NO₃⁻) and denitrification (conversion NO₃⁻ to N₂) pathways (Figure 3a) [67]. The N₂O efflux is influenced by soil pH, salt concentration, temperature, redox potential, O₂ concentration, etc. Studies conducted on N₂O efflux under salt-affected conditions are summarized in Table 2. N₂O emissions from low salinity wetland soils (0.060 mg kg⁻¹) were reported higher as compared to high salinity wetland soils [26]. It is reported that the addition of salts up to a certain threshold may enhance the N₂O emissions and decrease thereafter [85]. The lower salinity level inhibits both the steps of nitrification (conversion of NH₄⁺ to NO₂⁻ and NO₂⁻ to NO₃⁻), however, the inhibition of NO₂⁻ to NO₃⁻ conversion is stronger than that of NH₄⁺ to NO₂⁻ causing higher NO₂⁻ accumulation and enhanced N₂O emissions (Figure 3b) [85]. Li et al. [25] reported 110% higher N₂O emissions in slightly saline soil (1.0 dS m⁻¹) but 20% lower in moderate salinity (5 dS m⁻¹) soil as compared to non-saline soil (0.3 dS m⁻¹) under urea and ammonium sulfate fertilizer application. Ghosh et al. [30] observed that ammonical fertilizers, at low salinity, may enhance the N₂O emissions as compared to non-saline soil because that low salinity reduces the activity of N₂O reductase enzymes, therefore restricting the conversion of N₂O to N₂. In addition, low salinity can also serve as strong inhibition of nitrite oxidizers than ammonia oxidizers (Figure 3b) [36].

Soil moisture has a significant effect on N₂O emissions from saline soil. For instance, 4.7–37.6 and 5.0–15.3 times higher N₂O emissions at 100% soil moisture level than at 50 and 75% soil moisture levels, respectively, was reported by Li et al. [85], and it was mainly due to an increased rate of nitrification with increasing soil moisture [85]. Thapa et al. [86] reported that increasing salinity from 0.81 to 4.65 dS m⁻¹ increased N₂O flux at 90% of Water-filled pore space (WFPS) but reduced at 60% of WFPS in sulfate-dominated saline soil and it was due to that denitrifying bacteria performed more efficiently even at higher salinity levels because water addition reduces the adverse effect of salinity [86]. The quality of irrigation water plays important role in GHG emissions. Ma et al. [54] reported that saline water irrigation (8.01 dS m⁻¹) significantly reduced cumulative N₂O emissions than freshwater irrigation in calcareous soil. Wei et al. [87] reported that N₂O emissions decreased by 29.1 and 39.2% in 2 and 8 g L⁻¹ saline water irrigation, respectively, but increased by 58.3% in 5 g L⁻¹ treatments under N120 conditions as compared to freshwater irrigation. This investigation envisages that the sensitivity of N₂O production and consumption differed significantly with the degree of irrigation water salinities [87]. Nitrification is highly sensitive to salinity and lower N₂O emissions are expected with higher salinity levels. However, few researchers also reported the reverse trends [24,27,29].

Figure 3. (a) Overview of the N₂O production pathways. (b) Inhibition of nitrification pathways due to soil salinity. The lower level of soil salinity slightly inhibits the activity of ammonia-oxidizing bacteria (AOB) and strongly inhibits the activity of nitrite-oxidizing bacteria (NOB) leading to the accumulation of NO₂⁻ in soils and thereby high N₂O emissions. Adopted and modified from [18,25].

Figure 3. (a) Overview of the N₂O production pathways. (b) Inhibition of nitrification pathways due to soil salinity. The lower level of soil salinity slightly inhibits the activity of ammonia-oxidizing bacteria (AOB) and strongly inhibits the activity of nitrite-oxidizing bacteria (NOB) leading to the accumulation of NO₂⁻ in soils and thereby high N₂O emissions. Adopted and modified from [18,25].

Table 2. Effect of soil salinity and sodicity on GHG emissions from saline ecosystems.

References	Systems and Study Locations	Treatments Details	Observation of the Study (GHG Emissions)	Key Findings/Reasoning
Field experiments
Li et al. [25]	Field experiment at Kunshan field station, Suzhou, China.	Nonsaline (S0), salinity-S1 (1 dS m−1), and salinity-S5 (5 dS m−1)	N2O emission increased by 89–110% at S1 and decreased by about 20% at S2.	Saline soils could be a potential source of N2O emissions when cultivated. So, mitigation options should be explored.
Ma et al. [54]	Long-term (2009–2018) field experiment in calcareous soil at the experimental station of Shihezi University, China.	N0 (no N) + SF (fresh water, 0.35 dS m−1); N0 + SH (saline water, 8.04 dS m−1); N360 (360 kg N ha−1) + SF; N360 + SH	Irrigation with saline water inhibited N2O emission, by 45.19% (N0) and 43.50% (N360) compared with irrigation with fresh water	Saline water irrigation altered community structures of denitrifying bacteria with nirK, nirS, and nosZ
Capooci et al. [31]	Field experiment at temperate salt marsh connected to the Atlantic Ocean.	Control (17 ppt); treatment (12.4–18.6 ppt)	Soils subjected to low salinity had greater GHG emissions than control soils (17 ppt). Treatment soils had a 23% increase in GWP.	The storm surges can produce pulses of GHG emissions.
Zhang et al. [88]	GHG emissions from three rice (R1, R2, and R3) and maize (M1, M2, M3) fields with different salinity at Songyuan City of the Western Jilin Province, Northeast China.	R1 (pH: −7.6, EC: −0.16); R2 (pH: −8.6, EC: −0.27); R3 (pH: −9.1, EC: −0.41)	GWP of rice fields ranged 1070.0–1996.2 kg CO2 eq ha−1.	Higher pH and salinity conditions consistently resulted in lower CO2, CH4, and N2O emissions and CH4 uptake.
		M1 (pH: −7.34, EC: −0.10); M2 (pH: −7.76, EC: −0.19); M3 (pH: −8.43, EC: −0.25)	GWP of maize fields ranged 600.5–1149.8 kg CO2 eq ha−1.
Poffenbarger et al. [89]	Metadata analysis based on secondary in-situ studies (31 nos.) of CH4 emissions from tidal marshes	Fresh (salinity < 0.5 ppt); Oligohaline (0.5–5.0 ppt); Mesohaline (5–18 ppt); Polyhaline (>18 ppt)	Oligohaline marshes had the highest and most variable CH4 emissions (150 ± 221 gm−2 yr−1). Negligible CH4 emissions in polyhaline, and no significant difference between fresh and mesohaline marshes.	Need to estimate or monitor CH4 emissions in lower-salinity marshes.
Laboratory experiments
She et al. [90]	Laboratory experiment with different textured soil.	Sandy clay loam + S1 (0.10–1.0% soil salinity); Sandy loam + S1; Silty clay + S1	Cumulative CO2 emissions in the coarse-textured (sandy clay loam and sandy loam) soils were more (206–231 and 176–204 mg CO2 kg−1) affected by salinity than in the fine-textured (silty clay) soil.	Soil texture controlled the negative effect of salinity on C mineralization by regulating the soil microbial community composition.
Jia et al. [26]	Low salinity wetland (LW-Pragmites australis) and High salinity wetland (HW-Suaeda sals) soils were collected and incubated.	LW soils (3.18 ppt salinity, 0.28 SAR); HW soils (13.30 ppt salinity, 0.03 SAR)	N2O emissions were promoted in low salinity wetland (0.060 mg N2O kg−1) but significantly inhibited in high salinity wetlands (0.008 mg N2O kg−1)	Study suggests the complexity and uniqueness of N2O emissions responses to nitrogen inputs related to the salinity levels.
Zhang et al. [91]	Samples of saline-alkali soils were collected from four different locations in Yellow River Delta	Bare land soil (no vegetation) S0 (Control); S1 (1 mg g−1); S3 (3 mg g−1); S5 (5 mg g−1)	CO2 emission ranged from 88.55 (S0)–51.77 (S3) mg CO2 kg−1 and N2O 0.030 (S0)–0.012 (S3) mg N2O kg−1.	The N2O and CO2 emissions of were highest in herbage communities, intermediate in woody communities, and lowest in bare land under all treatments. The salinity effect on GHG emissions was stronger in soils with low salt levels. Higher GHG emissions at high soil moisture were found in all soils.
		Land covered with woody (T. chinensis) community vegetation.	CO2 emission ranged from 282.28 (S0)–231.46 (S4) mg CO2 kg−1 and N2O 0.252 (S4)–0.163 (S2) mg N2O kg−1.
		Land covered with herbage (S. sals) community vegetation	CO2 emission ranged from 504.33 (S0)–400.39 (S4) mg CO2 kg−1 and N2O 0.08 (S1)–0.036 (S4) mg N2O kg−1.
		Land covered with herbage (P. australis) community vegetation	CO2 emission ranged from 518.46 (S0)–391.27 (S4) mg CO2 kg−1 and N2O 0.153 (S1)–0.041 (S3) mg N2O kg−1.
Ghosh et al. [30]	Soil samples from three different locations within a salt affected agricultural land and incubated for 30 days.	S1 (0.44 dS m−1); S2 (7.20 dS m−1); S3 (4.55 dS m−1)	The N2O emissions significantly increased by 39.8% and 42.4% in S2 and S3, respectively. The addition of N significantly increased cumulative N2O and CO2 emissions.	Saline-sodic soils can be a significant contributor to N2O. Further, N fertilizer, irrigation, and precipitation may enhance GHG emissions.
Maucieri et al. [24]	Vertisol was collected from the experimental farm of University of Sydney and incubated for 30 days.	ECiw (0.09 dS m−1); ECiw (5 dS m−1); ECiw (10 dS m−1)	Saline water irrigation reduced CO2 emissions by 19% (5 dS m−1) and 28% (10 dS m−1). However, N2O emissions increased by 60% with salinity	Salinity decreased CO2 and increased N2O emission
Thapa et al. [86]	Soil samples from Soil Health and Agriculture Research Extension (SHARE) farm were collected and incubated for 25 days.	ECe < 0.50 dS m−1 (60% WFPS); ECe 4.65 dS m−1 (60% WFPS); ECe 0.81 dS m−1 (90% WFPS); ECe 4.65 dS m−1 (90% WFPS) WFPS is a water-filled pore space	Relative decline in CO2 at higher ECe was smaller at 60% WFPS than at 90% WFPS. N2O emission decreased by 45% at 60% WFPS and increased by 223% at 90% WFPS.	Higher soil moisture increased substrate availability, salt dilution, and enhance microbial activity, causing higher CO2 and N2O emissions.
Reddy and Crohn [29]	Collected soil samples from abandoned field of Coachella Valley, California and incubated for 60 days	S3 (2.8 dS m−1) (control); S15 (15.2 dS m−1); S30 (30.6 dS m−1)	Increased N2O emission by 18–24% (at S15) and 34–87% (at S30), but decreased CO2 emissions	The use of active organic amendments to remediate salt-affected soils can prove to be beneficial in mitigating N2O emission
Marton et al. [27]	Soil samples were collected from tidal forests along the Altamaha, Ogeechee, and Satilla Rivers in southeast Georgia and incubated in the laboratory.	0% (control); 2% saline water; 5% saline water	CH4 emission inhibited by 77% in the 2% and 89% in the 5% saline water treatment whereas CO2 generally increased with salinity, though exhibited a variable response between the three rivers.	Short-term salinity exposure enhanced anaerobic C mineralization, a decline in CH4 production, and a varied response in N2O production
Pattnaik et al. [92]	Soil samples from three locations (a) CRRI, Cuttack (alluvial soil), (b) Ernakulam, Kerala (acid sulphate saline soil (Pokkali)), and (c) Canning, West Bengal, (coastal saline soil) of India were collected and incubated for 35 days	Alluvial soil (0.35 dS m−1) (control); Acid sulfate soil (5.01 dS m−1); Coastal saline soil (17.23 dS m−1)	CH4 production in non-saline alluvial soil was 630.86 ng CH4 g−1, and reduce remarkably in acid sulphate saline soil (12.97 ng CH4 g−1), and coastal saline soil (142.36 CH4 g−1)	High sulphate content of acid sulphate saline soil attributed to lower emission
		Alluvial: (0.35 dS m−1) (control); (4 dS m−1); (8 dS m−1); (16 dS m−1); (20 dS m−1)	Addition of salts to the non-saline alluvial soil at 4, 8, 16 and 20 dS m−1 progressively decreased CH4 production.	CH4 inhibition due to low microbial activities and soil microbial population including that of methanogens
Pot experiments
Khatun et al. [28]	Pot study in net house at Bangladesh Agricultural University, Mymersingh, Bangladesh	Control (100% NPK); control + 25 nM NaCl; control + 50 nM NaCl; control + 75 nM NaCl	Decreased yield scaled CH4 emission from 7.5% (25 nM NaCl) to 25% (75 nM NaCl)	Phosphogypsum and biochar with recommended fertilizers in saline soils could mitigate yield scaled CH4 emissions
Wei et al. [87]	Collected soil samples from the greenhouse of Nanjing Vegetables Scientific Institute, China, and conducted pot experiments.	Freshwater (0.3 dS m−1) + N120 (120 kg N ha−1); S1 (3.5 dS m−1) + N120; S2 (8.1 dS m−1) + N120; S3 (12.7 dS m−1) + N120	Irrigation with S1 water lowered N2O emission and S2 enhanced emission by 58.3%	the effect degree of salinity on consumption and production of N2O might vary among irrigation salinity ranges

Table 2. Effect of soil salinity and sodicity on GHG emissions from saline ecosystems.

References	Systems and Study Locations	Treatments Details	Observation of the Study (GHG Emissions)	Key Findings/Reasoning
Field experiments
Li et al. [25]	Field experiment at Kunshan field station, Suzhou, China.	Nonsaline (S0), salinity-S1 (1 dS m−1), and salinity-S5 (5 dS m−1)	N2O emission increased by 89–110% at S1 and decreased by about 20% at S2.	Saline soils could be a potential source of N2O emissions when cultivated. So, mitigation options should be explored.
Ma et al. [54]	Long-term (2009–2018) field experiment in calcareous soil at the experimental station of Shihezi University, China.	N0 (no N) + SF (fresh water, 0.35 dS m−1); N0 + SH (saline water, 8.04 dS m−1); N360 (360 kg N ha−1) + SF; N360 + SH	Irrigation with saline water inhibited N2O emission, by 45.19% (N0) and 43.50% (N360) compared with irrigation with fresh water	Saline water irrigation altered community structures of denitrifying bacteria with nirK, nirS, and nosZ
Capooci et al. [31]	Field experiment at temperate salt marsh connected to the Atlantic Ocean.	Control (17 ppt); treatment (12.4–18.6 ppt)	Soils subjected to low salinity had greater GHG emissions than control soils (17 ppt). Treatment soils had a 23% increase in GWP.	The storm surges can produce pulses of GHG emissions.
Zhang et al. [88]	GHG emissions from three rice (R1, R2, and R3) and maize (M1, M2, M3) fields with different salinity at Songyuan City of the Western Jilin Province, Northeast China.	R1 (pH: −7.6, EC: −0.16); R2 (pH: −8.6, EC: −0.27); R3 (pH: −9.1, EC: −0.41)	GWP of rice fields ranged 1070.0–1996.2 kg CO2 eq ha−1.	Higher pH and salinity conditions consistently resulted in lower CO2, CH4, and N2O emissions and CH4 uptake.
		M1 (pH: −7.34, EC: −0.10); M2 (pH: −7.76, EC: −0.19); M3 (pH: −8.43, EC: −0.25)	GWP of maize fields ranged 600.5–1149.8 kg CO2 eq ha−1.
Poffenbarger et al. [89]	Metadata analysis based on secondary in-situ studies (31 nos.) of CH4 emissions from tidal marshes	Fresh (salinity < 0.5 ppt); Oligohaline (0.5–5.0 ppt); Mesohaline (5–18 ppt); Polyhaline (>18 ppt)	Oligohaline marshes had the highest and most variable CH4 emissions (150 ± 221 gm−2 yr−1). Negligible CH4 emissions in polyhaline, and no significant difference between fresh and mesohaline marshes.	Need to estimate or monitor CH4 emissions in lower-salinity marshes.
Laboratory experiments
She et al. [90]	Laboratory experiment with different textured soil.	Sandy clay loam + S1 (0.10–1.0% soil salinity); Sandy loam + S1; Silty clay + S1	Cumulative CO2 emissions in the coarse-textured (sandy clay loam and sandy loam) soils were more (206–231 and 176–204 mg CO2 kg−1) affected by salinity than in the fine-textured (silty clay) soil.	Soil texture controlled the negative effect of salinity on C mineralization by regulating the soil microbial community composition.
Jia et al. [26]	Low salinity wetland (LW-Pragmites australis) and High salinity wetland (HW-Suaeda sals) soils were collected and incubated.	LW soils (3.18 ppt salinity, 0.28 SAR); HW soils (13.30 ppt salinity, 0.03 SAR)	N2O emissions were promoted in low salinity wetland (0.060 mg N2O kg−1) but significantly inhibited in high salinity wetlands (0.008 mg N2O kg−1)	Study suggests the complexity and uniqueness of N2O emissions responses to nitrogen inputs related to the salinity levels.
Zhang et al. [91]	Samples of saline-alkali soils were collected from four different locations in Yellow River Delta	Bare land soil (no vegetation) S0 (Control); S1 (1 mg g−1); S3 (3 mg g−1); S5 (5 mg g−1)	CO2 emission ranged from 88.55 (S0)–51.77 (S3) mg CO2 kg−1 and N2O 0.030 (S0)–0.012 (S3) mg N2O kg−1.	The N2O and CO2 emissions of were highest in herbage communities, intermediate in woody communities, and lowest in bare land under all treatments. The salinity effect on GHG emissions was stronger in soils with low salt levels. Higher GHG emissions at high soil moisture were found in all soils.
		Land covered with woody (T. chinensis) community vegetation.	CO2 emission ranged from 282.28 (S0)–231.46 (S4) mg CO2 kg−1 and N2O 0.252 (S4)–0.163 (S2) mg N2O kg−1.
		Land covered with herbage (S. sals) community vegetation	CO2 emission ranged from 504.33 (S0)–400.39 (S4) mg CO2 kg−1 and N2O 0.08 (S1)–0.036 (S4) mg N2O kg−1.
		Land covered with herbage (P. australis) community vegetation	CO2 emission ranged from 518.46 (S0)–391.27 (S4) mg CO2 kg−1 and N2O 0.153 (S1)–0.041 (S3) mg N2O kg−1.
Ghosh et al. [30]	Soil samples from three different locations within a salt affected agricultural land and incubated for 30 days.	S1 (0.44 dS m−1); S2 (7.20 dS m−1); S3 (4.55 dS m−1)	The N2O emissions significantly increased by 39.8% and 42.4% in S2 and S3, respectively. The addition of N significantly increased cumulative N2O and CO2 emissions.	Saline-sodic soils can be a significant contributor to N2O. Further, N fertilizer, irrigation, and precipitation may enhance GHG emissions.
Maucieri et al. [24]	Vertisol was collected from the experimental farm of University of Sydney and incubated for 30 days.	ECiw (0.09 dS m−1); ECiw (5 dS m−1); ECiw (10 dS m−1)	Saline water irrigation reduced CO2 emissions by 19% (5 dS m−1) and 28% (10 dS m−1). However, N2O emissions increased by 60% with salinity	Salinity decreased CO2 and increased N2O emission
Thapa et al. [86]	Soil samples from Soil Health and Agriculture Research Extension (SHARE) farm were collected and incubated for 25 days.	ECe < 0.50 dS m−1 (60% WFPS); ECe 4.65 dS m−1 (60% WFPS); ECe 0.81 dS m−1 (90% WFPS); ECe 4.65 dS m−1 (90% WFPS) WFPS is a water-filled pore space	Relative decline in CO2 at higher ECe was smaller at 60% WFPS than at 90% WFPS. N2O emission decreased by 45% at 60% WFPS and increased by 223% at 90% WFPS.	Higher soil moisture increased substrate availability, salt dilution, and enhance microbial activity, causing higher CO2 and N2O emissions.
Reddy and Crohn [29]	Collected soil samples from abandoned field of Coachella Valley, California and incubated for 60 days	S3 (2.8 dS m−1) (control); S15 (15.2 dS m−1); S30 (30.6 dS m−1)	Increased N2O emission by 18–24% (at S15) and 34–87% (at S30), but decreased CO2 emissions	The use of active organic amendments to remediate salt-affected soils can prove to be beneficial in mitigating N2O emission
Marton et al. [27]	Soil samples were collected from tidal forests along the Altamaha, Ogeechee, and Satilla Rivers in southeast Georgia and incubated in the laboratory.	0% (control); 2% saline water; 5% saline water	CH4 emission inhibited by 77% in the 2% and 89% in the 5% saline water treatment whereas CO2 generally increased with salinity, though exhibited a variable response between the three rivers.	Short-term salinity exposure enhanced anaerobic C mineralization, a decline in CH4 production, and a varied response in N2O production
Pattnaik et al. [92]	Soil samples from three locations (a) CRRI, Cuttack (alluvial soil), (b) Ernakulam, Kerala (acid sulphate saline soil (Pokkali)), and (c) Canning, West Bengal, (coastal saline soil) of India were collected and incubated for 35 days	Alluvial soil (0.35 dS m−1) (control); Acid sulfate soil (5.01 dS m−1); Coastal saline soil (17.23 dS m−1)	CH4 production in non-saline alluvial soil was 630.86 ng CH4 g−1, and reduce remarkably in acid sulphate saline soil (12.97 ng CH4 g−1), and coastal saline soil (142.36 CH4 g−1)	High sulphate content of acid sulphate saline soil attributed to lower emission
		Alluvial: (0.35 dS m−1) (control); (4 dS m−1); (8 dS m−1); (16 dS m−1); (20 dS m−1)	Addition of salts to the non-saline alluvial soil at 4, 8, 16 and 20 dS m−1 progressively decreased CH4 production.	CH4 inhibition due to low microbial activities and soil microbial population including that of methanogens
Pot experiments
Khatun et al. [28]	Pot study in net house at Bangladesh Agricultural University, Mymersingh, Bangladesh	Control (100% NPK); control + 25 nM NaCl; control + 50 nM NaCl; control + 75 nM NaCl	Decreased yield scaled CH4 emission from 7.5% (25 nM NaCl) to 25% (75 nM NaCl)	Phosphogypsum and biochar with recommended fertilizers in saline soils could mitigate yield scaled CH4 emissions
Wei et al. [87]	Collected soil samples from the greenhouse of Nanjing Vegetables Scientific Institute, China, and conducted pot experiments.	Freshwater (0.3 dS m−1) + N120 (120 kg N ha−1); S1 (3.5 dS m−1) + N120; S2 (8.1 dS m−1) + N120; S3 (12.7 dS m−1) + N120	Irrigation with S1 water lowered N2O emission and S2 enhanced emission by 58.3%	the effect degree of salinity on consumption and production of N2O might vary among irrigation salinity ranges

Reddy and Crohn [29] reported 0.004–0.007 mg N₂O kg⁻¹ soil at 2.8 dS m⁻¹, which increased by 18–24% and 34–87% at 15.2 and 30.6 dS m⁻¹, respectively (Table 2), mainly because of denitrification being the main process behind N₂O emissions. Similarly, Maucieri et al. [24] observed higher N₂O emissions from increased irrigation water salinity. Marton et al. [27] also reported higher N₂O emissions from tidal forest soil in southeast Georgia irrigated with high levels of irrigation water salinities. The possible reasons behind the higher N₂O emissions at higher salinity levels were (a) decoupling of either denitrification and nitrification processes [93] and (b) the higher salinity levels have increased the sulfate reduction leading to a higher concentration of H₂S which is popularly known as the inhibitor for N₂O reduction [94,95]. Another possible reason is that high salinity suppresses the activity of N₂O reductase leading to N₂O accumulation due to denitrification under a saline environment [73,96].

Overall, it can be concluded that both soil and irrigation water salinity significantly affect the N₂O emissions from soils. Usually, the higher the levels of salts, the lower the N₂O emissions [26,66]. Although, soil salinity is a limiting factor affecting the nitrification and denitrification processes by changing the microbial growth and activity as well as the physical and chemical properties of soil. Beside this moisture is the key factor influencing nitrification and denitrification in salt-affected soils [65]. Denitrification had a significantly positive relationship with soil moisture and it increased with an increase in soil moisture when the soil water content was less than 27.03% and decreased with an increase in soil moisture when the soil water content was more than 27.03% [66].

There are several mechanisms behind this. Firstly, high salinity stress enhances the ammonification and inhibited the nitrification process resulting in reduced NO₃⁻ and a rise in NH₄⁺ concentration in soils, which restricted the concentration of substrate for the denitrification, the main process responsible for N₂O production [66,97]. Laura [98] reported that high salinity stress completely inhibited the process of nitrification due to a decrease in the nitrifying community in the soils. Further higher ionic strength at high salinity levels can adsorb the exchangeable NH₄⁺ ions [99]. Secondly, the diversity of denitrifying bacteria is reduced at higher salinity stress [100,101], which may lower down microbial-mediated denitrification process limiting the N₂O production. Finally, at higher salinity, the dissimilatory nitrate reduction to ammonium (DNRA) and denitrification processes compete with each other for a common substrate, i.e., NO₃⁻, and thereby the rate of nitrification is limited due to the unavailability of NO₃⁻ [102]. In contrast, several researchers reported higher N₂O emissions with increased salinity stress mainly due to the reduction of N₂O to N₂ conversion and higher N mineralization [24,36]. Jia et al. [26] reported an increased rate of denitrification with the addition of salts up to 1–5 ppt but decreased with 10 ppt salts level. Zhou et al. [103] reported a higher N₂O to N₂ ratio with high salinity treatments. Altogether, it is concluded that the relationship between the N₂O production and salinity levels significantly depends on various processes of production and consumption/reduction of N₂O under salinity stress, which is highly variable with variations in levels of salinity, moisture, soil pH, and concentration of NO₃⁻ and NH₄⁺, etc. in soils.

4.2. Methane Emissions

Methane is produced by methanogenic bacteria/archaea (methanogens) as the end product of organic matter decomposition under anaerobic conditions [104,105]. Under an anaerobic atmosphere, methanogens utilize the methanogenic substrates (methanol, formate, acetate, and CO₂) produced due to organic matter decomposition by a range of heterotrophic organisms (Figure 4) [13]. CH₄ is consumed by methanotrophs as a source of energy and carbon and they can grow both in aerobic and anaerobic environments [10,15]. Several factors such as pH, salt concentration, redox potential, soil organic matter, microbial community, etc. affect the CH₄ emissions from soils [10,106]. Soil pH and salt concentration significantly affect the methane emissions from soils and generally, it is inversely related to the salinity/sodicity. Usually, CH₄ emissions from normal/non-saline soil are higher as compared to saline and sodic soil [28,107]. CH₄ emissions reported from the saline soils of different ecosystems are given in Table 2. Sun et al. [108] reported 71% higher CH₄ emissions from non-saline inland soils than that from coastal soil in a meta-analysis. Datta et al. [109] reported similar results from a field experiment in India. They reported higher CH₄ emissions (279.79–378.20 kg CH₄ ha⁻¹) from non-saline soil as compared to saline soil (123.87–170.46 kg CH₄ ha⁻¹) under similar management practices. The findings of Sun et al. [108] and Datta et al. [109] suggest that higher concentrations of exchangeable cations (Na⁺, K⁺, Ca^2+, and Mg²⁺) were the main reason behind the lower CH₄ emissions from saline soils. High soil salinity suppresses the activity of methanogens resulting in lower CH₄ production and subsequent emissions [110,111,112,113].

Recently, Khatun et al. [28] reported an inverse relationship between salinity stress and CH₄ emissions and CH₄ emissions were 6.6, 6.1, 5.6, and 4.9 g CH₄ pot⁻¹ season⁻¹ with 0, 25, 50, and 75 mM NaCl stress, respectively. In China, Zhang et al. [88] investigated CH₄ emissions from rice soil reduced significantly by about 16 and 39 in 0.27 and 0.41% salinity levels, respectively, as compared to the 0.16% salinity (Table 2). The higher salinity levels inhibited the CH₄ oxidation potential of methanotrophs resulting in higher CH₄ emissions over lower salinity levels [88]. Marton et al. [27] collected tidal forest soils from three different locations and incubated them in the laboratory with 0, 2, and 5% salinity levels of irrigation water and reported that CH₄ emissions decreased from 0.51 to 0.07 mg CH₄ kg⁻¹ h⁻¹ by increasing salinity from 0 to 5% (Table 2).

Pattnaik et al. [92] reported significantly higher (3.78 mg CH₄ kg⁻¹ soil) mean CH₄ emissions from non-saline alluvial soil as compared to acid sulfate soil (0.02 mg CH₄ kg⁻¹ soil). The major reason behind this was the high sulfate content of acid sulfate soil which enhances the population of sulfate-reducing bacteria (SRB), and these SRB compete with the methanogens for common substrates (Figure 4) [110]. CH₄ emissions were significantly lower in the coastal saline soil (0.25 mg CH₄ kg⁻¹ soil) as compared to non-saline alluvial soil (3.78 mg CH₄ kg⁻¹ soil), which was mainly due to the higher salt content of coastal saline soils. Further, the non-saline alluvial soil was incubated with different levels of salinity, and it was reported that inhibition of the CH₄ production was directly proportional to the salinity levels. The average CH₄ emissions were reduced by 55% at 4 dS m⁻¹ salinity and almost inhibited at the 20 dS m⁻¹ salinity. Maucieri et al. [24] and Zhang et al. [88] reported that a higher level of soil salinity reduces the CH₄ uptake by methanotrophs. Zhang et al. [88] studied the CH₄ uptake/emissions from three maize fields (M1, M2, and M3) having different salinity and sodicity levels. The cumulative CH₄ uptake was 0.77 kg CH₄ ha⁻¹ from M1 (pH: −7.34, EC: −0.10), and it was reduced by 16% and 24% in the case of M2 (pH: −7.76, EC: −0.19) and M3 (pH: −8.43, EC: −0.25) soils. Similarly, Maucieri et al. [24] collected soil samples in pots and studied the CH₄ uptake/emissions by irrigating with three types of saline water (0.5/0.9, 5.0, and 10 dS m⁻¹). Total CH₄ uptake/emissions was 0.07, 0.07, and 0.06 mg CH₄ kg⁻¹ soil in 0.9, 5.0, and 10 dS m⁻¹, respectively. Earlier Poffenbarger et al. [89] did the metadata analysis for CH₄ emissions from tidal marshes [fresh (salinity < 0.5 ppt), oligohaline (0.5–5.0 ppt), mesohaline (5–18 ppt), and polyhaline (>18 ppt)] and reported that CH₄ emissions from the fresh, oligohaline, mesohaline, and polyhaline marshes were 419, 1500, 164 and 11.2 kg ha⁻¹ year⁻¹. Generally, CH₄ emissions decreased with increased salinity of tidal marshes, however, there is a need to monitor or estimate the CH₄ emissions from oligohaline marshes.

Figure 4. Schematic diagram showing the process of CH₄ production, mechanism of reduction of CH₄ production with the application of gypsum and phosphogypsum through competition with the sulfate-reducing bacteria, Modified from [114,115].

Figure 4. Schematic diagram showing the process of CH₄ production, mechanism of reduction of CH₄ production with the application of gypsum and phosphogypsum through competition with the sulfate-reducing bacteria, Modified from [114,115].

4.3. Carbon Dioxide Emissions

Both salinity and sodicity induce a significant impact on CO₂ emissions from soils and usually, CO₂ emissions have an inverse relation with salinity [90,111] and a positive relationship with soil temperature. Recently, Yu et al. [116] studied the process of CO₂ emissions under different levels of soil salinity and temperature and observed a positive correlation between CO₂ emissions, soil salinity, and temperature [116]. Soil temperature significantly affects the microbial population prevailing in the saline soil (Figure 5). At higher temperatures gram-positive bacterial and fungal populations dominated in the saline soil and these microbial populations effectively decomposed soil organic carbon pool into CO₂ [117]. She et al. [90] studied the effect of salinity levels on CO₂ emissions and found that under similar salinity levels (0.10–1.0%), the highest CO₂ emissions were reported from sandy clay loam soil (206–231 mg CO₂ kg⁻¹ day⁻¹) followed by sandy loam and lowest from silty clay. Zhang et al. [91] collected soils from four different soils having different salinity and vegetation types, i.e., bare soil (EC: 14.84 mS cm⁻¹), T. chinensis community (EC: 10.46 mS cm⁻¹), S. salsa community (EC: 5.18 mS cm⁻¹), and P. australis community (EC: 2.47 mS cm⁻¹). They incubated these soils with four salinity levels as control, 1 mg g⁻¹, 3 mg g⁻¹, and 5 mg g⁻¹. CO₂ emissions from the bare land were the lowest (51.76 to 88.55 mg kg⁻¹ soil). Whereas CO₂ emissions, from the soils with communities of T. chinensis, S. salsa, and P. australis: were from 231.46 to 282.25, 400.39 to 504.33, and 391.27 to 518.46 mg kg⁻¹ soil, respectively (Table 2). CO₂ emissions from the same soils with different salinity levels were decreased with increased salinity. It was observed that the CO₂ emissions from these soils were positively correlated with available labile soil carbon [91]. Degradation of above and below-ground biomass enhanced the labile carbon which results in higher CO₂ emissions from salt-affected soil with vegetation cover than bare salt-effected soil. Maucieri et al. [24] incubated the Vertisol soil in the laboratory adjusting irrigation water salinity to 0.09, 5.0, and 10.0 dS m⁻¹ using NaCl and studied GHG emissions. They reported that with increasing salinity, CO₂ emissions decreased by 19% (5 dS m⁻¹) and 28% (10 dS m⁻¹) as compared to control (0.09 dS m⁻¹). Reddy and Crohn [29] collected soil samples from Coachella Valley, California having three different levels of EC_e (2.8, 15.2, and 30.6 dS m⁻¹) and measured CO₂ emissions in an incubation experiment and reported 70–371 mg CO₂ kg⁻¹ soil in 2.8 dS m⁻¹, 38–259 mg CO₂ kg⁻¹ soil in 15.2 dS m⁻¹, and 12.47–187 mg CO₂ kg⁻¹ soil in 30.6 dS m⁻¹ salinity level.

5. Impact of Soil Amendments on GHG Emissions from Salt-Affected Soils

Reclamation and sustainable management of salt-affected soils for economic production of crops is a global challenge and expected climate change has further aggravated the task. Several management practices including gypsum, phosphogypsum, organic matter, biochar, vermicompost, etc. were evaluated and promoted for the management of these soils (

Table 3. Different amendments/materials used for the management of salt-affected soils and mitigation of GHG emissions.

S. No.	Amendment Details	References
01	Gypsum/phosphogypsum	[83,118,119]
02	Biochar	[81,114,115]
03	Humic acid	[118]
04	Rice straw compost	[120]
05	Cow manure	[81]
06	Deep tillage	[121]
07	Vermicompost	[121]
08	Azolla application	[122]
09	Cyanobacteria	[123]
10	Jatropha curcas	[124]
11	Sesbania green manure	[125]
12	Nitrification inhibitors (3,4-Dimethylpyrazole phosphate)	[126]

). These amendments are being used to improve salt-affected soils for agricultural performance. Besides, they also have an impact on soil microbial activities and thus may enhance or reduce GHG emissions. Therefore, the impact of various reclamation technologies/materials on GHG emissions needs a global scientific investigation.

5.1. Impact of Gypsum and Phosphogypsum Application on GHG Emissions

Gypsum and phosphogypsum are used for centuries to reclaim alkali soils. Once gypsum dissociates into calcium and sulfur, calcium has the greatest attraction for the soil particle displacing sodium and helps flocculate (aggregate) the soils to improve soil structure. Besides, it might also affect the soil microbes involved in GHG emissions. Several literatures are available on the gypsum application and reclamation of alkali soils. However, very selective studies are available that assessed the effect of these soil corrections on GHG emissions. Some studies [28,83,118] studied the CH₄ emissions in the rice ecosystems and observed its mitigation in reclaimed soils. Denier van der Gon and Neue, [127] reported 55–70% lower CH₄ emissions from gypsum amended rice fields and it is most likely due to the inhibition of the methanogenesis by the sulfate reductase bacteria (SRB) (

Table 4. Impact of different amendments practices on GHG emissions from salt-affected soils.

References	Experiment Type	Treatment’s Detail	Observation (GHG Emissions)	Key Findings and Reasoning
Application of gypsum and phosphogypsum
Khatun et al. [28]	Pot experiment Initial soil pH = 7.8, EC = 5.6 dS m−1, OC = 1.48%	25 nM salinity; 25 nM + phosphogypsum (P); 50 nM; 50 nM + P; 75 nM; 75 nM + P	Biochar amendment to saline soil reduced CH4 emission by 16.4% (25 mM) to 19.6% (at 75 mM)	Phospho-gypsum and biochar mitigate CH4 emissions due to improved soil redox potential (Eh), increased SO4− and decreased soil EC.
Sun et al. [118]	Field experiment growing rice conducted in Jiangsu Province, China	N1 (300 kg N ha−1); N1 + humic acid; N1 + gypsum; N1 + humic acid + gypsum	CH4 emissions increased with Humic acid (6.2%), gypsum (19.4%), decreased with gypsum + humic acid (27.3%). Humic acid and gypsum application increase N2O emission	Humic acid and gypsum application with N300 kg N ha−1 is the better management for coastal saline soils of China to mitigate CH4 emission.
Park et al. [131]	Field experiment with rice.	No by-product gypsum fertilizer (BGF); BGF (2 Mg ha−1); BGF (4 Mg ha−1); BGF (8 Mg ha−1)	CH4 flux decreased with increasing level of BGF, and BGF (8 Mg ha−1) reduced it by 60.6% compared to control.	BGF application could be a better management practice for CH4 mitigation from paddy soils.
Ali et al. [132]	Field experiment with rice in upland soil.	Urea (250 kg ha−1); Urea + Phosphogypsum (90 kg ha−1); Urea + silicate slag (150 kg ha−1)	Silicate slag and phosphogypsum reduced CH4 emission by 18.0–23.5% and 14.7–18.6%, respectively.	Silicate slag and phosphogypsum decreased CH4 due to high free iron oxide and SO42− content which acted as electron acceptors
Denier van der Gon and Neue, [127]	Field experiment with rice.	Urea (165 kg N ha−1); Urea + gypsum (6.60 t ha−1)	The CH4 emissions from gypsum amended plots were reduced by 55–70% compared to non-amended plots.	Inhibition of methanogenesis by sulfate-reducing bacteria caused a reduction in CH4 emission.
Application of organic amendments (biochar, organic matter etc)
Sun et al. [133]	Field experiment with rice	N1 (300 kg N ha−1); N1 + 20 t biochar ha−1; N1 + 40 t biochar ha−1	Biochar amendment increased N2O emissions by 13.7–38.1% and had no significant effects on CH4 emissions	Thus, long-term observations are needed to evaluate the environmental impacts of biochar and N fertilizers
Maucieri et al. [24]	30 days incubation experiment	Control; Biochar	Biochar amendment to saline soil decrease CH4 uptake (8.8%), CO2 (11.9%), and N2O (9.8%) emissions	Biochar amendment to soils mitigates GHG emissions where CO2 and N2O are driven by soil rewetting events.
Datta et al. [109]	Rice experiment in irrigated saline soils of Gadakujang (a fishing hamlet) of coastal Odisha, India	Prilled urea (40 kg N ha−1); Sesbania green manure (5 Mg ha−1) + prilled urea (20 kg N ha−1); Ipomoea lacunose (5 Mg ha−1) + prilled urea (28 kg N ha−1)	Sesbania and Ipomoea lacunose green manure reduced CH4 emission by 23.2 and 29.9%.	Locally available Ipomoea lacunose green manure can use CH4 mitigation and yield enhancement from the coastal saline rice ecosystems
Denier van der Gon and Neue, [127]	Field experiment with rice	GM (S. Rostrata: 20 t ha−1) + urea (30 kg urea ha−1); GM + urea + gypsum (6.60 t ha−1)	Green manure addition enhances CH4 emissions by 10 times than that of urea application alone, further gypsum addition reduced CH4 emission by about 71.1%	Database for CH4 emissions mitigation from rice grown on high-sulfate containing soils
Chen et al. [134]	Field experiment was conducted in saline-sodic soils in the upper Yellow River basin, Northwest China	Organic fertilizer (CK), sheep manure (FYM), lignite bioorganic fertilizer (LBF1) (1.5 t ha−1) LBF2 (3 t ha−1), LBF3 (4.5 t ha−1), and LBF4 (7.5 t ha−1)	LBF treatments decreased CH4 and CO2, and increasing N2O emissions beyond 3 t ha−1 application rate. FYM acted as a CH4 source, and LBF2 and LBF3 treatments acted as CH4 sinks.	The application of lignite bioorganic fertilizer at 3.0–4.5 t ha−1 is appropriate for GHG mitigation in saline-sodic farmlands.
Zheng et al. [135]	Microcosm experiments of 80 days incubation	Interaction of salinity (0 and 1.2% salt) with biochar	5–10 times higher N2O emissions occurred from saline soils than that from non-saline soils. Aged biochar decreased N2O emissions and increased CO2 emissions in saline soils.	Aged biochar could be a better option for mitigation of N2O emissions from saline soils.
Other amendments/interventions
Li et al. [126]	Field experiment with rice crop	Nonsaline (NS) soi; NS soil + DMPP (0.8% w/w of N); low saline (LS) soil; LS soil + DMPP; high saline (HS) soil; HS soil + DMPP	The nitrification inhibitor DMPP (3,4-dimethyl pyrazole phosphate) reduced cumulative N2O emissions by 61% in non-saline soil and by 75% in low saline soil	DMPP offsets low salinity-induced high N2O emissions by inhibiting ammonia oxidation.

). Gypsum application enhance SO₄²⁻ concentration, which gives rise to the competition of SRB with the methanogens for common substrate (H₂, CO₂, acetate) that otherwise be used by methanogens [127,128] as SRB has a high affinity for H₂ and acetate as compared to methanogens [129]. This inhibition of methanogenesis by SRB is incomplete and a considerable amount of CH₄ emissions still occurred [127]. The gypsum and phosphogypsum improve water infiltration through betterment in structure, thereby enhancing soil redox potential (less negative redox potential) and mitigating CH₄ emissions from saline/sodic soils [28]. The rate of gypsum application also plays a significant role in CH₄ emissions. Theint et al. [83] reported that CH₄ emissions from alkali soils significantly increased with the application of a lower dose (0.5 t ha⁻¹) of gypsum as rhizospheric exudates provide a sufficient amount of Organic C for methanogenesis. However, a higher dose (2 t ha⁻¹) reduced the CH₄ emissions by lowering the soil pH and increased sulfate concentration. The CH₄ mitigation is generally higher with the higher dose of gypsum application, and it is because of higher competition between SRB and methanogens at a higher rate. One mole of SO₄²⁻ is required for the reduction of one mole of CH₄ [130]. Overall, it can be concluded that gypsum is the best reclamation agent which can be used as a mitigating agent for CH₄ emissions from paddy cultivation in both sodic and non-sodic soils and mitigation is higher at a higher rate of application. Park et al. [131] reported about 60% CH₄ mitigation from paddy soils with the application of 8 MG ha⁻¹ by-product gypsum fertilizer (BGF) application. Similarly, Ali et al. [132] reported 18–23% CH₄ mitigation from the coastal paddy soils with the application of silicate slag (150 kg ha⁻¹). Both BGF and silicate slag had high free iron oxide and SO₄²⁻ content which acted as electron acceptors. Sun et al. [118] explored the potential of gypsum and humic acid on CH₄ and N₂O emissions from coastal saline soils and recorded mitigation of 19.36% CH₄ emissions and 9.43% N₂O emissions in gypsum amended N fertilized soils (Table 4). The application of humic acid in coastal saline soil served as electron acceptors, which result in higher CH₄ emissions as compared to no application of humic acid in soils. Further, the application of humic acid enhanced the soil redox potential which stimulate the higher N₂O fluxes from the soils [118].

5.2. Organic Amendments and GHG Emissions

Organic manure, green manure, biochar, compost, etc. are commonly used for the management of salt-affected soils and biochar has a great potential for N₂O mitigation [24,136]. The application of fresh organic matter and biochar improves soil’s physical, chemical, and biological properties and can either enhance or mitigate N₂O emissions [29,136]. Application of fresh organic matter (crop residue, manure, FYM) increases cumulative N₂O emissions due to enhanced nitrogen mineralization [137]. However, biochar application in saline soils inhibits the nitrification process through adsorption of substrate, i.e., NH₃/NH₄⁺ and resulting in a lowering of N₂O emissions [24,138]. Biochar at the rate of 1% of total N in saline-alkali soil reduced nirK and nirS gene copies of denitrifiers bacteria and resulted in low N₂O emissions [115]. The age of the biochar is also important in GHG mitigation, aged biochar further decreases N₂O emissions from saline soils. Therefore, aged biochar could be a better option for the mitigation of N₂O emissions from these soils.

Substituting inorganic fertilizer with organic matter in optimum portion can be useful in maintaining SOC in agricultural soil along with CH₄ mitigation [15,139,140]. The application of biochar significantly enhanced the community structure and abundance of methanotrophs which reduces the net CH₄ emissions from biochar-treated soils [136,141]. The application of FYM in rice grown in saline soil significantly enhances the population of methanotrophs [142]. Similarly, the use of FYM along with pyrite in alkaline paddy soil enhances the methanotroph population and CH₄ oxidation thereby reducing seasonal CH₄ emissions [142]. Wang et al. [143] reported three to six times higher methanotroph population and lower CH₄ emissions with the use of biochar along with steel slag. Nguyen et al. [81] observed that cow manure addition to salt-affected soil enhanced CH₄ emissions by 801%, however, the addition of biochar to cow manure amended soil reduced CH₄ emissions from 28 to 680%. The application of cow manure alone enhanced the population of methanogens leading to significantly higher CH₄ emissions. While application of biochar along with cow manure enhanced the methanotrophs population and thereby improved the net balance of methanogens (CH₄ production) and methanotrophs (CH₄ consumption) resulting in lowering the CH₄ fluxes from biochar + cow manure amended soils [10,81].

Sesbania aculeate and Ipomoea lacunose green manure reduced CH₄ emissions by 23.15 and 29.89%, respectively, as compared to urea application during the wet season (Table 4). However, this green manure enhanced CH₄ emissions by 382.68, and 300.57%, respectively, during the dry season [109] because the higher temperature in the dry season accelerates the process of fresh green manure. Maucieri et al. [24] reported 10% lower cumulative CO₂ emissions and 12% lower N₂O emissions from biochar amended soil as compared to without biochar amended soil. Supparattanapan et al. [144] conducted a field experiment in a saline patch (14.2 dS m⁻¹) and outside saline patch (4.7 dS m⁻¹) of a single field and reported that the addition of rice straw and cow manure enhanced the CH₄ emissions from both saline and outside saline patch over control. However, increased CH₄ emissions were higher from the outside saline patch (153–161%) as compared to the saline patch (33–19.5%). Overall, it can be concluded that biochar can be used as the best organic amendment for mitigation of N₂O and CH₄ emissions from both normal soils as well as salt-affected soils.

5.3. Other Interventions for GHG Mitigation from Saline-Sodic Soils

Several other materials were also tested for the GHG mitigation potential from the salt-affected Soils. Li et al. [126] investigated the role of 3,4-Dimethylpyrazole phosphate (DMPP) a new nitrification inhibitor in reducing N₂O emissions from saline soil and reported that the application of DMPP in non-saline, low saline, and high saline soil significantly reduced N₂O emissions by 61.19, 74.94, and 48.82%, respectively, over non DMPP treatment (Table 4). DMPP application reduced the NO₂-N accumulation and suppressed nitrifier denitrification processes and causes a lower N₂O emissions [126]. Sun et al. [108] reported that the application of an acid chemical (trade name—Hekang) at the rate of 22.5 kg ha⁻¹ along with urea application at the rate of 300 kg N ha⁻¹ did not reduce N₂O emissions from the saline soil but reduced the yield scaled emissions.

6. Future Research Directions

Based on the literature reviewed following thrust areas are identified which required future research attention for the low carbon/GHG emissions and sustainable crop production from the salt-affected lands/soils.

• The Impacts of excess salts and high pH on GHG emissions from salt-affected soils are well documented. However, impacts of the individual ion toxicity on microbial population, enzymatic activities, and GHG production processes required further investigation.
• Mostly, studies are conducted in the pot and laboratory under controlled conditions. However, in real field conditions the emissions may be affected by several other parameters, therefore, how salinity and sodicity in actual field conditions affect the soil GHG emissions needs further investigation.
• How the other parameters such as soil carbon and nitrogen level, soil moisture, redox potential, precipitation, temperature, cyclones, etc. affect the seasonal variation of GHG emission from salt-affected soils before and after reclamation needs systematic investigation.
• Systematic investigations are needed to understand and quantify the effect of different amendments and reclamation technologies such as gypsum, phosphogypsum, organic manure, green manure, biochar, etc. on GHG emissions from these soils to develop the low carbon emission reclamation technologies for the management of salt-affected soils.

7. Conclusions

Salinity and sodicity not only affect the soil’s physicochemical properties but also significantly affects the CH₄, N₂O, and CO₂ emissions from the soil. The production of GHG in the soil is mainly governed by the microbial-mediated processes in which several organisms are involved. Salinity and sodicity have detrimental impacts on the microbial population of nitrifying, denitrifying, methanogens, methanotrophs, etc., and enzymatic activities involved in GHG production and consumption. The magnitude of its impact depends on the level of salinity and sodicity. Microbial population and soil enzyme activities are generally decreased with increasing soil salinity and sodicity which restrict the C and N mineralization and thereby GHG emissions. Generally, CH₄ production and emissions from soils decrease with increasing soil salinity which is mainly due to the inhibition of methanogens activity. Similarly, N₂O emissions also decreased with increasing salinity due to strong inhibition of both steps of nitrification. However, N₂O emission enhanced at lower levels of salinity as compared to non-saline soils. Reclamation of salt-affected soils using various amendments normalizes the soil pH and reduces soil salinity which is favorable for the microbial population and can enhance GHG emissions from soils. However, reclamation of salt-affected soils using gypsum and phosphogypsum reduces the CH₄ emissions from soils mainly through the competition due to sulfate-reducing bacteria for the common substrates. The rate of gypsum application has a greater impact on CH₄ mitigation from salt-affected soils. Similarly, biochar amendments to soil reduce both CH₄ and N₂O emissions and mitigation is higher with aged biochar. The application of fresh organic matter and FYM may enhance GHG emissions. Although, the amendments and reclamation technologies are used to make crop cultivation possible from these soils. However, systematic investigations are needed for studying the impacts of various reclamation technologies on GHG emissions so that low carbon emissions reclamation technologies can be promoted in the policies for the reclamation of salt-affected soils, and the carbon sequestration potential of these soils can be explored.