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

: 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 (N 2 O) and methane (CH 4 ) emissions are of greater concern due to their 298 and 28 times higher global warming potential, respectively, compared to carbon dioxide (CO 2 ), on a 100-year time scale. Therefore, CO 2 emissions are given negligible/smaller signiﬁcance compared to the other two. Generally, nitrous oxide (N 2 O) 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 (CH 4 ) emissions from normal soils are higher than those of salt-affected soils. High salinity suppresses the activity of both methanogens (CH 4 production) and methanotrophs (CH 4 consumption). However, it imposes more inhibitory effects on methanogens than methanotrophs, resulting in lower CH 4 production and subsequent emissions from these soils. Therefore, reclamation of these soils may enhance N 2 O and CH 4 emissions. However, gypsum is the best reclamation agent, which signiﬁcantly mitigates CH 4 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 CH 4 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 CH 4 and N 2 O 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.


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 (CH 4 ) from enteric fermentation and rice cultivation, nitrous oxide (N 2 O) from the application of synthetic fertilizers, and carbon dioxide (CO 2 ) from tillage operations [10,11]. The emissions of N 2 O and CH 4 are of greater concern due to 298 and 28 times higher global warming potential (GWP) than that of CO 2 [10] on a 100-year time scale, respectively [5]. Global GHG emissions from agricultural activities were 9.3 Giga tonnes (Gt) of CO 2 equivalents (CO 2 eq) in 2018 [2]. The contribution of CH 4 and N 2 O emissions from crops and livestock was 5.3 Gt CO 2 eq with agricultural soils and enteric fermentation being major sources contributing 39.5 and 39.2%, respectively [2]. The emissions of CO 2 , CH 4 , and N 2 O occur from the agricultural soils through microbial-mediated processes/pathways. CO 2 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, CH 4 is produced by methanogens and consumed by methanotrophs under oxic and anoxic conditions [10,[13][14][15]. N 2 O 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 CH 4 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 N 2 O [25,26], CH 4 [27,28], and CO 2 [29,30] emissions with increased salinity and sodicity has been reported in several studies. However, reports on increased N 2 O [30], CH 4 [24,31], and CO 2 [31] emissions are also available. To our best knowledge, the review article concerning the N 2 O, CH 4 , and CO 2 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 Sustainability 2022, 14,11876 3 of 25 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.

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 (CO 3 2− ), bicarbonates (HCO 3 − ), chlorides (Cl − ), and sulfates (SO 4 2− ) of sodium (Na + ), calcium (Ca 2+ ), and magnesium (Mg 2+ ) [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. 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].
Sustainability 2022, 14,11876 3 of 25 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.

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 (CO3 2− ), bicarbonates (HCO3 − ), chlorides (Cl − ), and sulfates (SO4 2− ) of sodium (Na + ), calcium (Ca 2+ ), and magnesium (Mg 2+ ) [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 (ECe), pH of saturated paste (pHs), exchangeable sodium percentage (ESP), and sodium adsorption ratio in saturated paste extract (SARe) [33]. Saline soils contain high concentration of neutral salts mainly chlorides and sulfate of sodium, calcium and magnesium, and have ECe > 4 dS m −1 , pHs < 8.5, ESP < 15, and SARe < 13 [34]. Alkali/sodic soils possess excess contents of carbonates, bicarbonate and silicate salts of sodium, and characterized by ECe < 4 dS m −1 , pHs > 8.5, ESP > 15, and SARe > 13. While saline-alkali soils have high levels of both soluble salts and ESP, and these show ECe > 4 dS m −1 , SARe > 13, ESP > 15, and variable pHs due to collective effects of both salinity and sodicity [35]. Detailed characteristics of salt-affected soils are given in Table 1. 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) saltaffected 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].

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 O 2 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 N 2 O and CH 4 emissions [49].
High salinity levels inhibit the nitrification rate and decrease the availability of nitrate (NO 3 − ) which further limit the denitrification process and thereby reduces the N 2 O 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 (H 2 , formate, acetate, alcohol, and other compounds) which are used by methanogens for CH 4 production and thereby reduces CH 4 emissions [53]. The decrease in microbial enzymes and gene activities involved in organic matter decomposition, nitrification, denitrification, methanogenesis, and CH 4 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].
, 11876 5 of 25 may increase HS − ions. These HS − ions have a more inhibitory effect on denitrifying bacteria than DNRA [57].

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 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.

Nitrification
Nitrification is a two-step process involving (a) ammonia oxidation regulated by ammonia-oxidizing archaea (AOA) and (b) nitrite oxidation process regulated by ammoniaoxidizing 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].
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 N 2 O 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. N 2 O reductase (nosZ) is more sensitive to soil salinity and depressed significantly under salinity conditions hence N 2 O is not reduced into N 2 resulting in more N 2 O accumulation and effluxes from the denitrification [73]. Decoupling of nitrification or denitrification processes under saline conditions might be a possible mechanism for increasing N 2 O effluxes under salinity [71]. However, some other investigations also concluded that N 2 O emissions are negatively associated with soil salinity [74].

Effect of Salinity/Sodicity on Methanogenesis/Methanotrophs
Methanogenesis is the microbial-mediated process of CH 4 production from the complex organic material by bacteria and archaea. Bacteria hydrolyze the complex organic material into simpler substrates (H 2 , formate, certain alcohols, acetate, etc.) which are further consumed by methanogens as food and produced CH 4 [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 CH 4 from H 2 + CO 2 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 predomi-Sustainability 2022, 14, 11876 7 of 25 nance 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 CH 4 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.

Nitrous Oxide Emissions
N 2 O is produced through nitrification (conversion of NH 4 + to NO 2 − , and NO 2 − to NO 3 − ) and denitrification (conversion NO 3 − to N 2 ) pathways ( Figure 3a) [67]. The N 2 O efflux is influenced by soil pH, salt concentration, temperature, redox potential, O 2 concentration, etc. Studies conducted on N 2 O efflux under salt-affected conditions are summarized in Table 2. N 2 O 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 N 2 O emissions and decrease thereafter [85]. The lower salinity level inhibits both the steps of nitrification (conversion of NH 4 + to NO 2 − and NO 2 − to NO 3 − ), however, the inhibition of NO 2 − to NO 3 − conversion is stronger than that of NH 4 + to NO 2 − causing higher NO 2 − accumulation and enhanced N 2 O emissions (Figure 3b) [85]. Li et al. [25] reported 110% higher N 2 O 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 N 2 O emissions as compared to non-saline soil because that low salinity reduces the activity of N 2 O reductase enzymes, therefore restricting the conversion of N 2 O to N 2 . 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 N 2 O emissions from saline soil. For instance, 4.7-37.6 and 5.0-15.3 times higher N 2 O 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 N 2 O 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 N 2 O emissions than freshwater irrigation in calcareous soil. Wei et al. [87] reported that N 2 O 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 N 2 O production and consumption differed significantly with the degree of irrigation water salinities [87]. Nitrification is highly sensitive to salinity and lower N 2 O emissions are expected with higher salinity levels. However, few researchers also reported the reverse trends [24,27,29].  Oligohaline marshes had the highest and most variable CH 4 emissions (150 ± 221 gm −2 yr −1 ). Negligible CH 4 emissions in polyhaline, and no significant difference between fresh and mesohaline marshes.
Need to estimate or monitor CH 4 emissions in lower-salinity marshes.

Laboratory experiments
She et al. [90] Laboratory experiment with different textured 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.  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 ) CO 2 emission ranged from 88 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 CH 4 emission inhibited by 77% in the 2% and 89% in the 5% saline water treatment whereas CO 2 generally increased with salinity, though exhibited a variable response between the three rivers.
Short-term salinity exposure enhanced anaerobic C mineralization, a decline in CH 4 production, and a varied response in N 2 O production 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 N 2 O emission and S2 enhanced emission by 58.3% the effect degree of salinity on consumption and production of N 2 O might vary among irrigation salinity ranges  Reddy and Crohn [29] reported 0.004-0.007 mg N 2 O 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 N 2 O emissions. Similarly, Maucieri et al. [24] observed higher N 2 O emissions from increased irrigation water salinity. Marton et al. [27] also reported higher N 2 O emissions from tidal forest soil in southeast Georgia irrigated with high levels of irrigation water salinities. The possible reasons behind the higher N 2 O 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 H 2 S which is popularly known as the inhibitor for N 2 O reduction [94,95]. Another possible reason is that high salinity suppresses the activity of N 2 O reductase leading to N 2 O 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 N 2 O emissions from soils. Usually, the higher the levels of salts, the lower the N 2 O 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 NO 3 − and a rise in NH 4 + concentration in soils, which restricted the concentration of substrate for the denitrification, the main process responsible for N 2 O 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 NH 4 + 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 N 2 O 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., NO 3 − , and thereby the rate of nitrification is limited due to the unavailability of NO 3 − [102]. In contrast, several researchers reported higher N 2 O emissions with increased salinity stress mainly due to the reduction of N 2 O to N 2 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 N 2 O to N 2 ratio with high salinity treatments. Altogether, it is concluded that the relationship between the N 2 O production and salinity levels significantly depends on various processes of production and consumption/reduction of N 2 O under salinity stress, which is highly variable with variations in levels of salinity, moisture, soil pH, and concentration of NO 3 − and NH 4 + , etc. in soils.

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 CO 2 ) produced due to organic matter decomposition by a range of heterotrophic organisms (Figure 4) [13]. CH 4 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 CH 4 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, CH 4 emissions from normal/non-saline soil are higher as compared to saline and sodic soil [28,107]. CH 4 emissions reported from the saline soils of different ecosystems are given in Table 2. Sun et al. [108] reported 71% higher CH 4 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 CH 4 emissions (279.79-378.20 kg CH 4 ha −1 ) from non-saline soil as compared to saline soil (123.87-170.46 kg CH 4 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 Mg 2+ ) were the main reason behind the lower CH 4 emissions from saline soils. High soil salinity suppresses the activity of methanogens resulting in lower CH 4 production and subsequent emissions [110][111][112][113].
Recently, Khatun et al. [28] reported an inverse relationship between salinity stress and CH 4 emissions and CH 4 emissions were 6.6, 6.1, 5.6, and 4.9 g CH 4 pot −1 season −1 with 0, 25, 50, and 75 mM NaCl stress, respectively. In China, Zhang et al. [88] investigated CH 4 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 CH 4 oxidation potential of methanotrophs resulting in higher CH 4 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 CH 4 emissions decreased from 0.51 to 0.07 mg CH 4 kg −1 h −1 by increasing salinity from 0 to 5% (Table 2).
Pattnaik et al. [92] reported significantly higher (3.78 mg CH 4 kg −1 soil) mean CH 4 emissions from non-saline alluvial soil as compared to acid sulfate soil (0.02 mg CH 4 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]. CH 4 emissions were significantly lower in the coastal saline soil (0.25 mg CH 4 kg −1 soil) as compared to non-saline alluvial soil (3.78 mg CH 4 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 CH 4 production was directly proportional to the salinity levels. The average CH 4 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 CH 4 uptake by methanotrophs. Zhang et al. [88] studied the CH 4 uptake/emissions from three maize fields (M1, M2, and M3) having different salinity and sodicity levels. The cumulative CH 4 uptake was 0.77 kg CH 4 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 CH 4 uptake/emissions by irrigating with three types of saline water (0.5/0.9, 5.0, and 10 dS m −1 ). Total CH 4 uptake/emissions was 0.07, 0.07, and 0.06 mg CH 4 kg −1 soil in 0.9, 5.0, and 10 dS m −1 , respectively. Earlier Poffenbarger et al. [89] did the metadata analysis for CH 4 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 CH 4 emissions from the fresh, oligohaline, mesohaline, and polyhaline marshes were 419, 1500, 164 and 11.2 kg ha −1 year −1 . Generally, CH 4 emissions decreased with increased salinity of tidal marshes, however, there is a need to monitor or estimate the CH 4 emissions from oligohaline marshes. 1876 1 Figure 4. Schematic diagram showing the process of CH4 production, mechanism of reduc CH4 production with the application of gypsum and phosphogypsum through competition w sulfate-reducing bacteria, Modified from [114,115].

Carbon Dioxide Emissions
Both salinity and sodicity induce a significant impact on CO2 emissions from and usually, CO2 emissions have an inverse relation with salinity [90,111] and a p relationship with soil temperature. Recently, Yu et al. [116] studied the process o emissions under different levels of soil salinity and temperature and observed a p correlation between CO2 emissions, soil salinity, and temperature [116]. Soil tempe significantly affects the microbial population prevailing in the saline soil (Figure higher temperatures gram-positive bacterial and fungal populations dominated saline soil and these microbial populations effectively decomposed soil organic c pool into CO2 [117]. She et al. [90] studied the effect of salinity levels on CO2 emissio

Carbon Dioxide Emissions
Both salinity and sodicity induce a significant impact on CO 2 emissions from soils and usually, CO 2 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 CO 2 emissions under different levels of soil salinity and temperature and observed a positive correlation between CO 2 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 CO 2 [117]. She et al. [90] studied the effect of salinity levels on CO 2 emissions and found that under similar salinity levels (0.10-1.0%), the highest CO 2 emissions were reported from sandy clay loam soil (206-231 mg CO 2 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 . CO 2 emissions from the bare land were the lowest (51.76 to 88.55 mg kg −1 soil). Whereas CO 2 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). CO 2 emissions from the same soils with different salinity levels were decreased with increased salinity. It was observed that the CO 2 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 CO 2 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, CO 2 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]

Impact of Soil Amendments on GHG Emissions from Salt-A
Reclamation and sustainable management of salt-affec production of crops is a global challenge and expected clim

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

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 CH 4 emissions in the rice ecosystems and observed its mitigation in reclaimed soils. Denier van der Gon and Neue, [127] reported 55-70% lower CH 4 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). Gypsum application enhance SO 4 2− concentration, which gives rise to the competition of SRB with the methanogens for common substrate (H 2 , CO 2 , acetate) that otherwise be used by methanogens [127,128] as SRB has a high affinity for H 2 and acetate as compared to methanogens [129]. This inhibition of methanogenesis by SRB is incomplete and a considerable amount of CH 4 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 CH 4 emissions from saline/sodic soils [28]. The rate of gypsum application also plays a significant role in CH 4 emissions. Theint et al. [83] reported that CH 4 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 CH 4 emissions by lowering the soil pH and increased sulfate concentration. The CH 4 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 SO 4 2− is required for the reduction of one mole of CH 4 [130]. Overall, it can be concluded that gypsum is the best reclamation agent which can be used as a mitigating agent for CH 4 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% CH 4 mitigation from paddy soils with the application of 8 MG ha −1 byproduct gypsum fertilizer (BGF) application. Similarly, Ali et al. [132] reported 18-23% CH 4 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 SO 4 2− content which acted as electron acceptors. Sun et al. [118] explored the potential of gypsum and humic acid on CH 4 and N 2 O emissions from coastal saline soils and recorded mitigation of 19.36% CH 4 emissions and 9.43% N 2 O 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 CH 4 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 N 2 O fluxes from the soils [118].

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 N 2 O 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 N 2 O emissions [29,136]. Application of fresh organic matter (crop residue, manure, FYM) increases cumulative N 2 O emissions due to enhanced nitrogen mineralization [137]. However, biochar application in saline soils inhibits the nitrification process through adsorption of substrate, i.e., NH 3 /NH 4 + and resulting in a lowering of N 2 O 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 N 2 O emissions [115]. The age of the biochar is also important in GHG mitigation, aged biochar further decreases N 2 O emissions from saline soils. Therefore, aged biochar could be a better option for the mitigation of N 2 O emissions from these soils.
Substituting inorganic fertilizer with organic matter in optimum portion can be useful in maintaining SOC in agricultural soil along with CH 4 mitigation [15,139,140]. The application of biochar significantly enhanced the community structure and abundance of methanotrophs which reduces the net CH 4 emissions from biochar-treated soils [136,141]. The application of FYM in rice grown in saline soil significantly enhances the popula-tion of methanotrophs [142]. Similarly, the use of FYM along with pyrite in alkaline paddy soil enhances the methanotroph population and CH 4 oxidation thereby reducing seasonal CH 4 emissions [142]. Wang et al. [143] reported three to six times higher methanotroph population and lower CH 4 emissions with the use of biochar along with steel slag. Nguyen et al. [81] observed that cow manure addition to salt-affected soil enhanced CH 4 emissions by 801%, however, the addition of biochar to cow manure amended soil reduced CH 4 emissions from 28 to 680%. The application of cow manure alone enhanced the population of methanogens leading to significantly higher CH 4 emissions. While application of biochar along with cow manure enhanced the methanotrophs population and thereby improved the net balance of methanogens (CH 4 production) and methanotrophs (CH 4 consumption) resulting in lowering the CH 4 fluxes from biochar + cow manure amended soils [10,81].
Sesbania aculeate and Ipomoea lacunose green manure reduced CH 4 emissions by 23.15 and 29.89%, respectively, as compared to urea application during the wet season (Table 4). However, this green manure enhanced CH 4 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 CO 2 emissions and 12% lower N 2 O 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 CH 4 emissions from both saline and outside saline patch over control. However, increased CH 4 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 N 2 O and CH 4 emissions from both normal soils as well as salt-affected soils.

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 N 2 O emissions from saline soil and reported that the application of DMPP in non-saline, low saline, and high saline soil significantly reduced N 2 O emissions by 61.19, 74.94, and 48.82%, respectively, over non DMPP treatment (Table 4). DMPP application reduced the NO 2 -N accumulation and suppressed nitrifier denitrification processes and causes a lower N 2 O 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 N 2 O emissions from the saline soil but reduced the yield scaled emissions.

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.

1.
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.

2.
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.

3.
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.

4.
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.

Conclusions
Salinity and sodicity not only affect the soil's physicochemical properties but also significantly affects the CH 4 , N 2 O, and CO 2 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, CH 4 production and emissions from soils decrease with increasing soil salinity which is mainly due to the inhibition of methanogens activity. Similarly, N 2 O emissions also decreased with increasing salinity due to strong inhibition of both steps of nitrification. However, N 2 O emission enhanced at lower levels of salinity as compared to non-saline soils. Reclamation of saltaffected 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 CH 4 emissions from soils mainly through the competition due to sulfatereducing bacteria for the common substrates. The rate of gypsum application has a greater impact on CH 4 mitigation from salt-affected soils. Similarly, biochar amendments to soil reduce both CH 4 and N 2 O 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.