Around 50% of the land area is used for agricultural purposes and is characterized by tropical, dry, and temperate climates along with diverse ecosystems, land uses, and management practices. The region is densely populated, and per capita land availability in some countries is less than 0.1 ha and is continuously decreasing. The possibility of increasing crop area is limited. The region is undergoing rapid industrialization contributing to greater emission of GHGs. In addition, there is rapid degradation of soil quality with low SOM content due to fertility-mining practices . Lal reported a C sequestration potential of 7–10 Tg C yr1 and 18–35 Tg C yr1 from restoration of degraded land in India and South Asia, respectively. With the adoption of recommended management practices on the cropland of South Asia, SOC potential was estimated to be 11–22 Tg C yr1 . The underlying assumptions included were the implementation of appropriate policies to promote recommended management practices such as conservation agriculture , mulch farming, cover crops, integrated nutrient management with manuring and biological nitrogen fixation, weed dryers and water conservation and harvesting. Lal also reported a soil inorganic C sequestration potential of 19–27 Tg C yr 1 of secondary carbonates and 26–38 Tg C yr 1 of leaching of carbonates in the arid regions of South Asia. Using International Soil Reference and Information Centre Soil Grids 250 m and FAO GLC Share Land Cover database, Zomer et al. reported a C sequestration potential of 0.11–0.23 Pg C hr 1 in South Asia. Assuming that C sequestration continues for 20 years, the current soil C stock of 7.68 Pg is likely to increase to 9.87 or 12.18 Pg for medium and high sequestration scenarios, respectively.

Grace et al. , using IPCC methodology together with local data, calculated a sequestration potential of 44.1 Mt C over 20 years from the implementation of zero tillage practices in rice-wheat systems of India.SOC sequestration is a dynamic process, and the amount and duration of C storage depends on the pools and their cycling , the form of stabilization , and the physical location of the C in the soil . Rates of turnover of organic matter depend on soil properties such as clay content and nutrient status. Clay is one of the key carbon-capture materials and tends to bind organic matter in soil and helps to protect it from microbial breakdown . Yang et al. also showed that the quasi-irreversible sorption of high molecular-weight sugars within clay aggregates, inaccessible by the microbes is responsible for clay-C protection. In addition, temperature plays a crucial role, which is complex because of variation in the temperature sensitivities of different SOM fractions . The impact of temperature becomes more crucial with a rise in ambient temperature due to climate forcing, resulting in microbially-driven increases in decomposition. Therefore, there are limits to C sequestration which are not only biophysical but also include technical and economic barriers.Over time, SOC reaches a steady-state equilibrium, balancing C gains and losses. Since organic inputs vary in quality, quantity, and subsequent interactions with soil constituents and environment, the ability of a soil to retain C is not unlimited. Carbon saturation is often used to describe the maximum capacity of a soil to retain C as a stabilized fraction based on soil properties . Sanderman et al. opined that while the term ‘soil C saturation’ is conceptually and theoretically appealing, the results from some of the long-term experiments may not support it. For example, Blair et al. found that total C stocks increased linearly with input levels of up to 200 Mg dry weight ha 1 for 15 years, without showing any signs of saturating behavior. However, Stewart et al. found some evidence of saturation.

Likewise, Johnston et al. reported that the annual addition of farmyard manure in the Broadbalk long-term experiment at Rothamsted increased C over the 160-year period, but the higher increase in early years was followed by a slower increase in later years, arriving at a new equilibrium. The time taken for soil to reach a new equilibrium tends to vary not only between soils within a temperate or tropical environment but also between the environments. It has been suggested that SOC saturation depends on clay and silt content and that there is a critical C concentration below which a soil’s function is reduced . Nevertheless, most current SOC models assume first-order kinetics for the decomposition of various conceptual pools of organic matter , which means that equilibrium C stocks are linearly proportional to C inputs .Carbon stored in soils is non-permanent. With changes in land use and land management, soil loses C, which can only be maintained or increased with the continuous addition of C input. By changing agricultural management or land use, soil C is lost more rapidly than it accumulates . Soil clay plays an important role in retaining C. Agricultural soil with a 50% clay content requires >2.2 Mg C ha 1 annually to maintain a given C level, while agricultural soil containing a 30% clay content requires more than 6.5 Mg C ha 1 annually. In addition, the rate of C input must be higher at existing soil C levels to maintain a level stock of C in the soil . Microbial decomposition and mineralization to CO2 is the major outcome of organic C. Approximately 1–2% of crop residues are stabilized as humified SOM for a period that are composed of large complex macromolecules, carbohydrates, proteinaceous materials, and lipids. This could be 60–85% of the total SOM . However, this notion, which was based on chemical analysis of the extracted materials has been challenged, and recent understanding suggests that humic substances are marginally important . Based on direct high-resolution in situ observations with non-destructive techniques, it has been established that humic substances are rather simple, smaller biomolecules . Although Hayes and Swift however, strongly disagreed with these views. They presented a detailed account of decomposition processes leading to the formation of a range of products including soil humic substances with a degree of resistance to microbial degradation. The new thinking in SOM research suggests that the molecular structure of plant inputs and organic matter has a secondary role in determining C residence times over decades to millennia, and that C stability depends mainly on the biotic and abiotic environment . The biotic and abiotic factors along with dynamics of labile C pools are required to evaluate management, land use, and climate change effects on SOC changes and soil functionalities .

New findings suggest that microbial decomposition actually facilitates long-term C sequestration by maintaining C flow through the soil profile , and that infrequent tillage may not cause sufficient disruption of soil aggregates leading to C loss . Schmidt et al. proposed that a new generation of experiments and soil C models will be needed to make advances in our understanding of SOM and our responses to global warming.While many land and crop management practices are known to enhance SOC sequestration, benefit accrual is constrained by the existence of numerous adverse forces on the ground. Table 5 provides key adoption constraints to an effective SOC sequestration strategy, the existing practice, and their implications. There are major barriers for farmers to adopt SOC sequestration practices because of the trade-offs involved. For example, the removal of crop residues from the field for other uses such as fodder, fuel, and fencing are traditional practices for managing residues. Not only is this an economical option for farmers, but there is also a lack of knowledge and capacity which discourages the adoption of practices promoting SOC sequestration. Likewise, shifting to zero- or reduced-tillage requires altering farm implements/equipment and the substitution of conventional crop and weed-control methods. The adoption of practices to enhance SOC also involves additional costs and the risk of getting lower yields in the short term. Much remains unknown about SOC storage, so it is difficult to estimate total benefits and to know which soil management practices offer the most potential for a given soil type, climate, and crop.Not only does SOC sequestration involve economic and biological costs but there can also be environmental cost. When mismanaged, some management practices that are known to result in C sequestration and GHG mitigation risk losing SOC and/or enhancing GHG emissions. Notably, N fertilization, either from organic or inorganic source, drying cannabis has negative consequences when applied sub-optimally–used either insufficiently or excessively. On one hand, when applied in inadequate amounts over time, for example in Africa, then there is no or negligible soil C build up . On the other hand, when applied in excess, for example in China and India, then soil C decreases from enhanced decomposition, which increases N2O emission, NH3 volatilization, and/or NO3 leaching. No-till compared to conventional tillage is another example of a practice that is reported to result in higher N2O emissions . No-till adoption may also increase the use of herbicides and pesticides, potentially affecting the environment . Sub-optimal or excess organic amendment to soil can also have an adverse effect on grain yield from nutrient immobilization. A growing interest in bio-fuel, resulting in a competition for fixed C, could also be a threat to SOC sequestration , as the use of bio-fuel involves burning of C which originated recently from photosynthetic activity.Lal et al. proposed six soil C management strategies to increase SOC: minimum disturbance of soil, maintenance of permanent ground cover, intensification of nutrient recycling mechanisms, creation of a positive nutrient balance, enhancement of biodiversity, and reduction in losses of water and nutrients. These strategies are generally applicable in South Asia and could be achieved notably through conversion of degraded land to perennial vegetation, increasing the NPP of agricultural ecosystems, and converting conventional tillage to no-till farming opined that a C-management strategy should not only be able to increase SOC content, but also should have some potential for reducing GHG emissions. Carbon management practices are aimed at increasing the ecosystem C balance by adding more C into the soil , increasing below- and above-ground biomass , sequestering SOC , and also reducing C losses from the soil . In the eastern Indo-Gangetic Plains of India, CA management practices like zero tillage with partial residue retention in rice-wheat systems could increase SOC content by 4.7 Mg C ha 1 after seven years of practice . Avoidance of adverse land use, management strategies, and restoration of degraded land can help in maintaining SOC stocks in soil . Table 6 provides details of various management options for increasing soil SOC stocks and reducing GHG emissions.Proper land leveling is known to enhance input use efficiency, crop growth, and yield . In South Asia, the majority of agricultural lands are poorly leveled by traditional land-leveling practices . Precision land leveling is laser-assisted, and very fine leveling of land is achieved with the desired grade within 2 cm of its average micro elevation . PLL is known to lower GHG emission by improving water and N use efficiency . Under Indian conditions, PLL could reduce almost0.15 Mg of CO2-e ha 1 year 1 of GHG emissions due to less time spent for pumping irrigation water and decreased cultivation time . PLL is critical for efficient water use and for increasing water productivity, and improves crop productivity through better crop establishment practices . There has been 6–11% and 10–25% increases in wheat yields in Punjab, India due to PLL. The associated increase in NPP in terms of crop residues and below ground biomass can be a source of soil C if further managed properly.Loss of SOC is often attributed to the practice of tilling the soil. Adoption of zero or reduced tillage will enable SOC sequestration, and is believed to be one of the key global mitigation strategies of climate change. Zero tillage has been widely reported as a viable option in increasing the C storage in soils , although few have reported no change . Most of the cases where zero tillage showed SOC increase, were mostly sampled to a depth of 30 cm or less, thereby not-revealing changes down the profile. In limited studies, where soil sampling was beyond 30 cm, no apparent difference in SOC between conservation and conventional tillage was recorded . Soil aggregates are stabilized under reduced and zero tillage practice, which physically protect C from mineralization , however, the effect is realized over the long run .