It presents a detailed account of management practices to enhance soil C storage and GHG mitigation, and a meta-analysis of published appraisals in the cropping systems of South Asia.Agriculture in South Asia is predominantly cereal-based, i.e. the cultivation of about 40 million hectares with multiple cereal crops or a single cereal crop, followed by a non-cereal crop such as legumes, vegetables, or potatoes, in an annual rotation . Rapid population growth and climate unpredictability in South Asia will increase the demand for food by at least 40% by 2050 . Meeting this projected need is doubly challenging, considering that 94% of the land suitable for farming is already under production and that 58% of agricultural areas face multiple hazards such as water shortage and extreme heat stress . It is anticipated that the current situation will worsen with climate change, which includes rising temperatures . The region is undergoing rapid economic growth, resulting in an increase in the emission of GHGs into the atmosphere. As of 2017, South Asia accounted for 7.5% of the world’s total CO2 emission from burning fossil fuels, of which India’s share was 6.6% and the remaining less than 1% was shared by seven other countries in the region . A large proportion of the total GHG emission from agriculture in South Asia comes from CH4 and N2O, cannabis drying racks representing 17% of the world’s total in 2017 with 179% increase since 1990 . India accounted for 11.8% and the other seven countries for the remaining 5.2% of total global CH4 and N2O emissions. Among the major sources of GHG emissions, rice cultivation is responsible for both CH4 and N2O emissions .
In South Asia, on a CO2-equivalent basis, rice cultivation and N fertilization are responsible for the largest emissions. Other sources of CH4 emissions include crop residue burning , and other sources of N2O emissions include the application of manure and crop residues to soils. Meeting the increased demand for food during the Green Revolution was associated with intensive cropping, soil management, and the use of agrochemicals, hence, resulted in the gradual loss of SOM . Although crop productivity has doubled or tripled during the last decades, negative impacts on the environment, biodiversity, soil, and air quality are common consequences . Conventional cultivation practices with exhaustive tillage and removal of crop residues by burning or for other uses in South Asia have not only resulted in nutrient and C losses but have also created a severe air pollution problem . About 2 million farmers in northwest India burn an estimated 23 million tons of rice residues every year . In some of the cities of northwest India, particulate air pollution in 2017 exceeded by more than five times the safe daily threshold limit, causing severe health problems both in rural and urban areas . Continuous tillage with the removal or burning of crop residues has also brought about the loss of SOM, resulting in a lower threshold, and adversely affecting soil functioning .The term “C sequestration” has been defined in many ways but broadly it is used to describe both natural and deliberate processes by which CO2 is either removed from the atmosphere or diverted from emission sources and stored in the terrestrial environment , oceans, and geological formations . It is the process of capture and long-term storage of CO2 in a stable state. This process can be direct or indirect, and can be biological, chemical, geological, or physical in nature. When inorganic CO2 is sequestered directly by plants through photosynthesis or through chemical reactions in the soil, this process is often called “C fixation”. Biological processes that occur in soils, wetlands, forests, oceans, and other ecosystems can store CO2, which is referred as “C sinks”. Bernoux et al. argued that since soils are associated with CH4 and N2O as well as with CO2 fluxes, the concept of “soil C sequestration” should not be limited to considerations of C storage or CO2 balance. All GHG fluxes must be computed at the plot level, or preferably at the level of the entire soil-plant pools of agroecosystems in C–CO2 or CO2-e, incorporating as many emission sources and sinks as possible for the entire soil-plant system.
These fluxes may originate from different ecosystem pools: solid or dissolved, organic or mineral. Bernoux et al. proposed that “soil C sequestration” or better, “soil-plant C sequestration”, should be considered as the result of the net balance of all GHGs, expressed in C–CO2 or CO2-e, computing all emission sources and sinks of a given agroecosystem in comparison to a reference agroecosystem, for a given period. Beyond its role in climate-change mitigation, SOM is not only a key component in nutrient cycling, but also influences a wide range of ecosystem services including water availability and quality and soil erodibility and is a source of energy for the soil biota that act as biological control agents for the pests and diseases of plants, livestock and even humans . SOM is most beneficial when it decays and releases energy and nutrients, and therefore its turnover is more important than the accrual of non-productive organic matter deposits . We propose that a definition of C sequestration should encompass not only the components of SOM in C storage and GHG mitigation, but also the characteristic dynamic turnover that results in labile pools essential for maintaining soil health. Therefore, there are two highly related aspects of C sequestration that aim to attain food security under a changing climate: reducing GHG emissions for mitigating climate change, and increasing soil C storage and linked C recycling for improving the efficient use of resources .Soils act both as a C sink and a C source . Eventually, the ability of a soil system to sequester C lies in the balance between net gains and net losses. Before the dramatic increase in C emissions during the industrial revolution, the global C cycle, or “C flux” was maintained at a near balance between uptake of CO2 and its release back into the atmosphere . Therefore, soil organic carbon can be characterized as a dynamic equilibrium between gains and losses. Practices that either increase gains or reduce losses can promote soil C sequestration. The soil C gain occurs largely from photosynthetically captured C and from the recycling of a part of the NPP as crop residues, including root biomass, rhizodepositions or manure/organic waste. The loss of soil C occurs largely from respiration by plants and the microbial decomposition and mineralization of organic residues to CO2 and CH4. In addition, soil erosion and photo degradation of surface litter are other important forms of C loss. Natural ecosystems are undisturbed and strike a balance of C gains over C losses, hence maintain greater C storage or C sinks. But the conversion of stable natural ecosystems to disturbed agricultural systems promotes soil C loss, converting soil from a net sink to a source of GHGs. It is interesting to note that globally, weed dry rack about 50% of vegetated land surface has been converted to agriculture . A recent estimate indicated that since the beginning of agriculture about 10–12 millennia ago, 456 Gt of C has been lost from the terrestrial biosphere . There are two components: from the prehistoric era to about 1750, the loss is estimated as 320 Gt; and from 1750 to the present era, there has been a further loss of 136 Gt. Another estimate reported the reduction of soil C by 128 Gt during the 10,000 years of cultivation .
On the other hand, Paustian et al. reported a soil C loss of 0.5 to >2 Mg C per hectare per year following the conversion of a natural ecosystem to cropland. This would result in the loss of 30–50% of the total C stock in the top 30 cm layer of topsoil until a new equilibrium was established. The large historic losses over a large time frame, and the fact that soil possesses two to three times more C storage capacity than the atmosphere, have led to a belief that soil has the potential to mitigate GHG emissions and climate change via sequestering soil C. During the last few decades, several researchers have published a range of estimates of soil C sequestration/C storage potential in agriculture. Based on 22 published studies, Fuss et al. reported global estimates of technical potential annual C sequestration rates ranging from 0.51 to 11.37 Gt of CO2 . A large range of reported estimates represented diverse agroecologies/systems , and management practices . The discrepancies in the areas assumed for extrapolation were reported to be the main reason for the large variation in the reported rates of SOC sequestration. In addition, variations in soil depths and the SOC equilibrium durations used for extrapolation cannot be ruled out. Nevertheless, based on the median values of minimums/maximums ranges, the best estimate of technical potential was 3.8 Gt CO2 yr 1 or 1.03 Gt C yr 1 . It is encouraging that a strong interest in this area is not limited to the scientific community only. Recently in the global C agenda for climate change mitigation and adaptation, soils have become a part through the initiation of three high level programmes . Firstly, in 2015, the French government launched the “4 per 1000” initiative at the 21st Conference of Parties of the United Nations Framework Convention on Climate Change as part of the Lima Paris Climate Agreement. The agreement recommended a voluntary plan of 4p1000 to sequester C in world soils at the rate of 0.4% or 4‰ annually . Secondly, at COP23 in 2018, the Koronivia workshops on agriculture were launched, giving emphasis on soils and SOC for climate-change mitigation. And finally in 2019, the FAO launched a program for the recarbonization of soils, called RECSOIL . In 4p1000 initiative, the value of 0.43% is based on the ratio of global anthropogenic C emissions and total SOC stock . Annual GHGs emissions from fossil C are estimated at 3.7 Gt per year and a global estimate of soil C stock of 860 Gt at 40 cm of soil depth. The value of 3.7 Gt C of emissions per year comes from the range of 2–5 Gt C estimated by Fuss et al. . For agricultural soils, Smith estimated the value of 0.45%, which is based on 1.3 Gt C of emissions per year and an agricultural SOC stock of 286 Gt C at 0–40 cm depth . For a 0–30 cm depth, the same annual sequestration potential would be equal to 0.53% of emissions and 0.56% of global and agricultural soil stocks . Considering the land area of the world as 149 million km2 , the average amount of C is calculated to be 161 tonnes of SOC per hectare, and 0.4% of this would be 0.6 tonnes of C per hectare per year. It has been argued that the initiative’s target of 4p1000 is highly ambitious, and important questions have been raised as to whether it is feasible to increase SOC stocks by 0.4% per year on average around the world . Soussana et al. and Rumpel et al. mentioned that 4p1000 initiative is indeed an aspirational goal with much uncertainty about what is achievable but aimed to promote concerted research and development programs on good soil management that could help mitigate climate change. They discussed various specific criticisms of the initiative in relation to biophysical, agronomic, and socioeconomic issues, and provided a more realistic scenario of what was possible and not possible. Subsequently, Amundson and Beaudeu further elaborated on the challenges and complexities involved in achieving this goal, and opined that adaptation may be more relevant than mitigation. They proposed the concept of “weather proofing soils” which would involve the development and promotion of improved soil C management approaches that are more adaptable. Recently, Amelung et al. suggested a soil-specific perspective on feasible C sequestration and some of its trade-offs. They also highlighted that crop land soils with large yield gap and/or large historic SOC losses have major potential for carbon sequestration. A greater need for local, reusable, and diversified knowledge on preservation and restoration of higher SOC stocks has been suggested . A few promising sustainable management options with higher SOC sequestration potential were identified for farmers in America .South Asia accounts for less than 5% of the world’s total land area and supports around 25% of the world’s population .