Ecological Significance of Residues Retention for Sustainability of Agriculture in the Semi-arid Tropics

SAT region has a wide range of soils and agro-climatic conditions which provided the basis for co-evolution of different crop production and land use systems in South Asia. The region spread over 642 million hectares, has 218 Mha of crop lands and is home to around 1.6 billion people ⁶. In the SAT region, soil organic matter (SOM) is considered to play an important role in the development of stable soil structure, infiltration of rain water, its storage and regulating the release and uptake of nutrients and water by plants. Rainfall generally increases as we move from west to the easterly direction. High variability in the amount, intensity and distribution of monsoon rainfall events (spatial and temporal variations) is one of the main causes of agricultural uncertainty in South Asia. Our inability to manage spatial and temporal rainfall variations, related with onset and withdrawal of monsoon rains, results in a crisis of shorts and uncertainty of good crop harvests in rainfed dry lands, low lands, black soils and the hilly regions. The main crisis of subtropical agriculture in the Indian sub-continent is rooted in unimodal / bimodal nature of the rains wherein more than 85% is received through southwest monsoon in the summer season. During peak summers, surface soil layers attain temperatures upto 50°C. Hot summers desiccate and sterilise the soils and burn SOM. Pre-monsoon rains in summer deep ploughed bare fields promote slaking and break-down of soil aggregates facilitating erosion of fertile top soil with runoff water. Thus, in the SAT region, land degradation by erosion is largely a monsoon phenomenon spread over a period of 2-3 months (June- August) involving the loss of some or all of the following: soil sediments, soil productivity, vegetation cover, biomass, biodiversity, ecosystem services, and environmental resilience. An engineering bias for creating blue water has also encouraged widespread run-off, soil erosion and land degradation ⁷. Reversing processes that contribute to land degradation are central to water availability, soil health, adaptation to climate change and food security. Thus, the SAT agriculture faces a twin challenge namely: (i) dissipate raindrop impact causing soil erosion by creating a residue cover, and (ii) develop a planting strategy for the kharif crops before the onset of the monsoon rains such as to provide soils a crop cover and to make an efficient use of in situ green water supplies and reduce annual water deficits.

Monsoon rainfall anomalies are known to have a significant impact on overall kharif production ⁸. Negative impact of deficit rainfall has remained almost the same over time. Deficit rainfall impacts total food production more than the surplus rainfall ⁹. However, response of excessive summer monsoon rain anomaly on food grains production declined post 1980s ¹⁰. Therefore, an important aim of this article is also to evaluate whether Indian agriculture can be insulated against land degradation through recurrent annual rainfall anomalies by using appropriate conservation agricultural practices and adoption of residue management strategies ⁷. Roles of crop residues have been discussed in some details in the ensuing sections.

There is considerable uncertainty in the estimates of crop residue availability in India. National Plan for Management of Crop Residues report of the Ministry of Agriculture, Government of India ¹¹ has indicated production of about 500 million tonnes (MT) of crop residues, annually. Recently, TIFAC-IARI ¹² has published an elaborate survey on the availability of surplus residues of 11 major crops (rice, wheat, maize, sugarcane, cotton, gram, pigeon pea, groundnut, mustard, soybean and castor) grown in about 137 Mha. Four crops viz. rice, wheat, cotton and soybean occupied 72% area. The listed eleven crops generate about 683 MT of total dry biomass in the three crop growing seasons, of which 59% is generated during kharif, 39% during rabi and the remaining about 2% during summer season. The amount of residue produced in India is about 18% of the total global production on an area of 1502 Mha ¹³. The TIFAC-IARI ¹² report brings out that the total annual surplus crop biomass is approximately 178 MT, constituting about 26% of the total dry biomass generated in the country (Table 1). Five states (UP, Punjab, Maharashtra, Gujarat and Haryana) contribute 62.5% to the total annual surplus biomass. The surplus contributions followed the order: Uttar Pradesh (17.7%) > Punjab (17.3%) > Maharashtra (14.2%) > Gujarat (7.6%) > Haryana (5.6%). Large surplus availability of the residues particularly in rice, wheat and sugarcane is closely linked to crop production amounts and the number of residue fires ¹⁴. Development and availability of new seed-cum-fertilizer drills/ planters, shredders, spreaders attached to combine harvesters, and promotion of direct dry seeding no-till conservation agriculture practices have opened up new avenues for preventing farm fires in favour of on-farm management of crop residues.

Crop residue burning is not an isolated practice restricted to Indian subcontinent alone. In the crop harvest season, farm fires can be seen above the wheat fields of the Canadian Prairies and the US Great Plains, sugarcane fields of Latin America and rice fields in South Asia. Although a good portion of the crop residues, in India is used for domestic and industrial purposes yet about 92 MT are burned by the farmers across the country. The amount of residues burnt in India almost equals the residues jointly produced in its neighbourhood in Nepal, Bangladesh, and Sri Lanka etc. ¹⁵. Burning is an inexpensive means of removing crop residues from fields prior to tillage or seedbed preparation but is beset by several adverse effects. It weakens the local capacity of the agroecosystem services, ranging from protection of soils against erosion to recycling of nitrogen (N). Crop residue burning is known to hasten the decline of SOM levels ¹⁶ and decrease soil polysaccharides with consequent reduction in the percentage of water-stable aggregates ¹⁷. Heat from field burning of residues penetrates surface soils ¹⁸, which raises soil temperature upto 75°C in upper 2 inches ^{19, 20}. Open field fires burn about 75% of the total residues and an equal amount of N is also oxidised. Residue burning significantly changes pore size distribution, reduces pore space volume and significantly decreases moisture storage and soil hydraulic conductivity ²¹. Residues burning increases bulk density, reduces soil porosity and water intake of the soils thereby increasing their erodibility ²². Thus, residue burning seems to reduce the permeability, increase compaction and susceptibility of the soil to water erosion ¹⁸. It is commonly accepted that burning crop residues promotes soil water repellency, caused by hydrophobic, long-chained organic molecules released from decomposing or burning plant litter or by microorganisms ²³. The root zone and the leaf surfaces of living plants have also been acknowledged as possible sources of hydrophobic compounds. The view that heat during a fire markedly changes and intensifies water-repellency ²⁴ is now widely accepted and it has been observed that fire could induce hydrophobicity in a previously hydrophilic soil. Debano ²⁵ suggested that heating of any hydrophilic soil containing more than 2–3% organic matter would induce water repellency. Residue burning not only redistributes and concentrates hydrophobic substances in the soil, the heat during a fire is also thought to improve the bonding of these substances to soil particles ²⁶ and make them chemically more hydrophobic by pyrolysis ²⁷. Given a limited supply of hydrophobic substances to coat soil particles, coarser particles are more susceptible to developing water repellency because of their smaller surface area per unit volume compared with soils of finer texture ^{28, 29}. Debano ²⁵ concluded that water repellency is most likely to develop in soils with less than 10% clay content, and it is now well established that the addition of dispersible clay can be very effective in reducing water repellency in sandy soil ³⁰. Song et al. ³¹ pointed out that accumulation of hydrophobic compounds, such as lignin and lipid components, not only enhance water repellency but also provide a molecular mechanism for stabilization of organic C.

Besides impacting the soil processes and ecosystem services, residue burning causes substantial loss of embodied plant nutrients and atmospheric pollution due to emission of green house gases (GHGs). and toxic volatile organic compounds (VOCs). Straw carbon, nitrogen and sulphur are completely burnt whereas the other nutrients are partially lost in the particulate matter emitted to the environment. Burning of one tonne straw (dry mass) releases 1515, 92, 2.7, and 0.07 kg of CO₂, CO, CH₄, and N₂O, respectively ³². Open field burning of rice and wheat straw in India in 2000 was estimated to result in gaseous emissions of 110 Gg CH₄, 2306 Gg CO, 2 Gg N₂O, and 84 Gg NO_X³³. However, in the year 2016-17, open field burning of rice and wheat residues was estimated to result in C equivalent emissions of 7151 Gg C in the north Indian state of Punjab alone ³⁴.

From the above discussion, it is apparent that residue burning not only impacts SOM but also influences rain water storage and erosion hazards of the runoff water during the monsoon season besides causing loss of environmental pollution.

Although the amount of SOM in dryland SAT soils is typically less than 1%, but even at low contents, SOM serves as a major pool for essential plant nutrients and facilitates aggregation and structural stability of soils. In many dryland cropping systems, depending on the fertiliser additions, 50% or more of the N required by the crop comes from mineralization of SOM. The microbial action mediating the mineralization-immobilization turnover of organic matter produces SOM- a process which is regulated by tillage, crops and, residue management practices ³⁵. For improving C content in Indian soils, perhaps the most important priority is to devise tillage and crop residue management approaches and actions that promote soil health, improve soil C storage, in situ rain water storage and its use for growing more crops and provide surface cover to soils to prevent run-off rainwater mediated soil erosion during the monsoon season ⁷. Therefore, under continental monsoonal climates, rainwater management has to be an important element of the strategy for enhancing productivity, resilience and reversal of land degradation.

Increasing acreages of no-till agriculture and large adoption of reduced till methods by farmers can be seen as reducing the area burned to remove residues, particularly for seeding winter crops. However, the mind-set of many farmers is proving a deterrent to adoption of shifting paradigms of conservation agriculture. Practicing zero tillage after burning crop residues, negates the many benefits of conservation agriculture and is no good a practice. Augmenting the farm advisory services can prove helpful in the matter. Residue retention generally increases the mineralizable C and N compared to when residues are burned. Continuous retention of high C/N ratio cereal residues increase the microbial activity resulting in improved availability of nutrients.

Several studies have indicated that residue management systems have a significant impact on the levels of C and N in soils and hence on the crop production. Rice-wheat, a dominant cropping system of the Indian subcontinent, has a turn-around time of 50-60 days for rice and 35-40 days for wheat crop. Some farmers grow a green manure (GM) crop before rice and incorporate it to improve N availability. The practice of dry seeding of Sesbania with rice in which the GM crop is knocked down after 30-35 days with a herbicide molecule (2,4 D) is still in early adoption stages. Several studies have evaluated the effect of time of incorporation of rice/ wheat residues, starter-N application and combination of GM (Sesbania cannabina) on decomposition rates of crop residues ^{36, 37, 38, 39}. Rice residue incorporation after 10 to 40 days had no effect on wheat yields. Rice yields increased (0.18–0.39 Mg ha^-1) when wheat residue was incorporated with GM. Starter N applied at residue incorporation did not influence wheat yields but decreased N recovery efficiency. Rice straw is a poor source of N when used alone, but its combination with fertilizer (applied as urea) resulted in agronomic efficiency just 15% lower than for the use of fertilizer nitrogen alone. This slight disadvantage was offset by several compensating factors: Rice straw provided greater residual benefit (i.e., it provided N over a longer time period) and with its high C:N ratio was a better source of organic C and was able to increase bacterial fixation of nitrogen. Recycling of rice straw may thus have a greater potential for reducing requirements for applications of inorganic nitrogen than the use of green manure. ^{40, 41}. Besides improving plant N availability, the rice straw is an important source of potassium (K) as it has high concentration of this nutrient (> 2% K). Potassium is taken up in large amounts from the soil and a negative balance of −141 and −61 kg K₂O ha⁻¹ has been reported for the intensive rice-wheat cropping systems in India ⁴². Long-term addition of rice straw leads to improved K-fertility of soils and the effect is realized in various K-forms such as water soluble, exchangeable, NH₄OAc-extractable and lattice K ⁴³. Rice straw besides being source of K for the plants minimizes soil K depletion.

When residues are incorporated immediately before planting the next crop, the grain yields are lower than where residues are removed or burned. This is attributable to the slow decomposition rates of crop residues and resulting N immobilization ⁴⁴. Other potential problem of residue incorporation just before rice transplanting include accumulation of phenolic acids in soil and increased CH₄ emissions under flooded conditions ^{45, 46}. However, early incorporation of wheat residues at shallow depth enables their aerobic decomposition, obviates problem of N immobilization, gas emission and facilitates degradation of phenols ⁴⁷ and avoids any adverse effect on germination of young rice seedlings.

Evidently, incorporation of crop residue in the soil has a number of demonstrated benefits: it increases SOM content, nutrient availability, crop yield, and soil aggregate stability , and importantly, fuel soil’s life.

During decomposition of crop residues, soluble humic molecules and low molecular weight aliphatic organic acids are released from the residues and/or are synthesized by the decomposer microflora. These compounds detoxify the phytotoxic monomeric Al in soil solution, by blocking P adsorption sites on Al and Fe oxide surfaces and/or through precipitation of Al as insoluble hydroxy-Al compounds ⁴⁸. Thus organic residues could be used as a strategic tool to reduce the rates of lime and fertilizer P required for optimum crop production on acidic, P-fixing soils. Surface retained residues cut back evaporation and capillary rise thereby the salinization rates of root zone soil from the shallow and saline water tables. This improves crop growth and reduces the need for additional water for leaching of soluble salts out of the root zone ⁷. Recently, it has been shown ⁴⁹ that cyclic use of brackish water in salt tolerant growth stages of spice crops can significantly enhance C sequestration rates even in saline environments. Additional research is warranted to investigate the use of organic residues in the management of problem soils.

Residues control erosion primarily by two modes of action: reducing wind speeds below the threshold level for soil particle movement, and intercepting falling raindrops, preventing them from detaching soil particles. In addition, presence of residues reduces surface runoff of soil particles by increasing the water infiltration rates. Relatively low amounts of residues can be effective in enhancing infiltration but more are necessary to reduce evaporation losses. Crop residues retard heat loss from the soil during winter and hinder soil warming during summer. The extent of thermal insulation afforded by the residues depends on the amount, thickness and orientation of residues. Residue orientation can range from standing stubble to residue randomly lying prostrate on the soil surface. Standing stubble dissipates wind energy at the soil surface and thus minimizes the effectiveness of heat and water vapor transfer by convection from the soil to atmosphere. Stubbles are important in trapping snow and also influencing the interception of solar radiation; taller stubble generally traps more incident radiation and thus reduces the proportion that is reflected from the residue surface. Residues reduce both evaporation and runoff.

Straws are known as good absorbers of water, averaging 2-3 kg of water per kg of straw; shredding further enhances this capacity to 3-3.8 kg per kg of crop residue ⁴¹. Snow trapping by surface residues also significantly enhances soil water storage, with a more pronounced effect as stubble height increases. Retention of crop residues on soil surface has mulching effects of reducing soil water evaporation and moderating soil temperature besides suppressing weeds. Surface residues are reported to reduce soil water evaporation in wheat by 35 percent, thus helping save irrigation water. An analysis of published results on the effects of tillage and crop establishment methods in maize-wheat-mungean sequence ⁵⁰ revealed that compared with the conventionally tilled system, surface retention of residues in zero tillage and raised bed systems saved 400-600 mm water annually in the cropping sequence (Figure 1).

Figure 1.Effect of change in annual evapotranspiration on total productivity of maize-wheat-mungbean cropping system practiced with three tillage and crop establishment methods. Red dots refer to conventional tillage, blue dots to zero tillage and black dots to raised beds

Savings in irrigation water were because repeated irrigation cycles keep the surface soil moist longer and in the first stage of drying. When soil surface is wet (after the rainfall or irrigation event) and is in first stage of drying, the benefits from surface residues in reducing evaporation are greatest. Evidently, the presence of surface residue cover reduces energy input for evaporation and vapour exchange with atmosphere ⁵¹. After formation of a dry soil surface layer, conductivity to the surface limits water loss and hence the benefits of residues may be little or decrease during the dry season. It has been suggested that during periods of extended dryness, evaporation losses from a residue covered surface can actually exceed that from a bare surface. This is because first-stage drying under the covered surface takes longer time to form a dry layer than under bare surface conditions. Thus, in irrigated agriculture with repeated cycles of watering, surface residues can result in a large saving of stored soil moisture from losses due to evaporation. Use of crop residues as surface mulch in spring and summer crops regulates hydrothermal regime through reduction in soil water evaporation and moderation of soil temperature. Mulching is reported to reduce seasonal evaporation loss by 15 cm in maize and 20 cm in sugarcane ⁵² besides increasing crop yields.

In a field experiment conducted on a deep black soil, effect of surface residues and tillage and crop establishment (TCE) methods on plant stand and yield of rice was studied in tilled and permanent zero-till conditions under rainfed and irrigated conditions ⁵³. They observed that surface mulching and zero tillage and TCE methods significantly improved the plant stand of rice crop under rainfed conditions. Rice plant stand in presence of residues under rainfed conditions was generally similar to one observed under irrigated condition without residue covers. Provision of irrigation water masked the ill effects of poor agronomy (no residue cover, excessive deep tillage) and a few supplemental irrigations to rainfed rice crop significantly improved its productivity (Figure 2). These results point out that the early direct dry seeding in surface retained residues has the potential of making Indian agriculture (kharif season planting) independent of monsoon rains and thus helps prevent rainy season fallows and reduce the acreage of fallow lands in the Vertisols. Thus dry seeding is the single most desirable agronomic practice for harnessing the potential of early rains during hot summers and provides the soil surface covers against erosion during the rainy season

Figure 2.Effect of tillage and crop establishment methods, residue mulch and irrigation provisions (+/-) on productivity of a direct dry seeded rice crop on Jabalpur deep black soil (Adapted from Gupta et al., 2019)

Soils in the semiarid tropics generally have SOM in the range of 0.5 to 3%, and typically less than 1% ³⁵. Even at low SOM contents, it significantly influences aggregation and structural stability of soils. Globally, soils have been considered a large C sink but Indian researchers have long lived with a notion that C status of Indian soils cannot be enhanced under tropical climates. Recently, it has been indicated that even though soils are almost essential for us to survive climate change, they are unlikely to help remediate this change ⁵⁴. Therefore, it would appear that the current emphasis on C sequestration as the primary goal of mitigating climate change is somewhat misplaced and an ‘inverted’ priority. This is because what were considered as secondary benefits (improved rainwater storage, reduced soil erosion, and producing more food) must be viewed as primary objectives of research and development towards better farming. Several studies in recent years have examined the potential of Indian soils to sequester C ^{55, 56, 57} and have reported that ‘better-bet’ soil and crop management practices can mitigate more than half of the total GHG emissions in India ^{58, 59, 60}. Generally, farmers adopt different practices to increase SOC stocks such as (i) integrated use of inorganic fertilizers and organic manure ^{3, 61}; (ii) application of green manure, cover crops and compost; (iii) addition of anchored and trashed plant residues with reduced or zero tillage ^{62, 63}; or (iv) introduction of black C or biochar to the soil ^{38, 64, 65}. Results of long-term experiments conducted for more than 4 decades on Mollisol, Inceptisol, Alfisol and Oxisol in India have shown that use of organics together with balanced application of fertiliser nutrients, improve SOC, moisture retention and infiltration rates ^{5, 66, 67}.

Geographically, distribution of SOC is very heterogeneous and strongly dependent on soil type, land use and climatic conditions. Black soils contain a mollic horizon having high quality humus down to a depth of 60-80 cm. This high quality humus is the result of high base saturation, stable aggregate structure, SOC losses and intense biological mixing ⁶⁸. These soils are, however, very sensitive to degradation due to erosion, crusting, nutrient mining and therefore, need to be managed carefully. Improvement in water content, swiftly changes the bulk density and macro-porosity of fine textured black soils. Above field capacity, changes in soil water content impacts low-tension oxygen diffusion processes more than the water content itself ⁶⁹. We have observed that low oxygen in wet black soils adversely affects seed germination during rainy season even on the ridges. Surface retention of residues in the presence / absence of a crop stand bring about a significant change in cracking behaviour of deep black soils. Deep wide cracks a common phenomenon of deep vertisols disappear after few seasons by surface mulching of crop residues. Disappearance of cracking behaviour in presence of crop residues, greatly improves the irrigation efficiency and water use in black soils.

Above discussion, presumably suggests that oxygen diffusion rate in black soils could possibly be a better parameter than moisture content per se in computing SOM decomposition rates. Influence of surface residues on cracking behaviour of black soils, likely affects the moisture, temperature and aeration regimes and hence the organic C decomposition rates and C storage potential of these soils. Therefore, Q₁₀ concept needs some additional research. Dry lands have large potential to sequester SOC but C storage in drylands is affected and limited by a number of bioclimatic elements including availability of the organics for recycling.

Organic matter levels in soils are determined by a number of interacting factors including bio-climate, soil type, tillage and crop establishment methods (TCE), cropping systems and methods of returning residues to the soils (incorporation, surface retention/ removal), and the types of residues added to the soils (straws, green manure, farm yard manures or composts). Extent of erosion hazards also influence SOM levels and transformations. Carbon sequestration and stabilization regulate SOC levels and have important implications on soil productivity and the potential of using soils to enhance soil C storage and mitigating predicted climate changes ⁷⁰. In the decomposition of crop residues, factors such as water, temperature and the biochemical composition govern microbial activity responsible for decomposition and accumulation of SOM. In general, low temperature and high moisture favour increases in SOM and vice versa, implying thereby that semiarid regions have high SOM decomposition rates. Microbes can function over a wide temperature range but exhibit optimum growth and activity in 20-30^o C range at field capacity moisture contents. As soils dry out, bacterial activity decreases but fungi can still act on the residues even at low soil water contents ⁷¹. At constant temperature and moisture conditions, the residue composition influences the rate and extent of its decomposition. Crop residues with a high C/N ratio (e.g., wheat) decay at a slower rate than residues with a low C/N ratio (e.g., corn). The decomposition rates of relatively labile substrates have been related to C/N ratio and that of more recalcitrant organic residues to lignin content or lignin-to-N ratios ^{39, 72}. Microbes produce SOM through decomposition of organic residues and release CO₂ through heterotrophic respiration. They also utilise SOM and humus carbon as energy source. Lignaceous rice and wheat residues of high C/N ratio (80-120:1) induce a high microbial demand for nitrogen and if not met by straw, may immobilise native soil N or fertiliser N. The effect of temperature on decomposition rates of organic matter is often described using a variety of models including the van’t Hoff, Arrehenius, Lloyd and Taylor, and Gaussian ⁷³. A comparison of various models has revealed that for field situations, these models differed greatly in predicting response to temperature ⁷⁴. In several publications, effects of factors such as residue composition (C:N ratio), soil moisture and temperature variations, residue orientation (anchored and flat placements) and mineral matrix (surface area) on organic matter decomposition rates have been considered ^{37, 39, 63, 75}.

There are ample evidences to show that SOC stocks are strongly influenced by residue management and a soil’s ability to protect (or stabilise) carbon ^{41, 76, 77}. Charged mineral surface facilitate formation of organo-mineral complexes that protect organic molecules both physically and chemically from microbial decomposition ^{78, 79, 80}. The amount and type of soil clay greatly influence the quantity of C stabilized; the soils with higher clay content, particularly with higher exchange capacities retain greater amount of residue C ⁸¹. For instance, montmorillonite clay resulted in greater C stabilization particularly in the later stages of decomposition whereas kaolinite did not influence C stabilization in soil ⁸¹. Clays besides increasing microbial biomass and activity improve C use efficiency by reducing C loss as CO₂ thus leading to greater C stabilization in soil. For reasons of charge density, SOC in New Zealand was poorly correlated with clay content but well-correlated with the apparent specific surface area of the mineral matrix of the soils ^{82, 83}. The estimates of specific surface area based on measurements of water adsorption of soils and fluctuations of water contents seemingly provide a simple and cost effective method of working out the SOM decomposition rates. Using field capacity soil moisture variations with time, decomposition rates of SOM could be adequately defined by Mohammed et al. ³⁷. Since SOM itself can also adsorb soil water, Kirschbaum et al. ⁸⁴ estimated the soil carbon-based contribution to water adsorption to reach unconfounded estimates of mineral specific surface area in arriving at the realistic decomposition rates of SOM.

Anchored standing crop residues decompose slowly in the arid and semi-arid regions via photo-oxidation of organic matter ⁸⁵. In photo-oxidation, lignin plays a key role in regulating plant litter decomposition ⁸⁶, humic substance formation ^{87, 88} and production of dissolved organic matter (DOM) ⁸⁹. Photo-oxidation reduces the molecular size of lignin or DOM ⁹⁰, which potentially increases the biodegradability of DOM ⁹¹. In the initial stages (1-2 weeks), residue decomposition was related to crop residue organic N, C/N ratio, size of water soluble organic C pool and intermediately available C pool ⁹². Photo-degradation exercises a dominant control on decomposition of above-ground residues in a manner that can possibly short-circuit the carbon cycle ⁸⁵. This implies that a substantial fraction of plant biomass carbon may be lost directly to the atmosphere without cycling through SOM pools. Photo-oxidation of the anchored crop residue in rainfed dry lands could play a significant role in organic matter degradation. Above-surface residue (anchored) decay very slowly than those lying prostrate on the surface ⁶³. Studies have shown that soils high in SOM retain more moisture, especially when residues are retained on the soil surface as compared to when they are incorporated. Placement methods of residues have a significant influence on the decomposition rates. Incorporated residues generally decompose faster than those retained / placed on the soil surface. Reinertsen et al. ⁹² reported that indigenous microflora, which colonise the cereal straws provide adequate inoculums to facilitate decomposition. Thus the slow decomposition rates of residue placed on the surface is mainly due to prevailing suboptimal temperature and moisture regimes

The burned C is primarily biochemically stable and unlikely to play a significant role in C cycle. So the labile C and the mineral associated C (recalcitrant) remains. Its amount should be proportional to surface area of mineral and the C inputs after decomposition of the labile C. Sowing of zero till wheat in surface residues is a better alternative strategy to avoid residue burning, improving crop productivity, increasing C-sequestration, and enhancing the sustainability and soil quality in rice-wheat system. Residue returned to the soil besides enlarging SOC pool, impacts its quality in terms of persistence, turnover rates and functionality. Crop residues and organic amendments improve SOC pool directly through addition of embodied C and indirectly via greater crop mediated C input resulting from enhanced crop yield ⁹³. The quality of SOC is generally characterized by separating into various physical (based on size, density, protection or accessibility), chemical (extractability or oxidizability) and biological (microbial activity, kinetically defined) pools ^{4, 94, 95, 96}. The separated pools with different turnover times considered labile/ active, non-labile/ stable and recalcitrant/passive are associated with specific stabilization mechanism (Table 2). Recent works have suggested that C accrual and persistence in soil can be better described if SOM is broadly separated into particulate organic (POM) and mineral-associated organic matter (MinOM) pools ^{3, 97, 98} with further sub-divisions into coarse POM and fine POM. The POM represents more labile pool and MinOM, because of mineral association, a stable pool of SOC ³ though both the fractions are subject to occlusion within macroaggregates that slows decomposition ^{94, 99, 100}. Long-term SOM stabilization is thought to be through (i) physical protection by micro-aggregates, (ii) chemical protection by binding with oxyhydrates ¹⁰¹ and inter-molecular interactions with organic or inorganic substances ¹⁰², and (iii) molecular recalcitrance promoted stabilization of hydrophobic particulate organic matter contributing to C sequestration in paddy soils ¹⁰³.

Surface retained residues or buried in soil undergo fragmentation and degradation, adds to coarse POM pool, which decomposes progressively to more resistant finer particle sizes with the narrowing of the C/N ratio ^{3, 97, 104}. A number of studies have reported the accrual of added organic C as POM-C in soil. The light fraction POM responds to residue application to greatest extent followed by sand-sized heavy fraction and silt and clay sized MinOM-C ³. Surface residues are known to improve aggregate stability and formation of macro-aggregates, which provide protection to the associated organic C fractions from decomposition thus increasing their residence time ^{105, 106}. The effect is enhanced with no-till which promotes aggregate stability and the formation of recalcitrant SOM fractions within micro- and macro-aggregates ¹⁰⁷. The improved aggregate stability results from increased microbial activity due to metabolism of carbohydrates and the interaction of plant phenolic acids released during decomposition of residue structural components ¹⁰⁸ and the less oxidative biochemical environment of no-till soils ¹⁰⁹. Therefore, conservation agriculture results in higher C accrual and persistence in soil.

Several early studies had assumed that soils have a finite capacity to store C even with the better-bet practices. This capacity often referred as maximum C stabilization capacity ^{82, 110} is a function of rate and duration over which C sequestration continues. Most published studies have predicted C sequestration potential of different soils assuming a time span of about 25-50 years ^{111, 112}. The Intergovernmental Panel on Climate Change ¹¹³ suggested 20-yr period for estimating the C sequestration rates following a change in management practices. An analysis of global data from 67 long-term experiments indicated that C sequestration rates with no-till (NT) could peak in 5 to 10 years with SOC reaching a new equilibrium in 15 to 20 years ¹¹⁴. In a subsequent analysis, C sequestration rates for cropland management have been predicted to peak at about 10 years and continue at lower rates over another 40 years ¹¹⁵. However, it has been argued that sequestration duration does not indicate soil C sequestration potential rather it reflects only the time to attain new steady state when C input equals C output with a given management. The soils could still sequester more C after adopting additional management changes till maximum soil C sequestration capacity, or soil C saturation, is reached ¹¹⁵. Although the formation of mineral associated C may have an upper limit as determined by the quantity of fine silt and clay particles, but the accumulation of particulate organic C (POC) apparently has no saturation limit ^{76, 98}. On the contrary, some researchers have suggested that C sequestration potential of soils cannot be predicted because C storage takes place not only through clay-organic complexes but also through organic-organic complexes.

From the foregoing discussions, it would appear that most soils are far from the saturation threshold. Management practices that protect existing C stocks and bring additional C inputs can significantly maximize the C sequestration potential. When this threshold is reached, SOC sequestration comes to an end and soils stops being a net carbon sink and become a net carbon source. For this reason, SOC sequestration is a reversible process ¹¹⁶. Degraded soils having minimum SOC, are the ones that have the largest potential to gain C with appropriate management practices ^{117, 118}. Sub-soils have large potential to sequester C because of a large SOC saturation deficit. In context of climate change it is interesting to know whether SOC stocks will continue to change in line with changing C input rates, or the SOC changes will be constrained by limits of C that a soil can stabilise and protect.

In India, a vast majority of the land holdings are with marginal and small farmers, located in the dryland / rainfed areas. These farm lands have low productivity owing to high decomposition rates of SOM and low water availability. While summer monsoon rains are the life-line for Indian agriculture, runoff water mediated soil erosion at the same time results in loss of soil carbon, nutrients, moisture and contributes to reduced biomass production as well as restricting the farmers’ choices for diversification (biodiversity). Loss of surface soil in monsoon rains by far is a major process of soil degradation in the Indian subcontinent. Reversing processes contributing to land degradation are central to water availability, soil health, adapting to climate change and food security. It also appears (Figure 3) that rain water management has to be a crucial element of any strategy that enhances gene diversity and sequesters more carbon to offset the climate change effects, builds resilience and reverses land degradation ⁸. Thus the overall strategy for land degradation neutrality should favour actions that provide for keeping soil covered with residues and make kharif season planting independent of the onset of monsoons (e.g. Direct dry seeding in residues before pre-monsoon showers) for protecting soils against rain drop actions and promote in-situ rain water storage and use to enable farmers grow more crops and prevent run off and soil erosion.

Figure 3.Priority actions for handling land degradation-food security-climate change nexus (Adapted from Abrol and Gupta, 2019)

Increasing the use efficiency of rainwater and/or irrigation support increased crop production resulting in increased allocation of carbon to the soil through residues and root biomass returned to the soil. No-till together with residue retention on the surface reduces soil erosion to rates close to those found in natural ecosystems ¹¹⁹. Thus, the no-till conservation agriculture water centric management practices help build carbon in soils.

Fertilizer application rates have a profound influence on SOC contents and the health of the soils. Results of a large number of field trials conducted under the aegis of Indian Council of Agricultural Research- AICRP Long Term Fertilizer Experiments (LTFE) and the Modern Agronomy Experiments in different agro-ecoregions of India have indicated that fertilizer use has been a primary driver of increasing crop yields. It has also been observed that recommended dose of NPK plus FYM or GM application maximized crop productivity and enhanced SOC stocks for sustaining ecological functions on most soils in the semi-arid and sub-humid situations. However, several critical questions concerning balanced and efficient use of fertilizers, plateauing yields and reluctance of farmers to retain crop residues in the field and use of FYM remain unanswered. We have endeavoured to draw attention and discuss some of the underlying issues impacting soil health, nutrient use efficiency and production system sustainability.

How does the continuous use of chemical fertilizers and FYM impact the carbon dynamics in soils?

How does use of organics assist achieve soils their optimal ecofunctionality that cannot be compensated with fertilizers alone?

How should available organic resources be optimized considering the specifics of hot summer season and monsoon rains that make Indian soils extremely prone to soil degradation through soil erosion?

Finding appropriate answers to the above vexed issues appear fundamental to achieving the sustainability goals. Much of the fertilizer use and soil management research in India has focused on defining fertilizer use and management practices for improving crop productivity and their use efficiency. Most studies have indicated that balanced fertilizer use (100% NPK) improve crop productivity with simultaneous build-up of SOC by 20-600 kg C ha^-1 yr^-1 across several sites in India ⁵⁶. Further, the SOC accumulation in plots receiving FYM is nearly twice (100-1200 kg C ha^-1 yr^-1) of the plots receiving only chemical fertilizers. This implies that speeding up SOC build-up in Indian soils and improving use efficiency of the fertilizers would require simultaneous use of organics and chemical fertilizers. However, the range of SOC accrual values across India clearly show that C sequestration rate of “4 per thousand” ^{120, 121} is quite difficult to achieve in Indian sub-continent with prevailing soil and crop management practices.

Besides, balanced and integrated nutrient management, CA practices are reported to increase SOC accumulation and persistence. Published evidence suggests that C accumulation in soils from temperate region is nearly at double the rates than generally reported for subtropical soils. Generally, farmers apply tillage for incorporation of crop residues. Initially the practice destroys soil aggregates but subsequent litter decomposition can promote formation of water stable aggregates. Fresh litter C inputs provide readily bioavailable nutrients and energy for soil microbes to grow and exert a priming effect on decomposition of native SOC and play a significant role in governing the long-term dynamics of SOC. No-till (NT) cropping systems with residue retention usually favour increased macro-aggregate formation, from stable micro-aggregates facilitated by sequestered C, relative to conventional tillage ¹²². Therefore, conjunctive use of organics with chemical fertilizers under CA is an effective and important soil C and yield response management strategy for sustainability of agriculture in the subtropical regions. On the contrary, crop residue burning and harvest for bioenergy production will likely lead to SOC depletions over time with implications for CO₂ and N₂O emissions from soils ^{123, 124}, reduce plant nutrient supplies, adversely impact the biota that regulate some key soil functions. Therefore, crop residue harvest/burning has multiple influences on soil properties, water storage, biological activity and soil resistance to structural degradation ¹²⁵. Impacts of crop residue management on soil functions and plant growth has been summarized diagrammatically in figure 4. It shows that crop residues have multiple roles, and different strategies affect the soil ecofunctions and ecosystem system services differently. Cherubin et al. ¹²⁶ graded the soil functions and plant growth for 3 crop residue removal / harvest rates (high, moderate and low) on a 1-3 impact scale (1 - low, 2 – medium, 3-high).

Figure 4.Impact of crop residue management on soil functions and plant growth (Adapted from Cherubin et al., 2017). The 1-3 impact scale denotes 1 - low, 2 – medium, and 3-high.

This diagram allows an impact assessment associated with any residue management strategy. In Indian context, most of our soils are highly prone to summer monsoon rains mediated soil degradation each year during the kharif season. This is because agronomic fatigue does not allow crop planting before the onset of monsoon rains and reluctance of the farmers to keep soil surface covered with residues. Therefore better bet management strategies must include no-till, provide surface cover to soils via crop residues or plant cover established through direct dry seeding before the onset of monsoons, and use FYM during the rabi season to perk up the depleted carbon stocks. Apparently in figure 4, greater importance has been attached to large biomass production through efficient use of repeated cycles of green water supplies received through continental monsoons and the role surface residues play in cutting back the unproductive loss of water through evaporation.