A review on carbon pools and sequestration as influenced by long-term management practices in a rice–wheat cropping system

ABSTRACT

The drastic increase of atmospheric CO₂ concentrations and depletion of soil organic carbon (SOC) have prompted interest in exploiting the sink potential of soil to sequester carbon. The role of soils to mitigate climate change by the “4 per 1000” concept to increase global SOC stocks by 0.4% has been acknowledged. However, the potential of soils to sequester C depends on the cropping system, the magnitude of antecedent C depleted from soil, properties of the soil profile, climate and agricultural management practices. To formulate long-term agricultural management practices that lead to C sequestration, it is important to study their effect on SOC pools. Certain labile pools of C are considered sensitive indicators that show quick change after a modification in management practice. In contrast, changes in total SOC are relatively less detectable over the short to medium term. Most SOC pools are interrelated and vary in proportion. Rice–wheat, an intensive and dominant cropping system occupying 24 million hectares of cultivated land globally, significantly contributes to the global warming potential. Therefore, this review aims to identify the best management practices in the rice–wheat system that lead to C sequestration by improving SOC. These include the use of manure, compost, crop residues, balanced fertilization used conjointly with farmyard manure (FYM), mulch farming, conservation tillage, and inclusion of cover crops. The paper provides a comprehensive review of C pools and sequestration as influenced by long-term management practices under a rice–wheat cropping system.

Keywords:

• Carbon sequestration
• sustainable production technologies
• physical fractions
• chemical fractions
• biological fractions

Introduction

Carbon (C) circulates globally among three distinct pools: the atmosphere, the ocean, and the land biosphere. The atmosphere pool carries 762 petagrams (1 Pg = 10¹⁵ g) of C, chiefly in the form of CO₂ [1] . The atmospheric concentration of CO₂ reached a record of 417.1 ppm in 2020 or 147% of the pre-industrial level in 1750 [2]. Substantially more C is stored in the earth’s soils, about twice the amount of C (1500 Pg) that is contained by the atmosphere, and most of it is organic C with a turnover rate ranging from months to thousand years [3]. The soil, however, acts as an efficient sink for atmospheric CO₂ depending on the input–output C balance. Historically, soil has lost about 55–78 gigatons (1 Gt = 10¹⁵ g) C and thus offers an immense potential to sequester C [4]. The capacity of soil to sequester C depends on the land-use history, different management practices, the magnitude of antecedent C depleted from the soil, properties of the soil profile, climate, and management. The adoption of more sustainable production methods could improve C sequestration in the terrestrial ecosystem and mitigate climate change. It has been estimated that soils can sequester about 40 to 80 Pg of C over the next 50 to 100 years through the adoption of sustainable management technologies [5]. Carbon sequestration refers to fixing atmospheric CO₂ into the soil for a long mean residence time to avoid its discharge into the atmosphere. Agriculture is a promising sector to mitigate climate change by removing atmospheric C through sequestration. The possibility of C exchange in the soil–atmosphere continuum is important not only in the global warming context but also for soil productivity and ecosystem services, providing “win–win” situations [3].

Soil organic matter (SOM), the primary pool of C, is a crucial attribute of soil quality as it influences soil physical, chemical, and biological properties [6]. It is a source of plant nutrients (N, S, and P) via mineralization. It serves as an energy source and provides nutrients to soil biota. It is a key indicator not only for agricultural productivity, but also for environmental resilience. As a result, the quantity and quality of SOM are among the key factors in evaluating the sustainability of management practices [7]. The assessment of alteration in physical, chemical, and biological pools of C is a tool to examine the impact of management practice on soil organic carbon (SOC) turnover. Scientists have carried out various studies to correlate conceptual pools with recordable SOC fractions to understand C dynamics and the factors that affect them in the terrestrial ecosystem. A labile pool and a recalcitrant pool are two important constituents of total organic carbon (TOC) [8]. The labile C is a dynamic pool and the most sensitive C pool, while the recalcitrant fraction is unaltered by organic C gains or losses caused by crop production technologies. Therefore, changes to labile C are considered an early signal of the influence of cultivation technologies on TOC quality [8].

Rice and wheat is the world’s largest cropping system, occupying 24 million hectares (mha) of cultivated land, mainly distributed in southern and eastern Asia. This comprises 13.5 mha in the Indo-Gangetic Plains (IGP)/South Asia (includes 10 mha in India, 2.2 mha in Pakistan, 0.8 mha in Bangladesh and 0.5 mha in Nepal) and 10.5 mha widely spread across East Asia/China [9]. The area and the productivity of the rice–wheat system in the IGP increased substantially between the 1960s and 1990s, because of highly nutrient-responsive varieties, increased use of fertilizers, and availability of assured irrigation [10]. Rice–wheat is an input-intensive and highly productive system that could be a potential C sink if managed efficiently. Therefore, it is imperative to identify and adopt practices having a beneficial effect on the soil C pool and climate change through the reduction of greenhouse gas (GHG) emissions. Management-induced changes in SOC may take years to become noticeable; therefore, long-term experiments are vital assets to study such changes. Moreover, due to high resilience of the SOC, observations from long-term experiments could be a better way to understand the mechanisms and processes of C sequestration. Several studies showed that management practices could influence the labile C fractions of soil, and their impact depends on the environment, soil texture, straw management, and crop diversification [11,12]. Based on previous studies, management practices that ensure greater amounts of residue returns into the soil are expected to have a considerable effect on C stabilization. Most such studies observed that the combined application of inorganic and organic fertilizers improved both labile and recalcitrant SOM fractions in the rice–wheat system [13]. The long-term addition of organics in terms of compost, farmyard manure (FYM), and crop residues, besides leading to C build-up, improved C stabilization by imparting recalcitrance to SOC in a rice–wheat cropping system [14]. Conservation tillage has also favorably influenced SOC accretion.

Improving the benefits of greater SOC storage requires more information on management practices that increase C inputs and mitigate the loss of accrued benefits. Moreover, recent interest in soil quality, sustainable production, and, more importantly, ecological concern warrants comprehensive study of the existing knowledge, research, and challenges that describe the relative potential of management practices for C redistribution to different fractions and stabilization of soil C in the studied regions [15]. This review provides a holistic view on the effects of long-term nutrient management practices on C pools and sequestration under a rice–wheat system.

Soil organic carbon

Maintaining the SOC level in the soil is critical for long-term sustainable productivity. Carbon is an essential factor of soil quality, which regulates nutrient cycling, soil structure, water availability, and other important soil properties [16], and has a close relationship with crop productivity, since declining C levels generally lead to decreased crop productivity. Although a small quantity of soil C originates from mineral sources, a significant proportion of C is of plant origin. C-based organic compounds of varying size and chemical composition are left in the soil after plants die. Under suitable conditions, the soil fauna metabolizes these C materials, consume part of the C and transfer it to new compounds for use in their own body; excess C is respired to the atmosphere as CO₂ or returned to the soil through excreta. The constant flow of C in the soil–plant–atmosphere continuum is a result of its continual cycling among the different global C pools in various forms.

SOC includes a series of substances varying from very decomposable to highly recalcitrant fractions. Depending on the length of time that C remain undecomposed, a term referred to as “mean residence time,” soil C is categorized into different pools. These include three separate classes: the labile pool (low mean residence time), the slow pool (medium mean residence time), and the stable pool (very long mean residence time) [17]. The labile C pool is composed of freshly added plant residues and simple C compounds of root exudates that decompose and release to the atmosphere within a few days to a few years. The labile C pool serves as an energy source for soil microorganisms and helps in nutrient cycling for higher soil quality and productivity [18]. The slow C pool is composed of moderately processed plant residues and microbial by-products of the labile C pool. This C pool has physical protection against microbial and biological decomposition. The mean residence time of the slow C pool ranges from years to decades and can be influenced by soil type, management practices, and climate conditions.

Meanwhile, the stable C pool has mean residence times ranging from centuries to millennia because of its strong resistance to any change. C compounds in the stable C pool can withstand microbial breakdown and are protected from microbial decomposition [19]. The relative proportions of these C pools in different soils are not uniform.

Nevertheless, in general, the concentration of the stable C pool remains relatively unaffected in soil, while labile and slow C pools are sensitive to management. The labile C pools, such as microbial biomass C, mineralizable C, water-extractable organic C, and oxidizable organic C, may be considered a primary index of te soil quality because these pools are susceptible to management and land use [20].

Optimum levels of SOM can be managed through crop rotation, fertility maintenance including the use of inorganic fertilizers and organic manures, tillage methods, and other cropping system components [21,22]. As SOC changes are generally directly related to the quantity of crop residues returned to the land, agronomic practices that influence yield and affect the residues returned to soil are likely to influence SOC [23]. A strong positive relationship between the amount of C incorporated into soil, either from crop residues or from external sources such as manure, and total SOC content has been observed. The net change in SOC depends not only on the current management practices but also on the management history of the soil. Long-term experiments are the primary source of information to determine the effects of continuous cropping, and retention of residues in soils and fertilizer/manure addition, on SOC [24]. These experiments are usually the only source of information for determining agricultural sustainability and to define the effect on SOC [25]. Therefore, quantification of SOC in relation to various management practices is of value in identifying the pathways of C sequestration in soils.

Improvement in crop yields under degraded soils could be related to enhancement in the SOC pool, which positively affects available water content, availability of nutrients, soil structure, and other physical properties (Figure 1). Incorporation of organic amendments influences soil characteristics by the modification of biological, chemical, and physical properties [27]. A positive correlation between the SOC pool and the plant available water content indicates the higher potential of soils to withstand drought [28,29]. With an increase in SOM of 1 g, the available moisture content in soil increases by 1–10 g [28]. Management of these properties has the capability to optimize crop production.

Figure 1. Soil quality enhancement by improving the soil organic carbon content (reconstructed from [26]).

Carbon sequestration and stocks

Carbon sequestration is the practice of mining atmosphere CO₂ naturally or anthropogenically and transforming it into stable terrestrial C pools. There is no net gain in the atmospheric C pool by anthropogenically driven sequestration because the CO₂ that is sequestered comes from the atmosphere. Carbon sequestration can be achieved by both abiotic and biotic means. The abiotic method involves throwing CO₂ into deep oceans, geological strata, old coal mines and oil wells. The biotic techniques, on the other hand, involve higher plants and microorganisms to extract CO₂ from the atmosphere and convert it into a relatively more stable C pool in soil. Biotic C sequestration is further sub-grouped into oceanic and terrestrial C sequestration. In oceanic C sequestration, CO₂ is captured by photosynthetic activities of phytoplankton which convert the C into particulate organic material and deposit it on the ocean floor. It has been estimated that the ocean has the potential to sequester about 45 Pg C y⁻¹ [30].

Terrestrial C sequestration is the direct and indirect fixation of atmospheric CO₂ into a recalcitrant form in the soil. The conversion of CO₂ into carbonates of calcium and magnesium is direct C sequestration, and the sequestered C is called soil inorganic carbon (SIC) [31]. Indirect sequestration involves the production of plant biomass through photosynthesis. Subsequently, a portion of plant biomass is indirectly sequestered as SOC during decomposition processes. The improvement in the SOC pool indicates the long-term balance between C influx and outflux. The C sequestration feedback of the terrestrial system can be altered through land-use change or the adoption of right management practices (RMPs) in agricultural, pastoral or forest ecosystems. Soils in natural ecosystems have a higher SOC pool than those in managed ecosystems, due to their lesser oxidation or mineralization, leaching and erosion [31]. The terrestrial ecosystem includes forests, soils, and wetlands, which act as significant C sinks; they sequestered 3.5 ± 0.7 Gt C year⁻¹ in 2018 [32].

According to the IPCC, the capacity of agricultural soils for sequestering C is up to 1.2 billion tonnes per year [33]. Carbon sequestration is a promising solution for reducing atmospheric CO₂ concentration by trapping it in the soil for an extended period. Agricultural land has an enormous potential to sequester C, as over 33% of the world’s arable land is under agriculture. Agricultural land can sequester 8–10 Gt year⁻¹ and could offset at least 10% of the current annual emissions if appropriately managed [34].

Carbon sequestration potential in soil is affected by several factors such as climate and soil conditions [35], cropping systems [36], and managements including tillage and fertilization [22]. There is a need to recognize and take on the best management practices to improve SOC levels through sequestration, particularly in soils that are low in organic C, like those in tropical climates [37]. The most frequently recommended management practices that lead to an improvement in soil C sequestration under long-term nutrient management include the use of manure, compost, crop residues, fertilizers, mulch farming, conservation tillage, diverse cropping systems, and cover crops [3]. All of these practices can alter the C storage capacity of agricultural soil by returning the plant biomass to the soil (Figure 2), the addition of ex-situ organic materials, intensification of agriculture, reduced decomposition, or soil respiration [38]. In Figure 3, various processes such as removal by erosion sediment-associated POC, deposition with sediment in down-slope areas, decomposition through biological activity, the formation of soil humus and stable micro- and macro-aggregates, as well as translocation and incorporation of SOC take place deep in the soil profile. All these factors lead to a net increase in total SOC leading to soil C sequestration [40]. Management practices that enhance these processes will, therefore, result in increased SOC accumulation and sequestration over time.

Figure 2. Management practices of C sequestration. Reconstructed from [4]

Figure 3. Transformation and change of carbon among various pools leads to C sequestration (POC: Particulate organic carbon; DOC: Dissolved organic carbon). Reconstructed from [39].

SOC under agriculture is positively affected by the regular application of N, P and K fertilizers alone or integrated with FYM (Table 1). The balanced use of fertilizers and integrated nutrient management has been reported to increase SOC concentration by 8–200% and 13–232%, respectively (Table 1). The application of fertilizers improves crop productivity, and increases below-ground biomass and the quantity of crop residues [50]. Incorporation of FYM and green manure (GM) along with fertilizers/crop residues on a long-term basis sequesters more C compared to chemical fertilizers [51]. The rate at which SOM changes in agricultural soils is slow and can take a decade to centuries [52]. Variations in C inputs can immediately influence the changes in SOC fractions like labile C, water-soluble C, and microbial biomass C, relative to recalcitrant fractions which take more time to change [53]. A linear relationship was observed between C sequestration and rate of C input (Figure 4); added C was much higher in the combined straw or manure additions with fertilizers than in control.

Figure 4. A linear relationship between SOC sequestration rate and C input rate (reconstructed from [54]).

Table 1. Effect of inorganic fertilizers and farmyard manure application on soil organic carbon (SOC) build-up in a rice–wheat cropping system.

Location	Duration (years)	SOC (% of control) after specified duration		Reference
		NPK	NPK + FYM
Cuttack (India)	10	119	122	[41]
Mohanpur (India)	20	118	124	[41]
Varansi (India)	12	113	128	[41]
Faizabad (India)	14	200	232	[41]
Kanpur (India)	14	139	150	[41]
Samastipur (India)	8	113	125	[41]
Ludhiana (India)	15	107	180	[41]
Ludhiana (India)	11	115	162	[41]
Hisar (India)	10	107	117	[41]
Pantngar (India)	14	114	118	[41]
Bhubneshwar (India)	20	44	85	[42]
Faizabad (India)	20	111	163	[42]
Karnal (India)	20	7	17	[42]
Pantngar (India)	20	71	204	[43]
Pantngar (India)	20	50	140	[43]
Bhairahawa (Nepal)	20	21	–	[44]
Barrackpore (India)	20	8	13	[45]
Ludhiana (India)	20	85	–	[46]
Ludhiana (India)	15	20	47	[47]
Karnal (India)	10	34	65	[48]
Suzhou (China)	10	19	27	[49]

Long-term experiments improve assessments of the C budget. For example, in a 33-year long-term experiment with rice–wheat, it was noticed that the crop biomass C was highest (7.85 t ha⁻¹year⁻¹) under integrated balanced fertilization with manures in comparison to treatment with N alone (5.21 t ha⁻¹year⁻¹). The average crop biomass C was increased by 40–77% due to balanced fertilization compared to the unfertilized control [55]. Similarly, in a 7-year trial, integrated use of inorganic fertilizer and FYM sequestered 0.44 t ha⁻¹ C in the surface soil under a rice–wheat system [56]. Carbon sequestration could be as high as 1.53 t ha⁻¹ with the integrated application of rice straw and FYM applied with inorganic fertilizers annually. Long-term addition of FYM and crop residue along with fertilizer N caused the highest (83.5%) increase in SOC pool in soils under a rice–wheat cropping system [57]. Results of a long-term fertilizer experiment indicate that the C sequestration rate was higher in the initial years. For example, the rate of C sequestration (Mg C ha⁻¹year⁻¹) for the initial 9 years under NPK application was 0.37; it declined to 0.24 by the 15th year of the long-term experiment (Figure 5) [47]. This indicates that soils have a defined capacity to accommodate C depending upon soil properties, management practices, and climate.

Figure 5. Effect of duration of fertilizer application on soil C sequestration under a rice–wheat system (reconstructed from [47]).

Mazumdar et al. [58] explored the importance of long-term balanced fertilization (in a 43-year experiment) on soil C stabilization in a rice–wheat cropping system. They observed that balanced fertilization along with FYM significantly improved the total soil organic carbon (TSOC) over control. Integrated nutrient management enhanced SOC content and stock, and the soil C management index, in rice cropping systems in southern China [59].

In comparison to solitary application of NPK in a rice–wheat system, adding organic amendments over 10 years increased SOC stocks the most with rice straw compost (RSC) (12.2 t ha⁻¹), followed by FYM (9.1 t ha⁻¹); ad the lowest results were achieved with vermicompost (VC) (8.5 t ha⁻¹). The discrepancies between RSC and FYM were attributable to RSC’s lower C/N ratio and higher lignin concentration. The findings revealed that compost addition increased C stabilization via imparting recalcitrance to SOC, in addition to causing C build-up. This shows that composting rice straw before applying it to the soil could improve soil C stability while also conserving the environment [14]. Cropping with only NPK fertilization simply maintained SOC content, while NPK plus organics increased SOC by 24.3% over the control in a rice–wheat system [60].

Agronomic practices that can be helpful in SOC sequestration include: adoption of no-tillage (NT) or minimum tillage, incorporation of cover crops, use of mulch in the form of either crop residues or synthetic materials, and adoption of integrated nutrient management practices. Crop residue management is the key to soil structural development and stability, since organic matter is an important factor in soil aggregation. Residue incorporation or retention resulted in a significant increase of 15.65% in total water-stable aggregates in surface soil (0–15 cm) indicating that residue management could improve water-stable aggregates [61]. The rice residue mulch may have improved soil structure by stabilizing aggregates and protecting SOM against microbial degradation, and reduced the rate of SOC decomposition [62], which ultimately led to a significant increase in TSOC content.

Plowing is the basic cause of SOC oxidation. Conventional tillage (CT) practices led to a decline in soil C of to 20% [63]. Application of zero tillage (ZT) resulted in 46.5% higher water-stable macroaggregates in the surface soil as compared to CT [64,65]. The decline in the size of macroaggregates in CT could be credited to the disruption of the macroaggregates, which may have exposed previously protected SOM to oxidation. The macroaggregates are highly susceptible to oxidation, but, simultaneously, they are rich conservers of SOC. Their presence in higher proportions ensures more C sequestration and nutrient availability by regulating proper aeration and water infiltration within the root zone. A large number of studies have shown that NT can increase soil C rapidly, particularly at the soil surface, and this increase is linked to increases in aggregation [62,66,67]. Other studies demonstrated the benefits of reducing intensity and frequency of soil disturbance – leading to an increase in SOC content by 17–56% (Table 2) – to improve soil quality and agricultural sustainability.

Table 2. Effects of alternative management on soil organic carbon (SOC) content.

Location	Cropping system	Duration (years)	SOC (% of conventional tillage) after specified duration	Reference
			Alternative management (no tillage, residue incorporation)
Bihar, India	R-W	6	56	[68]
Sheikhupura, Pakistan	R-W	5	17	[69]
Patna, India	R-W	3	18	[70]
Uttranchal, India	R-W	4	26	[71]
Chitwan, Nepal	R-W	3.5	28	[72]
Delhi, India	R-W	4	26	[73]

Carbon sequestration is improved by the intensive cultivation of a rice–wheat sequence because of enhanced crop yield, C transport to roots, and decreased organic matter oxidation during the anaerobic condition in paddy soils [74]. The differences in the soil environment under rice–wheat cropping could lead to differential stabilization of SOM in different pools [75]. Some reports showed both a positive and a negative influence on C sequestration by the rice–wheat cropping system. The effects of cropping systems on the quality and quantity of SOC are not uniform. For example, although SOC stocks in maize–wheat and agroforestry systems were higher by 65–88%, about 56–60% of the TOC was labile C compared to the rice–wheat sequence [76]. Decreasing or paused crop yield indicates that the sustainability of the rice–wheat cropping system became critical [77]. Furthermore, a gradual decline in the nutrient supplying capacity of soils, and alterations in quantity and quality of SOM, can also correlate with declining or paused yield trends of rice–wheat rotation [77–79]. Some studies attributed the decreased productivity of the rice–wheat system to declining SOM content and diminished soil fertility [78,79].

Meanwhile, various studies report an improvement in soil C stocks under a rice–wheat sequence in the IGP [3,36,80]. Soil C stocks increased significantly at both surface and sub-surface depths following balanced fertilization along with FYM in 5 years of a rice–wheat cropping system [74]. Any change in land management can alter soil C stocks, which is at a relatively more steady state in agricultural systems [81]. At a regional level, SOC content increased by 38% after intensive cultivation of a rice–wheat cropping system for 25 years in Punjab state in India [80]. In contrast, another study revealed the decreasing SOC responsible for decreasing crop productivity of the rice–based cropping system in the IGP of India [82]. About 26% of C was transported to the soil stores in the surface soil, and the rest was released from the soil by root and microbial respiration [83]. A comparison of these estimates with numerous published reports implies that assessment of C stock is influenced by crop growth stages, environmental status, soil type, and microbial activity.

Root and leaf plus stubble biomass adds a considerable amount of C to the soil. This biomass production depends on different management practices. Integrated use of NPK and FYM enhanced root yield of rice and wheat crops by 132% and 135%, respectively, over the control [74]. Leaf and stubble biomass in rice and wheat improved by 0.53 and 0.32 Mg ha⁻¹ (Figure 6). These additions ultimately increase C stocks in the soil. Tong et al. [84] also found that the integrated application of fertilizers and organic manures increased root and leaf stubble biomass.

Figure 6. Total organic stocks in different treatments through annual addition from root and leaf plus stubble biomass and farmyard manure under a 5-year long-term rice–wheat cropping system (Reconstructed from [74]).

Bulk density considerably influences C sequestration by changing soil porosity. A number of studies have reported an increase in C stocks with an increase in bulk density, or vice versa [36,57,85,86]. In a long-term rice–wheat system, Brar et al. [36] reported a higher increase in C content (10.7%) as compared to C sequestration (8.7%) with the application of balanced use of fertilizers along with organic manures over NPK alone. This was due to the lower bulk density in the integrated nutrient mangement (INM) treatment compared with NPK alone. Other studies have also reported a decrease in bulk density due to the application of FYM alone or along with balanced fertilizer application.

Nonetheless, adoption of an integrated nutrient management strategy that uses organic sources, and balanced application of inorganic fertilizers and adoption of CT interlinked with residue management, can increase SOC and sequestration and the sustainabilty of the rice–wheat production system.

Physical fractions of SOC

Many studies have reported the influence of long-term management practices on qualitative changes in SOC as a whole, but quantitative analysis of SOC fractions is rare [87]. The total SOC is not always a sensitive indicator to assess management-induced changes in SOC [19], because soil C is a dynamic and complex material composed of various fractions that differ in their physical, chemical, and biological degradation [32]. The quantification of SOC stabilized in individual particle-size fractions, rather than the total SOC pool, may better reflect the management-induced changes in SOC dynamics. Therefore, a better comprehension of the effects of long-term management practices on the chemical composition of SOC and its physical fractions is essential. The study of physical fractionation in SOM turnover has increased over the past few decades. This is because the turnover is due to biological processes under the overall regulation of soil structure and because the availability of substrates to decomposers depends not only on the intrinsic chemical nature of the substrate but also, and more importantly, on the nature of its association with the soil’s mineral components. This association is manifested in the formation of organomineral complexes. Thus, mechanisms responsible for the retention of OM in soil include inherent chemical recalcitrance of the organic component, stabilization of potentially available OM by chemical reactions with mineral surfaces, and protection of substrates through the creation of physical barriers between substrates and decomposer organisms.

Physical fractionation of soil according to the size and density of particles is achieved by applying various degrees of dispersion to break bonds between the elements of soil structure, and it allows the separation of un-complexed OM and of variously sized organo-mineral complexes (Figure 7). On the basis of size and/or density of soil constituents, these are: (i) un-complexed organic matter/particulate organic matter (POM), (ii) primary organo-mineral complexes and (iii) secondary organo-mineral complexes/aggregates [88]. POM, a primary energy source for heterotrophic microorganisms and a reservoir of labile C [89], is more strongly influenced by land use and soil management practices than the total soil organic matter (SOM) pool is. Particulate organic carbon (POC), a physical fraction, has been considered one of the sensitive indicators of soil management effects on SOC [90].

Figure 7. Physical fractionation of soil organic matter. Reconstructed from [88]

All physical fractions of SOC respond differentially to the alterations caused by different nutrient management technologies. The effects of fertilizer management on physical fractions of C in the soil are not consistent. For example, the continuous application of chemical fertilizer alone may or may not influence physical fractions of SOC compared with zero fertilizer control [91,92]. Soil type may change the intensity and direction of the impact caused by integrated application of straw and chemicals on physical fractions of C, where the effect was positive for an Inceptisol and negative in a Mollisol [91].

In contrast, all SOC fractions in a Ultisol [84] and an Inceptisol [91] were increased significantly solely by manure application. That soil aggregation and aggregate stability are improved by the addition of crop residues and organic manure is a well-known fact [93,94]. The influence of residues on soil aggregation depends upon soil texture. For example, macroaggregates in a loamy soil increased with residue incorporation [95], whereas water-stable aggregates were unaffected [96]. Application of organic along with inorganic fertilizer was more beneficial in improving the physical C fractions in comparison to inorganic fertilization alone [54].

Moreover, the continuous application of organic manures in rice–wheat for about 7 years positively affected the C associated with different aggregate sizes fractions [56]. All aggregate fractions are not equally capable of accumulating and protecting soil C from degradation. The TOC has a positive and linear correlation with the proportion of C associated with macroaggregates [74]. The concentration of organic C in different sized aggregates and the mineral fraction increased significantly with the application of inorganic fertilizers and FYM, compared with the unfertilized control, in a loamy soil under a rice–wheat system [74].

In a long-term experiment on a rice–wheat system, the coarse particulate organic carbon (cPOC) stocks in the 0–7.5 cm soil layer were higher by 22.1–33.8%, with a greater increase in RSC (33.1%) and FYM (33.8%) compared to VC and INM. The increase in fine particulate organic carbon (fPOC) stocks with different treatments ranged from 23.2 to 68.9% in the 0–7.5 cm layer. The mineral-associated organic carbon (MinOC) responded significantly (p < 0.05) to the application of organics and was higher by 21.9–31.1%, 36.2–51.3% and 10.7–32.8% in FYM-, RSC- and VC-amended plots, respectively, compared to NPK (2.81–3.16 g kg⁻¹) treatment [14]. These findings are consistent with previous studies [57,74,97], in which application of organic sources enlarged cPOC and fPOC pools. Improvement in MinOC with the application of organic amendments compared with solitary application of NPK, which is in agreement with published research [57,84], could be ascribed to the chemical stabilization of SOM as a result of the formation of new organo-mineral complexes on the previously free mineral surfaces.

Soil aggregation and C build-up in response to tillage and residue management practices have been studied adequately [98,99]. In a study, conservation tillage (both reduced and ZT) caused 21.2%, 9.5%, 28.4%, 13.6%, 15.3%, 2.9% and 24.7% higher accumulation of SOC in >2, 2.1–1.0, 1.0–0.5, 0.5–0.25, 0.25–0.1, 0.1–0.05 and < 0.05 mm particles, respectively, than conventional tillage [61]. In comparison to residue management, tillage management had a significant impact on soil aggregation. Application of ZT with or without residue resulted in 46.5% higher water-stable macroaggregates in the surface as compared to CT. Comparatively limited research explained the influence of crop residue and manure application on C stabilization in different aggregate size fractions in rice–wheat.

Manure application facilitates the development of macroaggregates by reducing the proportion of micro-aggregates [100] that depends on the kind of binding agents. The higher stability of binding agents in micro-aggregates make them relatively more resistant to the destructive forces of tillage than macro-aggregates [101]. The presence of mucilaginous substances in soil binds micro-aggregates with macro-aggregates and protects them from degradation. The tillage disrupts water-stable aggregates (WSAs), and increased exposure of aggregate-protected C to degradation agents [61].

The tillage operation under soil wetting and drying conditions results in the breaking down of macroaggregates to microaggregates in the rice–wheat system. Figure [74 In another study, integrated nutrient management showed better results for associated C as compared to control [74, fig. 8]. Nevertheless, puddling made the distribution of SOC more uniform and improved the interaction between C and soil mineral components [102]. Moreover, in the paddy soils, puddling reduces soil aggregates to fine particles which form a coating on organic particulates [103].

Figure 8. Macro-aggregate (MacAC), micro-aggregate (MicAC) and mineral-associated C (MAC) influenced by different management practices under the R-W system (reconstructed from [74]).

In general, INM as well as conservation agriculture results in more organic matter being added in the soil and increases the C storage capacity of soil by improving soil aggregation, and vice versa. The MinOC resulting from the decomposition of POM provokes the development and stabilization of macroaggregates [94]. MinOC has recalcitrant biochemistry; therefore, it is more physically protected from degradation in soil [104].

Chemical fractions of SOC

Based on chemical extractants, SOC can be grouped into labile C, less labile C, and recalcitrant C pools (Figure 9). Recalcitrant C pools are relatively more stable and are unaffected by short-term management practices. These pools are a prerequisite for C sequestration in soil. In contrast, labile and less labile C pools are sensitive indicators that decompose/change readily with different management practices and also serve as a nutritional reservoir for soil microbes. The important labile C fractions include water-extractable organic C (WEOC), hot water-extractable C (HWEC), potassium permanganate oxidizable organic C (KMnO₄-C), and organic C fractions oxidized by acids of gradient strength [18,76]. Variation in the quality and quantity of C input to the soil can immediately impact these labile pools [53]; thus they can be considered reliable indicators of changes in SOC.

Figure 9. Different oxidizable soil organic carbon fractions. Reconstructed from [18]

A rice–wheat system under balanced fertilization sequestered relatively more recalcitrant C fractions compared to an imbalanced use of fertilizer. After five cycles of rice–wheat, most of the TOC was in stabilized pools, and approximately 38% was in the labile C pool [93]. Joint use of mineral fertilizers and organic manures (FYM/GM/straw incorporation (SI)) improved all the C fractions of different oxidizability [105] under a long-term rice–wheat cropping system (Figure 10). There was a significant increase in WEOC and KMnO₄-C with balanced fertilizer application as compared to imbalanced fertilizer application. FYM-treated soil resulted in higher WEOC concentration because of the higher content of soluble organic C in FYM [107]. WEOC has a rapid turnover rate because of the arrangement of the molecules that remains in a soluble form [108], responding immediately to the addition of organic inputs [107]. The concentration of WEOC is also sensitive to plant-induced changes as increased rhizodeposition could also contribute to this pool [109]. Soil KMnO₄-C comprises amino acids, simple carbohydrates, a portion of microbial biomass, and other simple organic compounds [110]. It was reported that RSC addition significantly increased KMnO₄-C under a rice–wheat cropping system [111]. Studies revealed that under a rice–wheat cropping sequence, the addition of FYM to soil made a substantial contribution to the KMnO₄-C pool [106,112]. Manure-applied soil showed a higher concentration of KMnO₄-C and HWEC because manure contains higher labile C compared with inorganic fertilizer [113,114].

Figure 10. Long-term use of fertilizer and organic amendments on oxidizable soil organic carbon fractions (g kg⁻¹) under an R-W cropping system (reconstructed from [105]).

Several studies indicated that the use of organic material positively influences Walkley and Black C (WBC) [115]. Easily oxidizable C (EOC) and dissolved organic carbon (DOC) were significantly affected by straw treatments in a rice–wheat rotation in the Yangtze River Delta of China [116]. Optimal application of manure and fertilizers adds a higher amount of residue inputs, leading to the build-up of the KMnO₄-C fraction of SOC [12]. The long-term use of FYM in the rice–wheat cropping system has a positive effect on the KMnO₄-C fraction [1].

NPK fertilizer combined with FYM, paddy straw (PS), or GM caused an increase in oxidizable C by as much as 12.4 and 20.9%, 10.4 and 18.8%, and 8.2 and 16.4% over that in the NPK and control treatments, respectively [60].

In a long-term experiment with a rice–wheat system, organic amendments and INM significantly increased non-hydrolysable carbon (NHC) stocks compared to NPK-treated plots. The increase in NHC stocks with the application of FYM was similar to that with INM (1.3 t ha⁻¹). In the INM treatment, the proportion of NHC grew by 25.2% over NPK [14]. The increase in NHC with the application of organic amendments is in line with published studies [117–119]. In a semi-arid region, on a sandy loam soil, the labile OC pool increased by 23% and the water-soluble organic C pool increased by 38.6% in INM compared to NPK alone [47].

It has been found that in a rice–wheat system, residue retention is better for both crops than conventional farming. In a rice–wheat system, results indicated improvement in all oxidizable components of OC in conservation agriculture as compared to conventional farming [68]. Conservation agriculture in a rice–wheat system increased the labile OC pool by 55% and the water-soluble organic C pool by 13.9% compared to conventional tillage [62]. Conservation agriculture promotes a continuous supply of fresh organic matter, often beyond the capability of microbes to act upon for humification or mineralization. This enhances the SOC lability. Several other researchers [71,120] reported a greater impact of residue retention and tillage reduction on the lability of SOC.

Very labile and labile C pools under puddled transplanted rice with 25% wheat stubble (PTRWS25)+GM increased by 80.5% and 63.7%, respectively, compared with puddled transplanted rice with no wheat stubble (PTRWS0). Similarly, very labile and labile C pools increased by 33.0% and 79.0%, respectively, under zero tillage with 100% rice straw retained as a surface mulch (ZTWRS100) compared with conventional tillage with rice straw removed (CTWRS0) [62].

A 26-year long-term rice–wheat experiment with different nutrient management practices showed a higher proportion labile C pool followed by very labile, non-labile, and less labile C pools, constituting about 46, 26.5, 20 and 7.3%, respectively, of the total organic C [121]. Conjunctive use of fertilizer and FYM resulted in significantly higher KMnO₄-C (59.45%) over control. The higher concentration of KMnO₄-C in fertilized crops having improved root and shoot biomass could be related to the higher microbial activities and secretion of root exudates [122]. Irrespective of the fertilizer management, the mean concentrations of different fractions of SOC were: very labile fraction (35.5%) > labile fraction (24%) > less labile fraction (23.8%) > recalcitrant fraction (16.6%). It was indicated that the influence of organic manures on soil LC is not uniform and the effect of FYM could be as high as 80.6% over control [122].

Categorizing SOC into various chemical fractions based on ease of oxidizability helps in studying the management effects on C dynamics and stability even on a short-term basis. Application of FYM and balanced fertilizer and conservation agriculture could improve both labile and recalcitrant C fractions and C sequestration.

Biological fractions of SOC

Cropping systems and management practices that ensure higher amounts of crop residue are returned to the soil are expected to cause a net build-up of the different C pools. Identification of such systems or practices is a priority for sustaining crop productivity and soil fertility. One study reported a higher proportion of recalcitrant C pool under a rice–wheat system in comparison to other land-use, indicating that C pools were more exposed to management practices in latter [76]. The categorization of SOC into different pools with varying residence times helps in understanding the processes of C sequestration in soil [123]. Organic matter can be partitioned into two main pools, i.e. recently added organic material such as plant residue or litter, and native SOM. Each of the main pool is further subdivided into different fractions or components. A generalized scheme of organic matter partitioning into different pools is presented in Figure 11 [124]. Plant residue or litter is generally divided into two compartments: “metabolic” or “labile” or “decomposable plant material” and “structural or resistant plant material.” The native SOM pool is further divided into soil microbial biomass (SMB) and one or more pools of dead SOM. The SMB pool is further subdivided into two or more components, such as non-protected (labile, dynamic) and physically protected (resistant or stable) biomass; cell walls and cytoplasm; or labile cell C and assimilated live biomass; or active and inactive biomass. The dead SOM is further divided into two or more pools based on stabilization mechanism, bioavailability, and biochemical and kinetic parameters. Generally, it is divided into “slow” or “physically stabilized” pools with turnover times of a few decades and “passive” or “chemically stabilized pools” that remain in soil for hundreds or thousands of years.

Figure 11. Schematic representation of organic matter partitioning into conceptual pools (reconstructed from [124]).

The labile C pool acts as a source of food for these soil microbes; therefore, it plays an important role in regulating the nutrient supply to maintain soil and crop productivity [18]. Soil microbial biomass C (MBC) and mineralizable C (Cmin) are considered critical biological fractions of SOC pools which are very dynamic and sensitive to different management practices [8,18,108]. Besides these biological and labile pools of SOC, activities of some enzymes such as dehydrogenase, phosphatase, β-glucosidase, and urease may also be used for the assessment of SOC [125]. Like SOM and microbial biomass, dehydrogenase activity is more often used to study the C mineralization and immobilization in soils. Dehydrogenase enzymes are involved in oxidation processes in soils and are critical indicators of microbial metabolism rates in soils [126]. Therefore, these biological C pools may be more useful in understanding the soil C dynamics in response to more effectivemanagement practices. However, there has been inadequate research on the effects of different long-term nutrient management practices on biological pools of SOC in the rice–wheat system.

Dehydrogenase activity was improved by the application of organic manures [127], whereas effect of soil tillage was not consistent [128]. Dehydrogenase enzyme activity increased by 488% with FYM + NPK treatment over the control in the 15th year under a rice–wheat cropping system Figure 12 [114]. Soil enzyme activities are easily affected by both natural and anthropogenic disturbances and are very receptive to the enforced changes [129]. The enzyme activities are susceptible to management technologies such as tillage, crop rotation, crop residue, organic manure, and organic farming [130,131].

Figure 12. Long-term effect of different management practices on soil dehydrogenase activity in an R-W cropping system (reconstructed from [114]).

It is a well-known fact that soil productivity is positively affected by crop rotation and organic matter additions to the soil. It is thought that this may result from increased fertility, microbial biomass, and microbial activity. It is thus critical to analyze the influence of different agricultural management practices on microbial responses, including soil enzyme activities that may affect SOM turnover. Because of concerns about climate change, such research will be beneficial in the development of strategies that will enhance C sequestration in soil.

The active C pool with short turnover times includes soil microbes and microbial products, and the passive C pool, that is very resistant to decomposition, includes physically and chemically stabilized organic C [132]. Carbon added through organic amendments is distributed into different C fractions in soil depending upon climatic conditions, agronomic practices, amendment composition, and soil type. Rhizodeposition, FYM, and residue application increase the available C concentration in soil. After mineralization of C sources, the microbial biomass is expected to be low because of temporarily released microbial biomass C, thus affecting crop and soil productivity in addition to environmental quality. Soil microbial biomass has a rapid turnover rate and response to management-induced changes in soils. The ratio of microbial biomass C to total C could be a useful criterion to quantify C-efficient cropping and tillage systems [133]. Studies indicated that the addition of organic manures like GM and crop residues along with inorganic fertilizers resulted in a considerable increase in microbial biomass C as compared to chemical N fertilizers alone [134,135].

Mineralizable C content was higher in NPK + FYM (12.4%) and fallow (12.3%) than in the other treatments, which may be due to the adequate supply of labile C substrate. Similarly, Lu and Lu [136] found that long-term mineral and organic fertilization had a positive effect on MBC. The mineralizable C content of the soil varied from 1.15 to 1.92, with a mean value of 1.65 g CO₂-C kg⁻¹ soil, constituting about 11.4% of the total C. This was higher in NPK + FYM (12.4%) and fallow (12.3%) than in the other treatments, and a similar trend was observed for basal soil respiration (BSR) [60]. On the contrary, the metabolic quotientqCO₂ was higher under the control and NPK than in the NPK plus organic treatments [60]. The availability of easily decomposable organic matter and readily available nutrients provide a conducive environment for microbial activity, resulting in a higher rate of respiration. Mineralizable C acts as a sensitive indicator of the effects of different management practices on soil quality [137].

Improvement of MBC in FYM- and straw-amended soils may be attributable to the ready availability of substrates from these amendments. Apart from depending on management, the magnitude of the effectiveness of the soil microbial biomass also relied on the amount and composition of SOM and environmental factors [138]. The addition of organic manure improves labile C and MBC of soils and leads to a higher basal soil respiration, C mineralization rate, and potentially mineralizable C compared to untreated soils. In unamended soils, higher qCO₂ could be the result of stress on the soil microbial population due to competition for substrate and diversion of energy from growth, maintenance, and reproduction [139]. The adoption of conservation agriculture in one study showed that SOC content increased by 13.3%, MBC by 38.5%, BSR by 32.6%, microbial quotient by 29.6% and mineralization quotient by 15.8% over conventional practices after 5 years of continuous rice–wheat cropping [140]. Therefore, conservation tillage can be speculated to promote the accretion of organic matter in the top soil layer, which leads to high microbial growth [141]. PTRWS25 + GM significantly increased dehydrogenase, beta-glucosidase, xylanase, and cellulase activities, by 32.2%, 52.5%, 125%, and 59.5%, respectively, compared with PTRWS0 in a rice–wheat system [62].

The balanced use of fertilizers along with organic sources like FYM and GM and residue management in conservation agriculture influences biological pool accrual and C dynamics in a rice–wheat cropping system.

Conclusions

Long-term integrated use of inorganic fertilizers and organic manures in a rice–wheat cropping system has a significant impact on soil C sequestration by altering the allocation of C among labile and recalcitrant C fractions. The continuous adoption of this cropping system even without any fertilizer application contributed toward C sequestration. The crop residue and manure inputs changed the distribution of C density fractions in soil. Organic matter inputs first caused the labile C pool to be enhanced, and then increased the more stable C fractions. Most of the SOC pools are interrelated and varied in proportion. The total SOC is not always a sensitive indicator with which to assess management-induced changes in SOC [19], because soil C is a dynamic and complex material composed of various fractions that differ in their physical, chemical, and biological degradation. The stability of physical and chemical C fractions varies depending upon management practices and the varying amounts of C added. Microbial population and activity are improved by alteration in SOM as a result of fluctuation in substrate supply, oxidation status, and physical properties of soil under the rice–wheat system. Conservation management systems such as reduced tillage and NT and crop residue addition increased SOC accumulation and improved the sustainability of agricultural systems. NT increased soil aggregation, and favorably influenced SOC accretion.

West and Post [98] demonstrated that transitioning from conventional tillage to NT could result in SOC sequestration of 0.57 ± 0.14 Mg C ha⁻¹year⁻¹; and in a rice–wheat system, reducing the intensity and frequency of soil disturbance increases SOC content by 17–56%. Applications of fertilizers with FYM and crop residues increased stubble and root biomass yield, and improved SOC accumulation, in some cases by up to 84% of the SOC stock in typically disturbed soils. The use of chemical fertilizers along with organic manure has increased SOC concentration by 13–232%, and thus could be a feasible option for restoring SOC and nutrient turnover, thereby improving the availability of nutrients in the soil, maintaining soil quality, and helping achieve sustainable productivity of rice and wheat crops in the long run. Both labile and recalcitrant pools of SOC improved under the long-term application of organic sources, emphasizing the need for continued use of organic amendments to enhance or sustain soil C status.

Evaluating the SOC dynamics of a long-term rice–wheat system under present and projected climate change scenarios, alternative management practices, and their potential impacts on agricultural system sustainability would substantially benefit producers, researchers, and policymakers. Improved understanding of SOC dynamics and soil–plant–atmosphere interaction of GHGs in the rice–wheat system would help in estimating the global warming potential of this agroecosystem around the world. More research evaluating the impacts of alternative management systems on SOC dynamics and GHG emissions is required. The study of different management practices suggests that application of organic manures along with a balanced fertilizer dose plays a great role in increasing the C pools; however C sequestration under conservation agriculture has been studied widely in temperate conditions whereas few studies have been reported from tropical and sub-tropical regions. Therefore, more studies should be carried out to investigate the effect of conservation agriculture on C sequestration as well as on SOC pools.

Disclosure statement

No potential conflict of interest was reported by the authors.