चावल-गेहूं की फसल प्रणाली के परिप्रेक्ष्य में मिट्टी की उर्वरता पर फसल अवशेषों का प्रभाव

Crop residues (CRs) are considered a vital natural resource for conserving and sustaining soil productivity. Addition of CRs to soil is a useful tool in maintaining and increasing amounts of soil organic matter. Therefore, soils have significant capacity for C storage and to mitigate atmospheric CO₂. As rice-wheat cropping system (RWCS) is one of the world’s largest agricultural production systems, covering an area of ~26 M ha spread over the Indo Gangetic Plains (IGPs) in South Asia and China.

Rice and wheat are harnessing enormous soil fertility therefore, to maintain the productivity of this system, replenishment of soil nutrients is necessary. It has not only resulted in mining of major nutrients (NPKS) from soil, created a nutrient imbalance, leading to deterioration in soil quality. Deficiencies of NPK are most extensive. One ton of wheat grains are estimated to remove 24.5, 3.8, and 27.3 kg N, P, and K, respectively, whereas similar production of rice grains removes 20.1 kg N, 4.9 kg P, and 25.0 kg K. Because of agricultural strength of the country, crop residues (CRs) production is also huge.

In India, over 500 Mt of agricultural residues are produced every year. Punjab alone produces ~20.8 Mt of rice residue and 23.3 Mt of wheat residue annually. Although during last three decades fertilization practices have started playing a dominant role in the RWCS, crop residues still play an essential role in cycling of nutrient. With availability of 37.87 Tg of rice and wheat residues for recycling, associated nutrient (N + P + K) potential was 0.634 Tg. RWCS accounts for nearly one-fourth of the total crop residue production in India.

One ton of rice residues contain ~ 6.1 kg N, 0.8 kg P, and 11.4 kg K, while one ton of wheat residues contain ~4.8 kg N, 0.7 kg P, and 9.8 kg K. Integrated uses of plant nutrients with mineral fertilizers have either maintain or enhance soil quality and improve performance of crop along with cropping system. Therefore, proper crop residue management (CRM) can play an important role in increasing  soil organic matter and nutrient supplying capacity, reducing ill effects of residue burning, as this leads to destruction of SOM as well as plant nutrients i.e. NPKS.

Incorporation of crop residues alters soil environment, which in turn influences microbial population/activity in soil and subsequent nutrient transformations. Through this chain of events, management of crop residues regulates efficiency with which fertilizer, water, and other reserves are used in a cropping system. No single residue management practice is superior under all condition.

Therefore, it is important to determine benefit and adverse effect of residue management options before these are recommended to the farmers for adoption. Research carried out in last few decades relating residue management to soil chemical, physical, and biological properties and consequent fertilizer management practices in RWCS provides valuable directions for efficient management of crop residues in rice-wheat cropping.

 Importance of residue retention in field in relation to C and N mineralization

Upon mineralization, CRs also supply essential plant nutrients. So, recycling of CRs is suggested as a potential means of sustaining soil fertility and productivity over long-term For recycling crop residues, in situ incorporation and mulching with reduced or no tillage are the major residue management options. Management of CRs in conservation tillage has crucial effect on soil C and N dynamics, a better knowledge about CRs decomposition and N mineralization dynamics of residue C and N is essential to quantify potential benefits of changes in tillage practices and residue management on soil quality and crop production.

CRM as practiced  in RWCS is of three types (1) wheat straw management in rice and its residual effect in following wheat, (2) rice straw management in wheat and its residual effect in following rice (3) wheat straw management in rice and rice straw management in wheat (cumulative effect).

Incorporation of CRs provides readily available C and N to soils depending upon the decomposition rates and synchrony of nutrient mineralization.Scientists reported that rice residues incorporation increased organic carbon content of sandy loam soil more significantly than straw burning or removal after 7 years (Table 1).

Table 1. Effect of crop residue management on SOC (%) and Total N (%)

There are two indices to calculate a carbon management index (CMI) with two fractions of organic carbon in soil. The more labile fraction (CL) was measured by oxidation with 333 mM KMnO₄, and the nonlabile C (CNL) plus the C not oxidized by 333 mM KMnO₄, (i.e., CT-CL). The total C (CT) was measured by combustion. On the basis of changes in CT between a reference site and the cropped site, a carbon pool index (CPI) was calculated:

CPI = CT_cropped/CT_reference

On the basis of changes in the proportion of CL in the soil (labiality = LI = CL/CNL), a labile index was determined.

 CMI = CPI X LI X 100

Incorporation of leaf litters increased the CMI from 9 in 1992 (initial) to about 20 after 3 years in 1996 and CMI in no-litter treatment increased to 13. Straw incorporation did not significantly affect the CT (4.44 versus 4.11 mg g^-1) and CL (0.78 versus 0.79) compared to straw removal treatments. The measurement of CL is a more sensitive indicator of SOM dynamics.

Total C measurement is still required to estimate bulk soil C change; however, CL more accurately and quickly detects the impact of management on soil C. Calculation of the CMI takes into account the change in CT pool size and its lability and gives a more definitive picture of soil C dynamics than when only a single parameter is used.

The quantity of SOM is not the sole factor that should be considered when devising management practices to optimize the agronomic benefits of SOM. A higher quantity of SOM does not automatically lead to a higher quality of SOM. It remains a difficult task to identify and quantify the intrinsic quality of an SOM pool in terms of nutrient supply power, microbial activity, or physical or chemical indices.

Labile SOM pools are key suppliers of nutrients to the crop, whereas other SOM pools are more recalcitrant in nature and will provide fewer nutrients, but their chemical and physical properties provide stability to the soil. The studies on soil organic matter dynamics suggest that soil texture, C inputs, and climatic conditions are the primary factors controlling stabilization of soil C.

The reduced soil C sequestration in the rice-upland rotation resulted primarily from an increased amount of microbially mediated C mineralization compared to the C mineralization rate in the rice–rice system.

A simplified model of the regulation of nutrient flux in the agoecosystem is presented in Figure 1. This conceptual model depicts the flow of carbon and nutrients among organic residues, organic and inorganic pools in soil, and the plant. Pathways of loss are also included. Decomposition and mineralization of plant residue are mediated by both soil faunal and microbial populations.

Some of the carbon and associated nutrients are mineralized immediately (pathway 1a) or are immobilized in the soil microbial pool (pathway 2a), later to be transformed into other soil organic pools via microbial by-products (3a). Recalcitrant plant material also may enter the soil organic pools directly (3b). The carbon and nutrients held in the various soil organic matter pools are subsequently decomposed and assimilated by soil biomass, resulting in

Figure 1. Conceptual model of nutrient pathways in crop residue amended soils

additional mineralization (1b). The inorganic nutrients released by mineralization may be assimilated by soil biota via immobilization (2). Immobilization occurs simultaneously with mineralization, and the rate at which nutrients are available for plant uptake depends on the net balance between mineralization (1a plus 1b) and immobilization (2).

The inorganic nutrients may also be taken up by plants (pathway 3), lost by leaching or volatilization (pathway 4), or remain in the soil. The size of the inorganic pool depends on the balance of the various processes that add to the pool (mineralization) and those that subtract (immobilization, plant uptake, and losses).

The proportion of N transferred from the residue to the plant and the rate at which it occurs are determined by the balance between the rates of the various processes represented by these flux pathways. This balance is regulated by a hierarchy of factors. Environment, which includes climate and soil, is an overriding control and determines the rate of the transfer betweenpools.

The rates also vary depending on the quality of the decomposing substrate. By manipulating the quality of crop residues, it should be possible to manage nutrient release to coincide with the time course of the nutrient requirements of the crop.

When low-quality crop residues (low N and P, high lignin or polyphenol contents) are incorporated into the moist soil, nutrients become available to the plants. With high-quality residues, nutrients are initially released rapidly in excess of plant demand with a risk of nutrients such as N being lost via leaching or denitrification or a nutrient such as P becoming chemically unavailable.

About 70% of the rice lands in south and south-east Asia contain<0.2% N and are considered N deficient. Incorporation of crop residues enhances the N content of several wetland rice fields. Within 3 years of incorporating the rice straw at 6–7 t ha^-1, total N content in soil increased by 0.021% over the straw removal treatment.

Conclusion

The intelligent management and utilization of crop residues is essential for the improvement of soil quality and crop productivity under rice-wheat cropping systems of the tropics. Crop residues, usually considered a problem, when managed correctly can improve soil organic matter dynamics and nutrient cycling, thereby creating a rather favorable environment for plant growth.

Due to intensive cropping of rice-wheat system prevailed in South Asia region it is necessary to manage the huge quantity of its residues, which are the good source of carbon, nitrogen and potassium. Greater knowledge in this area should improve our ability to manage soil nutrients efficiently.

Author:

Dr. Kirti Saurabh

Scientist, Division of Crop Research,

ICAR- Research Complex for Eastern Region,

Patna-800 014, Bihar, India

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