A two-cycle (2006–2008) organic field experiment at the Indian Agricultural Research Institute, New Delhi, found that diversifying an organic rice–wheat cropping system (RWCS) with summer mungbean (giving a rice–wheat–mungbean system, RWMCS) and incorporating crop residues jointly raised system productivity, protein yield, energy output, soil organic C, N, P and K, and culturable soil microbial populations, and recommended organic RWMCS with residue incorporation as an alternative to organic RWCS in the Indo-Gangetic Plains (IGP). That recommendation rests on two cropping cycles in one field block at one site, with one cultivar of each crop (rice cv. Pusa Basmati 1, wheat cv. HD-2643, mungbean cv. Pusa Vishal); the soil-biological advantage of the mungbean-inclusive system was reported without any tested explanation, and the economics were computed only as net returns within the organic systems over the same two cycles. The study therefore leaves open whether the superiority of organic RWMCS with residue incorporation — in productivity, protein and energy output, net returns within the organic systems, soil fertility and soil biology — persists and generalizes beyond that two-cycle, single-site, single-cultivar setting, and whether the observed soil-biological benefits can be explained.

Background

The rice–wheat cropping system (RWCS) — rice grown in the wet summer season followed by wheat in the dry winter season — occupies roughly 10–13 million hectares of the Indo-Gangetic Plain (IGP), the alluvial plain of the Indus and Ganges rivers spanning Pakistan, India, Nepal and Bangladesh, and supplies a large share of South Asia's food grains. Decades of continuous cereal–cereal double cropping, largely with mineral fertilizers, have led to declining soil organic matter, multi-nutrient deficiencies and stagnating yields, so agronomists have sought more sustainable rotations and nutrient sources. Two candidate remedies are central here. First, diversifying the rotation with a summer grain legume such as mungbean (Vigna radiata) adds biologically fixed nitrogen and a legume residue: the ratio of carbon to nitrogen (C:N) in plant litter controls whether the microbes decomposing it release (mineralise) or temporarily tie up (immobilise) soil nitrogen, and while cereal straw such as rice straw has a C:N ratio above 80:1, which commonly immobilises nitrogen and can depress the succeeding crop, legume residues decompose faster and release nitrogen. Second, incorporating post-harvest residues into the soil instead of removing or burning them returns the large nutrient pool held in straw (a substantial fraction of the nitrogen, phosphorus and potassium taken up by the crop) and builds soil organic carbon; but under organic management — where no mineral fertilizer may be applied to buffer a period of nitrogen immobilisation — the rate, timing and mixing of residues become critical management variables.

Within this context, Davari, Sharma and Mirzakhani (2012) reported a two-cycle field experiment (2006–2007 and 2007–2008) conducted under organic management at the Indian Agricultural Research Institute, New Delhi, on a Typic Ustochrept soil, comparing an organic RWCS with an organic rice–wheat–mungbean system (RWMCS, which includes a summer mungbean crop) with and without incorporation of crop residues, in combination with farmyard manure, vermicompost and biofertilizers; the cultivars grown were rice cv. Pusa Basmati 1 (a Basmati type), wheat cv. HD-2643 and mungbean cv. Pusa Vishal. Over two cycles, RWMCS out-yielded RWCS in productivity, protein yield and energy output, and carried higher available N, larger culturable populations of bacteria, fungi and actinomycetes (the standard plate-count assay that captures only the small, culturable fraction of soil microbes), higher microbial biomass, and higher CO2 evolution from the soil. Residue incorporation further increased productivity, soil organic C, Kjeldahl N (the classical total-soil-nitrogen assay), Olsen P (the standard bicarbonate-extractable soil-test phosphorus), ammonium-acetate-extractable K, and all measured microbial indicators. Economics for the same experiment were computed within the organic systems as net income (gross income minus cost of cultivation at organic output prices), and the mungbean-inclusive system gave higher gross and net income than RWCS in both years. The authors recommended organic RWMCS with residue incorporation as the more productive and soil-health-preserving alternative to organic RWCS.

The recommendation, however, rested on two cropping cycles in one field block at one site with one cultivar of each crop, with microbiology limited to counts, biomass, respiration and enzyme activity (not community composition), with an energy analysis in the journal paper, with the soil-biological effects reported but not attributed to any tested mechanism, and with no experimental variation of residue rate, timing or cereal-to-legume mixing ratio — even though the difficulty of using high-C:N cereal straw in organic farming, and the need to mix it with well-decomposed manures or legume residues to narrow the C:N ratio and avoid immobilisation of native nutrients, is explicit in the same body of work. The same research group had earlier shown analogous benefits of diversification and residue incorporation in a conventional rice-based system (Sharma et al. 2010), but the organic version of the question — where nutrient supply depends entirely on residues, manures and biofertilizers (preparations of beneficial microorganisms such as nitrogen fixers applied to seed or soil) — remained, and remains, much less deeply evidenced.

Problem Statement

The problem, as left open by the 2012 study, is to place its two-cycle recommendation — that organic rice–wheat systems of the Indo-Gangetic Plain be diversified with summer mungbean and managed with crop residue incorporation — on a sound long-term, mechanistic, economic and transferable footing. Concretely, for organic rice-based cropping systems of this region, it remains to determine each of the following, which are linked parts of the same open problem:

  1. Persistence. Whether the superiority of organic RWMCS with residue incorporation over organic RWCS — in system productivity, protein and energy output, net returns within the organic systems, soil organic C, Kjeldahl N, Olsen P, exchangeable K, and the measured soil biological indicators — persists, plateaus or reverses beyond the two cropping cycles studied.

  2. Soil biology beyond the reported indicators. Whether the differences between the two organic systems extend beyond the indicators the source measured (culturable counts, microbial biomass, CO2 evolution, dehydrogenase activity) to microbial community composition, and how those differences develop over time.

  3. Mechanism. Why the mungbean-inclusive system raised soil organic C, microbial biomass and dehydrogenase activity. The source reports these effects without isolating a cause; candidate contributors discussed in the surrounding literature — the low C:N of mungbean residue relative to cereal straw, the water and nutrient inputs and canopy of the summer legume crop, and the biofertilizers inoculated with it — co-varied in the original design, and this list is not closed.

  4. Economics within the organic systems. Whether the higher net returns of the mungbean-inclusive organic systems, computed as gross income minus cost of cultivation at organic output prices, hold over the long term once the additional cost of cultivation and labour that residue incorporation entails are accounted for. The source posed no comparison against conventional mineral-fertilizer systems and no certification or transition-cost analysis.

  5. Residue and amendment management. How crop residues and organic amendments should be managed in a fully organic system — the quantities of cereal and legume residues returned, their combination with farmyard manure, vermicompost and biofertilizers, and their timing — so that nitrogen immobilisation from high-C:N cereal straw is avoided where no mineral nitrogen is available to buffer it, while yield and soil benefits are sustained.

  6. Generalization. Whether the findings transfer to rice cultivars other than Pusa Basmati 1 (including non-Basmati types), to wheat and mungbean cultivars other than HD-2643 and Pusa Vishal, and to other soils, climates and regions of the Indo-Gangetic Plains.

References

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  15. Davari, M. R., Sharma, S. N., & Mirzakhani, M. (2012). Residual influence of organic materials, crop residues, and biofertilizers on performance of succeeding mung bean in an organic rice-based cropping system. International Journal of Recycling of Organic Waste in Agriculture, 1(1), 14. DOI: 10.1186/2251-7715-1-14. https://doi.org/10.1186/2251-7715-1-14 (open-access companion paper from the same 2006–2008 IARI experiment; treatment structure, economics within the organic systems, and the C:N/immobilisation rationale for mixing cereal with legume residues)
  16. Davari, M. R. (2009). Study on organic farming of rice-based cropping systems with respect to nutrient management. PhD thesis, Division of Agronomy, Indian Agricultural Research Institute, New Delhi (advisor S. N. Sharma). Full text: https://krishikosh.egranth.ac.in/server/api/core/bitstreams/f4ce1f23-a1d3-40d3-b688-272710782f6a/content (thesis of which reference 1 and reference 15 form part; cultivars Pusa Basmati 1, HD-2643 and Pusa Vishal, within-organic net returns, and the two-conclusion Summary and Conclusion chapter)