Conservation Agriculture and minimum soil disturbance slow the decomposition rate and release of nitrogen from rice shoot and root residues in rice-based intensive cropping systems

Conservation agriculture (CA) practices have been gaining popularity in the rice-based intensive cropping systems of the Eastern Gangetic Plains (EGP). Changes in nitrogen (N) mineralisation in these CA systems are predicted due to decreased soil disturbance and greater crop residue retention but they need to be quantified to guide changes in N fertilizer practice. The decomposition of rice residues was studied at three sites in sandy loam to silty clay loam paddy soils under three soil disturbance levels: conventional tillage (CT), strip-planting (SP) and bed planting (BP). The residues collected from wet season rice were buried in upland crop fields (mustard, wheat and lentil) in nylon mesh bags at each site, retrieved at 15, 30, 45, 60, 90, 120, 150 and 180 days after burial and measured for dry mass and N content. After 180 days of incubation (DAI), only 12.3, 25.5 and 17.8% of the rice straw mass remained at Alipur; 17.4, 32.8 and 23.1% at Digram; and 18.7, 32.4 and 23.5% at Baliakandi, under CT, SP and BP, respectively. The decay rate of rice roots was lower than the rice shoot especially under SP. The % N remaining in SP after 180 DAI in shoots were 37.2%, 26.0% and 18.6% higher at Alipur, Digram and Baliakandi, respectively, than the CT, while in roots they were 21.5%, 15.0% and 15.6% higher at Alipur, Digram and Baliakandi, respectively, than the CT. The higher biomass and N remaining in shoot and root residues with decreased soil disturbance were associated with lower decay rates of mass and slower N decomposition that are attributed to less contact of residues with soils and less favourable microbial decomposition conditions in soils. Greater biomass and N retention in rice residues in soil under SP and BP may prolong the supply of mineral N to better match with in-season crop demand in rice-based cropping systems.