Response of nutrient management practices on carbon mineralization in relation to changed soil biology under rice-wheat cropping in north-western India

Nutrient management plays a significant role in influencing carbon (C) dynamics in soils due to altered substrate availability. It influences soil respiration which is considered important for the quantitative assessment of C loss from soils, which inter-alia impacts the long-term sustainability of cropland ecosystems. A laboratory incubation study for 77 days was conducted with soils collected after completion of 5-years long rice-wheat cropping system to elucidate C mineralization kinetics under different nutrient management regimes viz. unfertilized control (CK), NPK, farmyard manure (FYM) and NPK + FYM for quantitative assessment of soil respiration rate in response to changed substrate availability and soil biological activity. The C mineralization was modeled for predicting C loss potential of soils under different nutrient management regimes using different models of variable complexity. These results revealed that C mineralization accounts for > 94% of total C mineralized (Cmin) during the initial 60 days. After reaching maxima, soil respiration rate started to drop, keeping a respiration plateau for 10-14 days before maintaining a static rate. The Cmin revealed noteworthy differences (392-911 mg kg(-1) soil) among different nutrient management treatments. The NPK application significantly (p < 0.05) increased Cmin by similar to 60.3% and the basal soil respiration (BSR) by similar to 53.8%, compared with CK. The mineralization (qM) and respiratory quotients (qCO2) for CK treatment were significantly lower, while the highest for treatments including NPK and FYM application, either alone or conjointly. The lower qCO2 value indicates high efficiency of soil microbial biomass in the fertilized soils involving high C input, but less losses of C through respiration. The amount of C mineralized was linearly related to total organic C (TOC) pool. The C mineralization data fitted to seven different kinetic models revealed that first order single compartment (FOSC) with lowest root mean square error (RMSE) had high C mineralization prediction potential in soils. Cumulative C mineralization exhibited a significant (R-2 = 0.703*, p < 0.05) linear increase with increase in TOC pool. The potential rate of C mineralization (C-O*k) was significantly lower for CK and highest for NPK + FYM, which indicates improved soil organic matter quality.