Carbon Sequestration Potential of Inceptisols under Long-term Soybean-Wheat Rotation in Sub-temperate Rainfed Agro-ecosystem of North-West Himalayas

Relationship between addition of organic C and storage under rainfed long-term (27 years) soybean-wheat cropping in a sandy loam soil (Typic Haplaquept) of sub-temperate agro-ecosystem of north-west Himalayas was studied. Investigation was comprised of 6 treatments, viz., no fertilizer or manure (unfertilized control) and three combinations of inorganic N, P and K (NP, NK and NPK) along with N + FYM and NPK + FYM. Fertilizers and FYM were applied annually to soybean crop only following different treatment combinations. Average annual contribution of C input from soybean and wheat was 30.3% and 30.1%, respectively of the harvestable above-ground biomass. The annual gross C inputs to soil were found to be 1097 kg C ha⁻¹ under control plots and 5201 kg C ha⁻¹ under NPK + FYM treated plots. The annual rate of total soil organic carbon (SOC) enrichments ranged from 84.3 to 964.3 kg C ha⁻¹ in 0–45 cm soil layer. The observed annual rate of change in SOC (δCs/δt, kg C ha⁻¹ yr⁻¹ at 0–45 cm) was positively correlated with gross annual C input (A, kg C ha⁻¹ yr⁻¹) to 0–45 cm soil, as described by a linear equation (δCs/δt = 0.226x A-68.8; R² = 0.978, P<0.01). The slope of the equation was the humification rate constant (h) and indicates that 22.6% of the gross C input was annually humified in the soil to contribute towards the increase in SOC content. The intersection of the curve with Y-axis at 68.8 suggests that in our experimental set up C losses during 27 years from native SOM averaged 68.8 kg C ha⁻¹ yr⁻¹. The calculated decay rate (k) of the native SOC was 0.0021 and estimated quantum of biomass C required to maintain equilibrium SOM content was 304 kg ha⁻¹ yr⁻¹. This information can provide a basis for effective soil organic carbon management in rainfed hill soils.