Long-term impact of organic and inorganic fertilizers on soil organic carbon dynamics in a rice- wheat system

Deep soil carbon (C) plays an important role in the global carbon cycle because it is more recalcitrant and remains in the soil for a longer period. Yet, very few studies have reported the impact of long-term fertilization on soil organic carbon (SOC) dynamics in deep soil in a rice-wheat system (RWS). Hence, the objectives of this study were to evaluate the long-term fertilization impact on carbon sequestration and C pools in RWS in the Indo-Gangetic Plain (IGP) of India. Application of 90 kg N, 20 kg P and 25 kg K ha(-1) through mineral fertilizers (NPK) + green gram incorporation (GR) + 30 kg N ha(-1) through farmyard manure (FYM) sequestered more C (25.86 Mg ha(-1)) in the 0-105 cm soil depth compared with mineral fertilizer alone (1.25 Mg ha(-1)). The C-sequestration rate ranged between 0.61 to 1.23 Mg C ha(-1) yr(-1) under treatments having organic fertilizer and mineral fertilizer, whereas it varied from 0.06 to 0.39 Mg C ha(-1) yr(-1) in the treatments receiving mineral fertilizer only. Total C and critical C input were 197 Mg kg(-1) and 1.94 Mg C ha(-1) yr(-1), respectively under NPK + GR + FYM treatment. Integrated application of mineral fertilizers and organics in treatments NPK + FYM, NPK + SPM (sulphitation press mud), NPK + GR, NPK + GR + FYM and NPK + CR (crop residue) resulted in higher sustainable yield index of RWS compared to mineral fertilizer alone. We conclude that application of organic material along with mineral fertilizers is essential to mitigate the atmospheric C load through C-sequestration in deep soils and to attain sustainable RWS productivity in the IGP.