Yield trends, soil carbon fractions and sequestration in a rice-rice system of North-East India: Effect of 32 years of INM practices

Abstract Soil carbon (C) is essential in improving the crop productivity and the quality of soils. However, the research reports on enhancing the system productivity and soil C status in the acid Inceptisol of north-eastern region (NER) of India under sustainable integrated nutrient management (INM) are not adequately available. Here, we studied a rice-rice system after 32-year (1987–2020) of continuous INM treatments with farmyard manure (FYM)/ rice stubbles/ Azolla and chemical fertilizer inputs. Yield data were measured yearly to derive the sustainable yield index (SYI) and soil samples were collected during the last three years to assess the different C fractions, C stocks and sequestration and C indexes. The recommended dose of fertilizers (RDF) + rice stubble significantly increased the total yield and the SYI over the unfertilized-control, followed by RDF + FYM and RDF + Azolla. The RDF + Azolla treatment effectively enhanced the very labile and labile C fractions, while RDF + rice stubble contributed to the less labile and non-labile C fractions in soil. The maximum SOC stock was found under RDF + FYM (67.1 Mg ha−1) and RDF + rice stubbles (66.6 Mg ha−1), whereas the lowest in the unfertilized-control (39.1 Mg ha−1). The soil C sequestration followed the same trend ranging between -4.0 and 24.1 Mg ha−1. The water soluble C and microbial biomass C fractions were found to be the most responsive to the different nutrient management practices. The INM treatments showed a higher lability index, C pool index, and C management index. While yield declined in the unfertilized control, the supply of organic and chemical fertilizers sustained rice-rice productivity with an enhancement in soil C status in the long-term.