Long-term effects of fertilisers and organic sources on soil organic carbon fractions under a rice–wheat system in the Indo-Gangetic Plains of north-west India

Abstract

Decline in soil organic carbon (SOC) content is considered a key constraint for sustenance of rice–wheat system (RWS) productivity in the Indo-Gangetic Plain region. We, therefore, studied the effects of fertilisers and manures on SOC pools, and their relationships with crop yields after 18 years of continuous RWS. Total organic C increased significantly with the integrated use of fertilisers and organic sources (from 13 to 16.03gkg–1) compared with unfertilised control (11.5gkg–1) or sole fertiliser (NPKZn; 12.17gkg–1) treatment at 0–7.5cm soil depth. Averaged across soil depths, labile fractions like microbial biomass C (MBC) and permanganate-oxidisable C (PmOC) were generally higher in treatments that received farmyard manure (FYM), sulfitation pressmud (SPM) or green gram residue (GR) along with NPK fertiliser, ranging from 192 to 276mgkg–1 and from 0.60 to 0.75gkg–1 respectively compared with NPKZn and NPK+cereal residue (CR) treatments, in which MBC and PmOC ranged from 118 to 170mgkg–1 and from 0.43 to 0.57gkg–1 respectively. Oxidisable organic C fractions revealed that very labile C and labile C fractions were much larger in the NPK+FYM or NPK+GR+FYM treatments, whereas the less-labile C and non-labile C fractions were larger under control and NPK+CR treatments. On average, Walkley–Black C, PmOC and MBC contributed 29–46%, 4.7–6.6% and 1.16–2.40% towards TOC respectively. Integrated plant nutrient supply options, except NPK+CR, also produced sustainable high yields of RWS.