Effect of organic inputs on aggregate associated organic carbon concentration under long-term rice–wheat cropping system

The present study investigated the effect of cultivation with or without inorganic fertilization, and farmyard manure (FYM), paddy straw (PS), and green manure (GM) along with inorganic fertilizers on soil aggregation, array and accessibility of aggregate associated C, specially the micro-scale location or stability of soil organic C (SOC) at 0–0.15, 0.15–0.30 and 0.30–0.45 m depths on a silty clay soil of Inceptisol in the hot, humid subtropics of eastern India, with a long-term 21 year old rice (Oryza sativa L.)–wheat (Triticum aestivum L.) cropping system. In general, structural indices were found higher in the soil receiving organic amendments (NPK + FYM, NPK + PS and NPK + GM) than the minerally fertilized (NPK) soil or the control and in the surface than in depth. Among treatments including organic amendments, GM seems to be less efficient in increasing aggregate stability. The aggregated C decreased with increasing soil depth and was distributed as ‘silt + clay’ C (< 0.05 mm) > coarse macroaggregated C (CMacAC, > 2.0 mm) = mesoaggregated C (MesAC, 0.25–2.0 mm) > coarse microaggregated C (CMicAC, 0.05–0.25 mm). Within a size class, aggregated C concentrations of the organically amended treatments were in the order of FYM ≥ PS ≥ GM, limiting the negative effect of ploughing but not to the level of native soil. A significant positive linear relationship was found between the ‘silt + clay’ C and the silt and clay content of this soil. Out of the added organics, C from FYM was stabilized more than PS and GM, respectively, and C from GM was stabilized more in ‘silt + clay’ sized fraction than FYM and PS. This study represents a further step towards understanding the bio-chemical nature of organics and their array with mineral particles for C stabilization.