Interrelationship of biomass yield, carbon input, aggregation, carbon pools and its sequestration in Vertisols under long-term sorghum-wheat cropping system in semi-arid tropics

We investigated dynamics of soil organic carbon (SOC) i.e. total soil organic carbon (TOC), oxidisable organic carbon (OC) and its different pools viz. very labile (C-VL), labile (C-L), less labile (C-LL) and non-labile (C-NL), and various sized aggregate associated carbon (AAC) and also its sequestration along depth (0-0.15, 0.15-0.30, and 0.30 -0.45 m) in a typical Vertisols under hot semi-arid agro-climatic conditions. Such dynamics were studied under different soil management practices viz., control, NPK (100% recommended doses of N, P and K fertilizers, RDF), NPK + FYM (50% N and whole of the RDF and 50% N through FYM) and fallow using a 28 year old long-term experiment with sorghum (Sorghum bicolour L) - wheat (Triticum aestivum L.) cropping system. On average, a higher proportion (70% of TOC) of SOC was found in passive pool (less labile + non labile) than active one (very labile + labile) throughout the profile. The proportion of active pool was, however, higher under balanced (NPK) and integrated nutrient management practices (NPK + FYM) over the others followed. Long-term intensive cropping with NPK (15.1%) and NPK + FYM (22%) caused a net enrichment in SOC stock over the control. To offset the loss of C and maintain the SOC level, a critical amount of 0.96 Mg C ha(-1)yr(-1) was needed to be incorporated into the soil. Out of the total water stable aggregates (WSA), mesoaggregates (0.25-2.0 mm) predominated (73.6%) followed by micro (16.4%), and macroaggregates (10.7%) at surface soil. Amount of carbon associated with different sized aggregates had the following trend: mesoaggregate (52.3%) > coarse microaggregate (13.7%) > coarse macroaggregate (13.5%) > (silt + clay) sized aggregate (13.1%) > fine microaggregate (12.2%). Balanced fertilization with organics thus provided not only higher yield but also increased C sequestration in Vertisols even with intensive cropping of sorghum-wheat system under hot semi-arid conditions.