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 (CVL), labile (CL), less labile (CLL) and non-labile (CNL), 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−1yr−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.