Impact of continuous organic manuring on mechanisms and processes of the stabilisation of soil organic C under rice-wheat cropping system

Understanding the mechanism of soil organic carbon (SOC) stabilisation may help in developing management strategies for SOC storage. A long-term organically managed rice-wheat cropping system was used for SOC stabilisation study. Soil samples were collected from control, FYM (farmyard manure to rice and wheat), GM (green manure; Sesbania aculeata to rice and Leucaena leucocephala to wheat), GB (GM with biofertiliser; blue green algae to rice and Azotobacter sp. to wheat), GF (GM with FYM), GFB (GM with FYM and biofertiliser). Sodium hypochlorite (NaOCl)-resistant C correlated significantly with ammonium oxalate and dithionate extractable Fe, Al and Si in soil. The GFB showed the highest enrichment of SOC (32%) as well as NaOCl-resistant C (22%) at 0-15 cm soil depth. At higher soil depth, GM alone showed the highest enrichment of SOC (39% at 15-30 cm, 84% at 30-60 cm). The NaOCl-resistant C was higher in FYM and GFB treatments at 15-30 and 30-60 cm depths respectively. The proportion of NaOCl-resistant C to SOC increased down the profile and was highest (30-52%) in 30-60 cm soil depth. A multiple regression model developed between ammonium oxalate extractable Fe, Al and Si and SOC could well predict the stable SOC content. There was a substantial improvement in prediction when extractable Fe, Al and Si were combined together. Among the organic treatments, GFB showed the highest humification and aromaticity in humic acid with least polarity and more reduced form. The combination of all the organic sources (GFB) could be a promising nutrient management strategy for enhancing the stability of SOC in rice-wheat cropping systems of semiarid subtropical India.