Critical Carbon Inputs to Maintain Soil Organic Carbon Stocks under Long-Term Rice-Rice Cropping System in Semi-Arid Southern India

Abstract

A long-term field experiment under a rice–rice cropping system was conducted for 18 years to evaluate the influence of inorganic fertilizers and organic manures on soil physico-chemical properties, carbon dynamics and crop productivity. Continuous application of fertilizers and organics brought notable changes in soil properties. Soil pH remained slightly alkaline, and electrical conductivity showed minor increases under higher nutrient inputs. Organic carbon improved considerably with the addition of farmyard manure and its integration with fertilizers compared to the control. Annual carbon inputs were relatively low with fertilizers alone, contributing about 2.3–3.1 Mg C ha-1 yr-1, but increased substantially with FYM addition (around 4.8 Mg C ha-1 yr-1), while sole FYM application provided the highest return (about 6.8 Mg C ha-1 yr-1). Fertilizer-only treatments often resulted in negative or near-equilibrium carbon balances, particularly under excessive nitrogen, whereas maintaining soil organic carbon required a minimum threshold of 3.6 Mg C ha-1 yr-1. Grain yield trends indicated sustained productivity under balanced fertilization and significant improvement with integrated nutrient management. Furthermore, analysis of cumulative carbon inputs identified a critical threshold required to maintain soil organic carbon stock. The findings suggest that long-term balanced fertilization combined with organic amendments is essential not only for sustaining crop productivity but also for enhancing soil health and organic carbon sequestration in intensive rice-based systems.