Sustainable Management of Soil Carbon in a Sub-Tropical Vertisol Rice–Wheat System: A 25-Year Analysis of Fertilizer Impacts on Crop Productivity and Carbon Sequestration

The rice–wheat cropping systems in sun-tropical Vertisols of India face major challenges in sustaining soil health and crop productivity due to climate variability and intensive agricultural practices, which accelerate soil organic carbon (SOC) depletion and soil degradation. Therefore, sustainable nutrient management that not only maximizes crop yields but also supports climate change mitigation through carbon (C) sequestration and soil health management is essential. In this context, a long-term fertilizer management (25 years) study was undertaken to assess crop productivity, C fractions, sequestration potential to achieve soil C 4 per mille under rice–wheat cropping system in sub-tropical Vertisol. The experiment was based on ten different fertilizer treatments including unfertilized control, varying levels of inorganic fertilizers, their combinations, and integrated nutrient management (INM) with organic amendments. Grain yield, system productivity, and the sustainable yield index (SYI) were significantly higher under the application of 150% of the recommended dose of nitrogen, phosphorus, and potassium (NPK) (150%NPK) and 100% of the recommended NPK dose with farmyard manure (FYM) (NPK + FYM) treatments compared to the unfertilized control. Long-term application of 100%NPK + FYM (NPK + FYM) increased SOC content by up to 16%, whereas SOC decreased by 34% under absolute control relative to the initial value. The increase in SOC stock was significantly correlated (R2 = 0.76) with system productivity, requiring a minimum C input of 2.7 Mg C ha−1 yr−1 to maintain C equilibrium. The INM treatment achieved the highest C sequestration (2.27 Mg ha−1), potential (7.04 Mg ha−1) and rate (0.10 Mg C ha−1 yr−1) compared to NPK. In the present study, the C sequestration rate under NPK + FYM and 150%NPK surpassed the annual increment requirement of 0.4% SOC to achieve the target of the “4 per mille”. Over 25 years, application of 5 Mg ha−1 FYM with NPK proved to be the most sustainable practice for SOC management. In addition to enhancing SOC stock, this practice maximizes crop productivity, thereby highlighting the potential of soil management as an effective voluntary carbon sequestration pathway for climate-change mitigation strategy in sub-tropical regions.