Long-Term Crop Diversification Enhances Soil Carbon Fractions and Sequestrations in Northwestern India

Prolonged cultivation of cereal-based cropping systems in the Indo-Gangetic Plain has contributed to soil degradation, groundwater depletion, and declining soil organic carbon levels, highlighting the urgent need for climate-resilient, sustainable crop diversification strategies that enhance soil carbon sequestration and improve overall soil health. A 6-year field experiment assessed 10 cropping systems (CSs) using a randomized complete block design with four replications, focusing on their effects on soil carbon stocks and sequestration at two soil depths (0–15 cm and 15–30 cm). It was inferred from the results that there is a significant variation in soil carbon stocks, with maize–peas–spring groundnut (CS6) having the highest surface carbon stock (13.0 Mg ha−1) and baby corn–potato–okra (CS10) having the highest sub-surface carbon stock (11.9 Mg ha−1). Carbon sequestration peaked in CS6 at 5.06 Mg ha−1 at 0–15 cm, and its sequestration rate was the highest (0.84 Mg ha−1 yr−1). Total organic carbon (TOC) ranged from 0.63% in Rice–Wheat (CS1) to 0.73% in CS6, with similarly high values in other diversified systems. Very labile carbon (VLC) was highest in basmati rice, late-sown wheat, and cowpea (CS3) and CS6, demonstrating the benefits of legume-based systems. At depths of 15–30 cm, trends were consistent but lower. Water-soluble carbon (WSC) and hot water-soluble carbon (HWSC) showed significant differences across systems, with CS3 recording the highest values. The findings indicate that cropping systems incorporating legume diversification and green manuring enhance carbon stocks, sequestration rates, and soil carbon stability, demonstrating that crop diversification is an effective means of increasing soil carbon storage, promoting soil health, and supporting sustainable agricultural production in Northwestern India.