Variability of Soil Organic Carbon Fractions under Different Cropping Systems

Abstract

Soil organic carbon (SOC) pools respond differently to cropping-system diversification, but evidence from farmer-managed fields in semi-arid India remains limited. This study evaluated the effects of 10 cropping systems on soil pH, electrical conductivity (EC), bulk density, and SOC pools at three depths (0-15, 15-30, and 30-45 cm) in semi-arid eastern Rajasthan. We hypothesized that diversified cropping systems involving legumes and pulses would sustain higher SOC concentrations and stocks than cereal-cereal rotations. Following the rabi-season harvest in April 2024, 360 soil samples were collected from four districts (Karauli, Sawai Madhopur, Bharatpur, and Dholpur) representing ten cropping systems: pearl millet-wheat, pearl millet-mustard, pearl millet-chickpea, sesame-chickpea, pearl millet-fennel, groundnut-wheat, maize chickpea, maize-wheat, tomato-cabbage, and sesame-mustard. Samples were analyzed using standard procedures for pH, EC, bulk density, SOC fractions (CVL, CL, CLL, and CNL), total organic carbon (TOC), and SOC stock. Soil pH and bulk density increased with depth across all cropping systems; sesame mustard recorded the highest pH (8.10-8.23), whereas groundnut-wheat showed the lowest bulk density (1.28-1.32 Mg m-3). SOC fractions showed strong depth-wise stratification, with concentrations decreasing significantly (p ≤ 0.05) from the surface to the 30-45 cm layer. Groundnut-wheat recorded high TOC concentrations of 8.92, 7.24, and 5.30 g kg-1 and corresponding SOC stocks of 17.17, 14.20, and 10.53 Mg ha-1 at 0-15, 15-30, and 30-45 cm, respectively. In this observational field survey, diversification with legumes and pulses was associated with higher SOC concentrations and stocks; however, causal inference is limited by potential confounding factors such as soil type, irrigation, and management history. Groundnut wheat and maize-chickpea warrant further controlled assessment as potentially effective systems for soil carbon management in semi-arid eastern Rajasthan.