Impact of 20-year-old agroforestry systems on soil organic carbon fractions and biological activity in Central Himalayan foothills

Limited data exist on the long-term effects of agroforestry systems (AFSs) on carbon fractions and biological activity in Central Himalayan soils. To fill this knowledge gap, this study was conducted using ten randomly selected 20 year-old AFSs (treatments) and pure croplands (Control) and compared using Tukey’s HSD test. Soil samples were collected from 0 to 60 cm depth under AFSs and pure cropland. The total soil organic carbon (SOC), carbon fractions, active and passive carbon pools, microbial biomass carbon (MBC), and enzymatic activity were analyzed. The results clearly showed that AFSs increased the total SOC, with the highest value under T. chebula (70.98 Mg ha−1), followed by T. bellirica and T. arjuna. The carbon fractions followed the order Cfrac1 > Cfrac4 > Cfrac3 > Cfrac2. The active carbon pool was higher under AFSs (32.22–39.81 Mg C ha−1), with T. bellirica having the highest value, while the passive carbon pool peaked under T. chebula (32.26 Mg C ha−1). The mean carbon pool management index (CPMI) was higher under AFSs (186.46–229.96), indicating improved soil organic carbon stability. Microbial biomass carbon (MBC), bacterial, fungal, and actinomyces populations, and enzymatic activities were significantly higher under AFSs at 0–15 cm depth. Total C correlated with Cfrac1(r = 0.92) and Cfrac2(r = 0.96), showing a high dependence of Ctot on labile fractions. PCA showed that carbon fractions, MBC, and enzymatic activity were key factors enhancing soil quality. This study demonstrates AFSs’ potential to enhance soil carbon sequestration and biological properties, thereby contributing to climate change mitigation in the Central Himalayas.