Soil organic carbon fractions, carbon stocks and microbial biomass carbon in different agroforestry systems of the Indo-Gangetic Plains in Bihar, India

A study was undertaken in the Vaishali district of Bihar, India, in 2020 to assess the effect of various agroforestry systems (AFS) on the distribution of different pools of soil organic carbon (fraction I – very labile, fraction II – labile, fraction III – less labile and fraction IV – non-labile), carbon stocking and soil microbial activity. The mean (0–45 cm) total organic carbon (TOC) in different AFS ranged from 5.55 to 6.64 Mg C ha^–1, with the highest under poplar-based AFS (PB-AFS). Across the AFS studied, the C stocks (0–45 cm) varied from 36.24 (mango-based AFS) to 41.43 Mg C ha^–1 (PB-AFS). Overall, the magnitude of C fractions showed the order: fraction I > fraction IV > fraction III > fraction II. Significantly higher soil microbial biomass carbon was recorded under PB-AFS (219.36 mg g^–1) in 0–15 cm depth. Basal respiration was also the highest under PB-AFS (0.54 mg CO₂-C g^–1 h^–1), followed by TB-AFS (0.50 mg CO₂-C g^–1 h^–1) in 0–15 cm depth. Principal component analysis result showed that PC 1 and PC 2 represented about 97% of the total variation. TOC and active carbon pool had the maximum loading in PC 1, while microbial metabolic quotient and bulk density had the maximum value in PC 2