Quantifying the impact of different agroforestry systems on soil carbon fractions lability and long-term carbon sequestration in Central Himalayas

Soil Organic Carbon (SOC) is a key indicator for understanding the carbon dynamics in agroecosystems. Carbon fractions, on the other hand, can be more sensitive over short periods and can detect changes in the distribution and relationship patterns of SOC pools. To test this hypothesis nine, 16-year-old agroforestry systems (AFS) were tested and compared with control at 0-15 cm and 15-30 cm soil depths for K2Cr2O7 oxidizable carbon (SOC), pH, Electrical Conductivity (EC), Microbial Biomass Carbon (MBC) and carbon fractions, viz. very labile (C1(frac)), labile (C2(frac)), less labile (C3(frac)), and non-labile (C4(frac)). Carbon lability index (CLI) and Stratification Ratio (SR) were calculated to assess the lability and vertical distribution of SOC. The study found that from the time of plantation, SOC stocks significantly improved in the range 19.82%-46.33% under different AFS with SOC sequestration ranging from 0.111 +/- 0.002 to 0.697 +/- 0.017 MgC ha(-1) yr(-1). Among different AFS, Bambusa vulgaris showed significantly higher SOC sequestration than all other treatments at both soil depths. AFS also demonstrated a significant improvement in SOC lability, leading to an increase in CLI by 0.08%-4.48% at 0-15 cm depth and a more pronounced improvement of 2.08%-18.32% at 15-30 cm depth. Among different AFS the highest improvement in CLI was observed in Bambusa vulgaris and the lowest in fallow land. The vertical distribution of carbon fractions in the soil was also affected by AFS, with the labile carbon fraction mostly concentrated in the topsoil layer as indicated by high SR values (1.133-1.203) for C1(frac). The highly significant positive correlation (p < .05) of labile carbon fractions with SOC (r = .872** for C1(frac), r = .900** for C2(frac) and r = .915** for C3(frac)) indicated the high dependence of SOC on labile carbon. The study revealed that AFS have the potential to significantly enhance carbon sequestration, while also impacting the lability and vertical distribution of SOC.