Assessment of soil organic carbon stocks and fractions under different agroforestry systems in subtropical hill agroecosystems of north-east India

Soil total organic carbon (TOC) is not sensitive enough to measure the short and medium term changes due to land use change practices, and thus meaningful fractions of soil organic carbon (SOC) should be measured. The objective of this study was to evaluate four agroforestry systems (AFSs) in order to test the hypothesis that agroforestry systems promote an increase in the soil organic carbon stocks and fractions, thus improving soil quality. For this, four 26 years old agroforestry systems consisting of multipurpose tree species (MPTs) (Michelia oblonga Wall, Parkia roxburghii G. Don, Alnus nepalensis D. Don, and Pinus kesiya Royle ex- Gordon) maintained at ICAR Research Complex, Umiam, Meghalaya, India were tested and compared with control plot for SOC stocks, microbial biomass carbon (MBC), readily oxidizable carbon (ROC), non-labile carbon (C-NL), particulate organic carbon (POC), and carbon management index (CMI). The soil samples were collected at five depths: 0-15, 15-30, 30-45, 45-60, and 60-75 cm. The TOC, POC, ROC, C-NL and MBC in soils of agroforestry land use, on average, increased significantly by 26.3, 60.1, 27.1, 46.4 and 34 %, respectively in relative to control plot. However, the AFS consisting A. nepalensis recorded the highest mean SOC stocks (60.2 Mg ha(-1)), TOC (3.60 g 100 g(-1)), POC (5.42 g kg(-1)), ROC (10.16 g kg(-1)) and MBC (548 mg kg(-1)) than other AFSs. Furthermore, the CMI was highest under A. nepalensis (129.6) followed by M. oblonga (119.4), whereas P. kesiya showed the lowest value (91.1). The SOC stocks and fractions were, on average, 35.3 and 70 %, respectively higher in upper soil layer (0-15 cm) compared to subsurface soil layers (15-75 cm) in all the AFSs. The strong and positive correlation (P < 0.05) between labile soil carbon fractions with TOC (r = 0.680** for POC; r = 0.683** for ROC and r = 0.758** for MBC) indicates that the changes in TOC content of soils is mainly influenced by the labile C pools and are the sensitive indicators of soil quality improvements. The AFSs, A. nepalensis in particular, can, therefore, be recommended as an alternative soil management strategy for food production, and for the maintenance of soil quality and agricultural sustainability through increased SOC sequestration in the highly fragile agro-ecosystems of northeast India.