Soil organic carbon (SOC) degradation is very common in northeast India due to shifting cultivation on hill slopes coupled with unscientific management practices and high rainfall in this region. Agroforestry has a potentially important role to play in climate change mitigation through increased carbon storage in the above ground biomass and below ground soil. A 25 years old agroforestry plantation consisting of four multipurpose tree species (MPTs) (Michelia oblonga, Parkia roxburghii, Alnus nepalensis and Pinus kesiya) maintained at ICAR Research Complex for NEH Region, Umiam were compared with a control plot (without tree plantation) for soil organic carbon (SOC) stocks and fractions. Soil samples were collected from 0-15, 15-30, 30-45, 45-60 and 60-75 cm and analyzed for SOC stocks and fractions. MPTs showed significant influence on SOC stocks with the mean values ranged from 47.8 to 60.2 Mg ha^-1 and followed the order: A. nepalensis>M. oblonga>P. kesiya>P. roxburghii>Control. Land conversion from fallow to agroforestry plantation significantly enhanced the total organic carbon (TOC), particulate organic carbon (POC), KMnO₄ oxidizable C (labile C) and microbial biomass carbon (MBC) fractions in soil. The increase in these fractions was greater with A. nepalensis compared to other MPTs including control. Overall, on average, MPTs increased the TOC, POC, labile C and MBC by 26.3, 54.9, 27.1 and 34%, respectively in relative to control plot. Similarly, approximately 17% increase in SOC stocks was observed under MPTs compared to control. All these C fractions including SOC stocks decreased significantly with soil depths. The increased values of lability index and carbon management index under MPTs revealed that land conversion from fallow to agroforestry plantation have more sensitivity to the changes in SOC and other C fractions in soil. The labile soil carbon fractions were significantly (P<0.05) correlated with TOC indicating that the changes in TOC content of soils is mainly influenced by the labile C pools. The correlation between the TOC and MBC (r=0.493**) was higher than that between POC and MBC (r=0.487**). The data support the conclusion that, conversion of fallow lands to agroforestry plantation is important to mitigate the climate change through increased carbon sequestration in soils besides their vital role in improving soil quality.