Soil organic carbon stocks under different forest types in Pokhare khola sub-watershed: a case study from Dhading district of Nepal

Abstract

Assessment of carbon stock in vegetation and soil is an essential step in estimating the carbon sequestration potential of an ecosystem. This study was carried out to quantify total carbon sequestration in different forest types of the Pokhare Khola sub-watershed (forested area of 312 ha), Dhading, Nepal. The inventory for estimating the above and belowground biomass of different forests was carried out using the stratified random sampling method with 0.5% sampling intensity for plant biomass. The dry biomass was calculated using allometric models. Four soil profiles from each forest types were excavated and soil samples were taken from the soil profile up to 1 m depth at intervals of 20 cm. The soil bulk density was collected using a core ring sampler of 9.5 cm long and 4.2 cm diameter and organic carbon content was assessed using the Walkley and Black method. The total soil carbon stock in all forest types was estimated at 42,523 t/ha, Shorea forest 62%, Schima-Castonopsis forest 25%, Pine-Shorea forest 5.5%, and degraded forest 7.5%. The distribution pattern of carbon stock was mainly due to the biomass of the stand, carbon content of soil and area coverage of these forest types. The total biomass carbon in forest was found as 77.68 t/ha and SOC sequestration 58.6 t/ha. The study showed that soil organic carbon was higher in the upper layer (0– 20cm) in all forest types and rapidly declined below the 20–40cm depth. The rate of SOC concentration is in decline trend as the depth increased. The study revealed that a Shorea forest and Schima-Castonopsis forest are better than a