Soil organic carbon fractions under different land uses in Mardi Watershed of Nepal

This study was conducted to investigate the organic carbon (C) content and its chemical fractions in soils under forest, grassland, and agricultural uses in the Mardi Watershed of Nepal. Surface soil samples from 0 to 15 cm were collected. Classical extracting procedure with alkali and acid solution was used to separate humic acid (HA), fulvic acid (FA), and humin fractions. Hydrogen peroxide was used to separate black carbon (BC) from humin. The results showed that higher amounts of total soil organic carbon (SOC), as well as of different SOC fractions, i.e., FA, HA, humin and BC, existed in the forest soils compared to those from grassland and agricultural areas. The carbon proportion of FA, HA, and humin fractions accounted for 25, 29, and 37% of the total SOC in forest soils. The corresponding values were 34, 27, and 36% in grassland, and 28, 18, and 54% in agricultural land. The grassland soil showed a higher FA fraction, while the agricultural soil contained lower HA fraction and higher humin fraction as compared to the forest soil. Forest soil showed higher HA/FA ratio and lower (HA + FA)/SOC ratio, indicating high degree of humification and large humification rate in this soil. Black carbon contents occupied 3.4, 3.4, and 11.5% of SOC under forest, grass, and agricultural lands, respectively.