Effects of grassland conversion to cropland and forest on soil organic carbon and dissolved organic carbon in the farming-pastoral ecotone of Inner Mongolia

Effects of grassland conversion to cropland and forest on soil organic carbon (SOC), dissolved organic carbon (DOC) in the farming-pastoral ecotone of Inner Mongolia were investigated by direct field sampling. SOC content and DOC content in soil decreased after grassland were shifted to forest or cropland, in the sequence of grassland soil > forest soil > cropland soil. SOC stock declined by 18% after grassland shifted from to forest. Reclamation of cropland for 10 years, 15 years and 20 years lost SOC in 0–30 cm soil layer, by 34%, 14% and 18%, respectively, compared with that of grassland. DOC in 3 soil layers was within 21.1–26.5 mg/L in grassland, 12.1–14.6 mg/L in forest soil, and 8.0–14.0 mg/L in cropland soil. Correlation analysis indicated that SOC content and DOC content were positively dependent on total nitrogen content (p < 0.05), but negatively on bulk density or land use type (p < 0.05). DOC was positively correlated SOC (p < 0.01). Moreover, SOC content could be quantitatively described by a linear combination of land use types (p = 0.000, r² = 0.712), and DOC content by a linear combination of two soil-related variables, land use types and SOC (p = 0.000, r² = 0.861).