Soil carbon fractions under different land use types in the tablelands of the Loess Plateau

Effects of land use types on soil organic carbon(SOC) sequestration in the tablelands of the Loess Plateau were studied in this paper. There were four vegetation types cultivated in pure and mixed planting models, namely arbor, shrub, grass and farmland. Soil total carbon(STC), soil inorganic carbon(SIC), density of organic carbon and dissolved organic carbon were analyzed. The results showed that concentrations of soil total carbon in 0-20cm and 60-100cm were significantly different, and the light fraction(LFC), heavy fraction(HFOC) and dissolved organic carbon(DOC) were in 0-40cm, while the soil inorganic carbon was in 40-100cm, LFOC/SOC and DOC/SOC were different in each layer, and HFOC/SOC was significantly different in 0-20 cm. Soil total carbon, light fraction, heavy fraction, dissolved organic carbon and LFOC/SOC of forest land and natural grass land were higher than those of farmland, while their HFOC/SOC were lower than those of farmland, and that of shrub forestland and natural grass land showed this tendency much more. Under different land uses there were significant positive correlations between soil total carbon, organic carbon, light fraction, heavy fraction, dissolved organic carbon and coarse particulate, fine particulate, easily oxidizable organic carbon, stable organic carbon and aggregate organic carbon, while there were negative correlations between inorganic carbon and organic carbon fractions. The correlation coefficients between light fraction organic carbon, dissolved organic carbon and coarse particulate organic carbon, easily oxidizable organic carbon, organic carbon in aggregates with the sizes of 2-0.25 mm were bigger than those of between fine particulate organic carbon, stable organic carbon and organic carbon in aggregates with the sizes of >2mm, while heavy fraction organic carbon was contrary. Light fraction organic carbon was more sensitive to the land use types than others, the sensitivities of organic carbon and heavy fraction organic carbon were almost same and rank the second, and the sensitivities of total carbon and dissolved organic carbon were the lowest. Therefore, light fraction organic carbon could be used as one of the effectiveness indicators of soil ferltility change assessment.