Soil organic carbon stocks in deep soils at a watershed scale on the Chinese Loess Plateau

An accurate evaluation of soil organic C (SOC) stocks is important to C management and to understanding fully the role of soils in the C cycle. However, SOC stocks in deep soils and the factors that affect them have been largely ignored. We measured SOC stocks and other soil properties to a depth of 500 cm (n = 73) under three land uses in the Lao Ye Man Qu watershed on the Chinese Loess Plateau. Similar patterns in the vertical distributions of SOC stocks were found under cropland, grassland, and shrubland. However, for the 0- to 200-cm soil layer, SOC stocks were significantly lower under shrubland than under either cropland or grassland. The SOC stocks under cropland, grassland, and shrubland in the 0- to 100-cm layer were 2.64 ± 0.67, 2.50 ± 0.69, and 1.99 ± 0.22 kg m^-2, respectively; in the 0- to 500-cm layer they were 8.34 ± 2.26, 8.37 ± 2.01, and 7.26 ± 1.00 kg m^-2, respectively. Significantly higher SOC stocks occurred in loamy than sandy soils (P < 0.01), and they were lower at the mid than at the upper and lower slope positions. However, SOC stocks were similar in sunlit and shaded soils. Pearson correlation analysis indicated that land use, soil texture, and soil water content significantly affected SOC stocks in each 100-cm soil layer to a depth of 500 cm. Effects of topographic features and pH varied with increasing depth. A considerable amount of SOC was stored in deep soils, indicating the SOC content dependence on time and depth. This information is essential for evaluating C fluxes, estimating C stocks, and for managing the C cycles in regions around the world with deep soils.