Soil organic carbon (SOC) content play a crucial role in sustaining soil quality, crop production, and environmental quality. It is strongly affected by land use types and management practices. Changes in state land use policy in China in the last two decades have resulted in intense changes in agricultural land use types and farm management. The effect of such changes on SOC content, SOC density and vertical distribution is of great concern. This study investigated changes in SOC in sandy loam soils (Ustochrepts) under orchard, vegetable, corn (Zea maize L.) and soybean (Glycine max L.) cultivation in northern China. The corn fields were further classified into three categories based on its inputs, i.e. high-input corn fields, mid-input corn fields and low-input corn fields. In April of 2005, a total of 197 soil samples were collected in Yanqing Basin, Beijing. The soil samples were taken from 42 soil profiles within 100 cm soil depth which represents the six land use types and management practices. SOC contents were determined via rapid dichromate oxidation and ANOVA statistical analysis was used to test for significant differences among land use types at p<0.05.
The results showed: (1) the effects of land use type on SOC occurred primarily within 25 cm soil depth, and below this it was not significant. SOC contents of each land use type and management practice decreased sharply with increasing soil depth. (2) SOC contents under orchard and high-input corn fields fluctuated from 4.16 g kg^－１ to 10.00 g kg^－１, and 4.73 g kg^－１ to 9.31 g kg^－１, respectively. Under vegetable cultivation to a depth of 40 cm, SOC varied from 6.42 g kg^－１ to 9.67 g kg^－１; however, it was lower under soybean fields (3.27 g kg^－１ to 7.73 g kg^－１), low-input corn fields (1.83 g kg^－１ to 7.67 g kg^－１), and mid-input corn fields (3.14 g kg^－１ to 8.33 g kg^－１), respectively, for up to 100 cm depth. (3) The variation patterns of SOC density were similar to those of SOC content for all land use types. SOC densities can be ranked as: orchard>vegetable fields>high-input corn fields>mid-input corn fields>soybean fields>low-input corn fields( 4.15 kg m-2 to 8.22 kg m^-2).
This investigation revealed that SOC contents and densities of orchard fields, vegetable fields and high-input corn fields are higher than those of soybean fields, mid-input and low-input corn fields. This is indicative of applied fertilizer, irrigation, tillage practices, and farm management. Increasing SOC content through intensive management practices in orchard and vegetable fields is a valuable option for improving soil quality and soil carbon sequestration.