Vertical distribution of STN and STP in watershed of loess hilly region

In order to explore the effects of land use change on the contents of total nitrogen and total phosphorus in deep soil, four land use types (cropland, grassland (7 years), grassland (30 years), and Jujube orchard) were selected from the Yuanzegou watershed in the loess hilly region of northern China. Soil samples at 0–10 m depth were collected to measure the contents of soil total nitrogen (STN) and soil total phosphorus (STP), and their stocks were estimated. The results showed that the STN content showed a decreasing trend with the increase in soil depth, and the lowest STN content of grassland (7 years) was 0.09–0.17 g kg . The range of STN content in the watershed was 0.12–0.22 g kg and the coefficient of variation was 10.52–25.90%, which belonged to medium variation. The STP content is stable regionally with the change in soil depth, and does not change much (except for grassland [30 years]). STP content of the watershed is 0.81–1.05 g kg and the coefficient of variation is 9.37–54.69%, which is a high variation. The change trend of STN and STP stocks is consistent with the nitrogen and phosphorus content. The results revealed the dynamic changes in STN and STP after land use change, and estimated the stocks of STN and STP in deep loess, which provided scientific basis for land and soil resource management and sustainable development of the project of returning farmland to forest or grassland in small watershed of loess hilly-gully region.