Soil nitrogen distributions for different land uses and landscape positions in a small watershed on Loess Plateau, China

The comprehensively managed Shanghuang watershed has been subjected to vegetation restoration since 1982 and is continuing through the Grain for Green project. Because land use and landscape position are the most important factors that influence soil nitrogen, understanding the effects of landforms and slope position on the distribution of soil nitrogen is an important prerequisite for estimating soil quality on the Loess Plateau. The results of our study showed that soil total nitrogen, nitrate nitrogen and ammonium nitrogen were highly variable in the small watershed. The soil nitrogen concentrations declined in the following order: natural grassland > shrub land > abandoned farmland > orchard > farmland > man-made grassland. Among the different landscape positions, soil nitrogen showed the highest values in valleys. There was a significant positive correlation between soil nitrogen and soil organic carbon, which were negatively correlated with soil pH and soil bulk density. The total soil nitrogen concentration decreased with increasing soil depth, and the highest nitrate nitrogen content and lowest ammonium nitrogen concentration were detected in the 10-30-cm layer. Regarding land use conversion, farmland to shrub land and abandoned farmland was more effective than orchard and man-made grassland for increasing total soil nitrogen and promoting soil nitrogen mineralization, which plays an essential role in ecological restoration. The potential contribution of land use change on soil organic carbon (SOC) and total N concentration could be estimated by assessing the land use conversion in the Shanghuang watershed. It was found that the implemented projects on the Chinese Loess Plateau were successful with regard to benefiting soil nitrogen.