Spatial patterns of soil total nitrogen and soil total phosphorus across the entire Loess Plateau region of China

Abstract

Soil total nitrogen (STN) and soil total phosphorus (STP), which vary spatially at different scales, play important roles in both agriculture and in the natural environment, especially those related to soil productivity and aquatic eutrophication. However, little information is available about the regional spatial availability of STN and STP and the influence of land use at the regional scale of the Loess Plateau (620,000km2) of China. Therefore, 764 soil samples were collected from 382 sampling sites across the region in order to determine STN, STP and other related soil properties and to relate them to site characteristics. Classical statistics and geostatistics were used to analyze the current status and spatial pattern of STN and STP. Mean STN and STP concentrations ranged from 0.50gkg−1 to 0.81gkg−1 and from 0.46gkg−1 to 0.61gkg−1, respectively, under different land use types. Mean STN and STP densities ranged from 0.27kgm−2 to 0.39kgm−2 and from 0.27kgm−2 to 0.38kgm−2, respectively, under different land use types. The concentrations and densities of STN and STP under different land use types were all moderately variable. Land use, precipitation and temperature significantly affected both STN and STP (p<0.05). The results varied among different precipitation and temperature regions for different land use types. Generally, cropland had higher concentrations and densities of STN and STP than forestland and grassland, and regions with higher precipitation and temperatures had higher STN and STP densities. Significant correlations were found between STN and STP, with selected variables, i.e. soil organic carbon, precipitation, temperature, elevation, latitude, longitude, slope gradient, clay content, silt content and soil pH. The results were not consistent within either STN, or STP, or the land use types. Therefore, land-use specific linear models were derived that predicted STN and STP. Both STN and STP demonstrated moderate spatial dependence. The spatial range of STN and STP ranged from 374km to 461km and from 546km to 664km, respectively, which were much greater than our sampling intervals (30–50km). Distribution maps of STN and STP densities, derived by kriging interpolation, showed similar patterns with a central area of low values surrounded by bands of higher values progressively increasing towards the region's boundaries. Stocks of STN and STP were estimated to be 0.217Pg and 0.205Pg in the upper 0–40cm soil layers, which were about 5.4% and 7.3% of the total nitrogen and phosphorus stocks in China. Our study suggests that it is important to take land use into account when considering variations of STN and STP at the regional scale. The spatial data of STN and STP could serve as initial inputs in regional nitrogen and phosphorus models and could be combined with soil erosion data to assess the risks of nitrogen and phosphorus losses to aquatic systems.