Distribution of soil nitrogen and its relationship with particle size along the Dan river valley, China

The Dan River valley is an important region for conservation of soil and water loss and is the catchment region of the middle route of the South to North Water Transfer Project in China. Understanding the distribution of soil nutrients and correlation with particle size distribution (PSD) will enable improvement of non-point pollution processes in this area. In this study, soil was sampled at the 0–40 cm in two soil horizons layers for four altitudinal gradients along the Dan River valley. The PSD was analyzed together with the soil total nitrogen (STN), ammonium nitrogen (AN) and nitrate nitrogen (NN). The dominant soil particle size in this area was silt, followed by sand and clay. The concentrations of AN and NN in the Liuling-Valley region (LV) and Riverhead-Valley region (RV) were higher than those in the Plain and Plateau-Valley region (PPV) and the Shang-Dan Basin region (SDB), which was contrary to the STN. Except for AN and NN in the RV, the nutrients showed an obvious decreasing trend with increasing depth in the various gradients. Both AN and NN were negatively correlated with sand and positively correlated with silt. However, the correlation between AN and silt or sand was only significant in the RV. The correlation between STN and silt or sand was not significant in the PPV. Furthermore, NN was not significantly correlated with sand and silt in the four altitudinal gradient regions.