Spatial variability of soil properties in fixed sand dunes of the Eastern Ningxia Province

As sand dunes gradually become fixed, soil particle size, soil organic carbon (SOC) and total nitrogen (TN) contents vary across different locations of the dunes. To investigate the spatial variation of soil particle size distribution and soil nutrition in the fixed sand dunes, we examined particle composition, SOC and TC features in different locations of dunes in the Eastern Ningxia. The results showed that the particle sizes of each soil layer were mainly characterized by medium and coarse sands. The SOC and TN contents were higher in surface soil layers, with a maximum of 5.781 and 0.412 g·kg-1, respectively, which were observed in interdune lands and dune ridges, while the leeward slope of the dunes showed the least. The SOC content of both the leeward and windward slope gradually decreased with increasing soil depth along the dune. By contrast, that of the interdune lands decreased first and then increased. At small scale, both the SOC and TN contents showed a clear spatial heterogeneity. There was a positive correlation between soil nutrition contents (SOC and TN) and silt and very fine sand contents in the fixed sand dunes, and a negative correlation with medium and coarse sand contents. Our results implied that soil particle size composition influenced SOC and TN contents. The contents of soil nutrition increased with increa-sing contribution of fine particles, highlighting the role of fine particles in adhesion and accumulation of organic matter.