Spatial distribution of soil salinity and potential implications for soil management in the Manas River watershed,China

Understanding the watershed-scale spatial distribution of soil salinity and its compositions is important for soil management. Here, we present the first study on the Manas River watershed in northwest China. In this study, we took soil samples in upper 20 cm of soil from 186 locations across the watershed and measured total salt concentration (TSC), salt ion composition and soil particle size distribution (PSD). We found that on average topsoil TSC tended to increase, from 3.55 g kg^?1 in upstream regions to 19.40 g kg^?1 in downstream regions. The stoichiometric analysis showed that the equivalence ratio of soil Cl^- to SO₄^2? increased from 0.53 in upstream regions to 2.12 in midstream regions, and further to 3.76 in downstream regions; thus, the soil types were classified into chloride–sulfate, sulfate–chloride and chloride soils types, respectively. Additionally, proportions of small (<2 μm in diameter) and large (>2,000 μm) soil particles increased, while that of medium sizes (2–50 μm) decreased from upstream to downstream, with an increasing coefficient of variance (CV) in PSD. Taken together, watershed-scale topsoil salinity may be horizontally characterized by increased TSC and Na⁺ & Cl^? proportions, greater equivalence ratio of Cl^? vs. SO₄^2? and more balanced distribution of PSD along with surface water flow. Results demonstrated that soil salinity and its ions compositions showed a great variation across the watershed scale, suggesting that soil management may consider the spatial heterogeneity of saline–alkaline soil types, and our results provided scientific guidance for local soil management and restoration.