Spatial distribution and variability of soil salinity in film-mulched cotton fields under various drip irrigation regimes in southern Xinjiang of China

In addition to the shortage of water resources, the sustainable development of agriculture is also largely limited by soil salinization in arid and semi-arid regions of China. However, the spatial information of soil salinity at the farm scale in these regions is still poorly understood, which is the basis for the rational utilization of saline soils and allocation of crops. The main objective of this study was to explore the spatial distribution and variability of soil salinity in drip-irrigated cotton fields with film mulch under four irrigation levels (W0.6: 60% ETc, W0.8: 80% ETc, W1.0: 100% ETc and W1.2: 120% ETc, where ETc is the crop evapotranspiration) in northwest China. The results showed that the distribution of soil salinity was significantly affected by irrigation amount. The increase of irrigation amount reduced the total dissolved solids (TDS) in the soil, while it increased the area of the desalination zone around the dripper and the depth of salt accumulation below the soil surface. TDS in the 0–80 cm soil layer did not vary significantly after irrigation compared to that before irrigation, but the distribution of TDS differed from that before irrigation. The salt accumulation zone below the soil surface obviously moved downward after irrigation under sufficient irrigation conditions (W1.0 and W1.2). The variability of soil salinity was affected by irrigation amount, growth stage and soil depth. With the increase of irrigation amount and soil depth, the variability of soil salinity in the root zone decreased. The largest spatial variability of soil salinity occurred at the middle growth stage of cotton. The salt tolerance threshold of cotton (32 Mg ha−1 of TDS) and the optimal irrigation amount (W1.0) were obtained based on the relationships between irrigation amount, TDS and seed cotton yield.