­Revealing the Scale- and Location-Specific Variations and Control Factors of Soil Salinity in Wet and Dry Seasons

The low agricultural productivity and ecological degradation in the majority of northwestern China is mainly attributed to the disordered accumulation and expansion of salinization. Soil salinization has a significant spatial and temporal variability. Therefore, it is essential to reveal the variability characteristics and control factors of soil salinization in wet and dry seasons to understand the mechanism of salinization occurrence and prevent soil degradation.First, a random forest model (RF) was used to predict soil salinity distribution and uncertainty in wet and dry seasons at 90-m grid nodes in the Wei-ku Oasis, China. Soil salinity variations at different scales and locations were then extracted using a two-dimensional empirical modal decomposition (BEMD) algorithm. Finally, correlation analysis, semi-variance functions, and redundancy analysis were applied to dissect the correlation and main control factors between soil salinity and environmental factors at three different scales.The results suggest that the salinization of the Wei-ku Oasis varies greatly in different seasons. Specifically, soil salinity was higher in the dry season than in the rainy season and spread mainly towards the desert-oasis interface, with more dramatic changes in slightly saline and strongly saline areas. The soil salinity was decomposed into five intrinsic modal functions with spatial variability scales of 6.2-50.61 km and 7.93-50.18 km during the wet and dry seasons, respectively. The relationships between environmental factors and soil salinity were scale- and location-specific, suggesting a complex interrelationship between topography, temperature, vegetation cover, and soil salinity. The normalized difference vegetation index and the temperature vegetation dryness index were the main factors controlling the soil salinity at small scales, whereas evapotranspiration and temperature had a more pronounced effect on soil salinity than other factors at a large scale. These findings highlight the effectiveness of BEMD in revealing the scale and location control of soil salinity