Evolution of Physicochemical and Microbial Characteristics During Natural Precipitation Leaching in Dongying’s Moderate–Severe Saline–Alkaline Soil and Its Ecological Significance

Soil salinization threatens agricultural production, and increasing extreme rainfall may alter natural leaching processes in coastal saline–alkaline soils. However, the relationships among salt ion migration, alkalinity changes, nutrients, and bacterial communities under natural rainfall leaching remain unclear. Therefore, a phased natural rainfall leaching experiment was conducted from June to September 2025 using moderate to severe NaCl-type saline–alkaline soil from Dongying in the Yellow River Delta. The results showed that natural rainfall leaching significantly reduced soluble salt ions, especially Na+, Cl−, and SO42−, and rapidly alleviated early salt stress. However, soil pH did not decline continuously with salt reduction, but fluctuated under the buffering effect of the carbonate system, indicating that desalination was not necessarily accompanied by alkalinity alleviation. Available nutrients showed stage-dependent changes, with HN and AK increasing around the middle leaching stage. Bacterial community composition and co-occurrence networks also changed during leaching, and these changes were more closely associated with salt ions and HCO3−/pH than with available nutrients. These results suggest that post-rain management of saline–alkaline soils should not rely only on total salinity, but should also consider major salt ions, pH/HCO3−, and nutrient availability.