Geospatial assessment of soil salinity–nutrient stratification and soil health responses to groundwater table dynamics in the Bohai Rim region, China

Soil salinization, a major form of land degradation, severely threatens ecosystem sustainability and agricultural productivity across the globe. Its dynamics, however, are strongly regulated by groundwater fluctuations. This study focused on the Bohai Rim, a representative coastal agroecosystem, to examine how groundwater table changes from 2010 to 2020 affected stratified soil salinity–nutrient structures and soil health. A groundwater–soil–ecosystem coupling framework was established by integrating groundwater depth monitoring, soil physicochemical profiling, and multisource remote sensing indicators, which was supported by multivariate statistics and machine learning analyses. The results revealed that 64.7% of the region experienced groundwater deepening, including 28.6%, with a significant decline (≥0.2 m/year). A decrease in groundwater facilitated surface soil (0–5 cm) enrichment of total nitrogen (TN) and phosphorus (TP), while cation exchange capacity (CEC) decreased with depth, suggesting that water–solute migration may play a major role. The soil health index (SHI) showed a clear vertical attenuation pattern, with low-health conditions reaching 35.73% at 15–30 cm. Principal component and random forest analyses identified the CEC, TN, and normalized difference moisture index (NDMI) as key explanatory factors. This study enhances the understanding of groundwater-driven water–salt–nutrient processes and offers scientific support for salinization control, soil-health improvement, and sustainable coastal agroecosystem management.