Analysis of factors influencing soil acidification in the Jiaodong Peninsula based on machine learning and the SHAP algorithm

Soil acidification has become a critical issue constraining the sustainable agricultural development of the Jiaodong Peninsula, yet the driving factors of its spatial differentiation remain unclear. To identify the key factors influencing soil acidification and clarify their roles, 144 farmland topsoil samples were collected from the Jiaodong Peninsula in May 2021. A Random Forest (RF) model combined with the SHAP (shapley additive explanations) algorithm was employed to systematically analyze soil pH and its 13 influencing factors. The results indicate that soil pH in the Jiaodong Peninsula exhibites a spatial differentiation pattern, decreasing from the northwest to the southeast, with prominent acidification issues in the southeastern region. The RF model achieves an accurate prediction of soil pH (R²=0.9999), while SHAP analysis further quantifies the contribution of each influencing factor. The primary drivers of soil pH variation are identified as nitrate-nitrogen (NO₃⁻-N) and the soil's intrinsic buffering capacity (calcium and magnesium base cations). Notably, the contribution of NO₃⁻-N (84.0%) is significantly higher than that of exchangeable Ca²⁺, Mg²⁺, and cation exchange capacity (CEC). Furthermore, a strong positive interaction is observed between NO₃⁻-N and Ca²⁺, along with a positive synergistic effect among exchangeable Ca²⁺, Mg²⁺, and CEC. A significant positive correlation between soil pH and NO₃⁻-N(P<0.001) is found, and the spatial distributions of soil NO₃⁻-N, Ca²⁺, Mg²⁺, and CEC values are highly consistent with that of soil pH. It is revealed that the synergistic retention effect of soils with high buffering capacity on nitrate and base cations effectively inhibits the decline in soil pH in the northwestern Jiaodong Peninsula. Conversely, the severe acidification in the southeastern region is primarily caused by nitrogen input into soils with low intrinsic buffering capacity. These findings provide a critical scientific basis for the prevention and control of soil acidification in the region.