Soil Multifunctionality Differences and Driving Pathways Among Plantations in Coastal Saline–Alkali Land

Afforestation of saline–alkali tidal flats is a key ecological engineering measure to mitigate soil degradation; however, the pathways through which different plantation types drive changes in soil multifunctionality remain unclear. We selected 15-year-old stands of 10 plantation types and a bare tidal flat (CK) in the Dafeng Forest Farm of northern Jiangsu and measured 20 soil physicochemical and biological indicators in three layers (0–20, 20–40, and 40–60 cm). Z-score standardization was used to construct carbon sequestration, nutrient cycling, enzyme activity, and comprehensive multifunctionality indices, which were analyzed via two-way ANOVA, SHAP, and PLS-PM. After afforestation, topsoil electrical conductivity decreased from 4563 to 83–202 µS cm−1 and mean weight diameter increased from 0.17 to 0.27–0.56 mm (both p < 0.001). The enzyme activity Z-score differed significantly among plantation types, and the comprehensive Z-score declined significantly with soil depth. SHAP identified R0.25, MWD, and bulk density as the dominant drivers of the comprehensive Z-score. PLS-PM (GOF = 0.615) showed that plantation types enhanced multifunctionality mainly by reducing salinity–alkalinity (path coefficient 0.96) and improving aggregate structure, whereas soil depth exerted the largest total negative effect (−0.84). U. thomasii and P. tomentosa showed preliminary potential for improving multifunctionality in this study, which requires verification across additional sites, stand ages, and seasons.