Vegetation Restoration in Karst Southwest China: Effects of Plant Community Diversity and Soil Physicochemical Properties on Soil Cadmium

In southwest China, vegetation restoration is widely used in karst rocky desertification control projects. However, mechanistic evidence explaining how plant community composition and species diversity regulate cadmium (Cd) bioavailability remains limited. Here, the plant community’s species diversity, soil properties, Cd, and available Cd contents were evaluated. Four plant community types, NR (natural recovery), PMC (Pistacia weinmannifolia + Medicago sativa + Chrysopogon zizanioides), and PME (Pistacia weinmannifolia + Medicago sativa + Eragrostis curvula), were selected as the research objects. The species composition was recorded, and dominant plant species and soil samples were collected to analyze Cd accumulation characteristics. Relative to NR, composite restorations increased plant diversity and soil nutrient availability and reduced soil compaction, with PMC showing the strongest remediation, decreasing total Cd by 49.4% and available Cd by 59.5%. Model-averaged regression and hierarchical partitioning analyses further identified nitrogen availability and community structure as the dominant drivers. Specifically, available nitrogen (AN), vegetation coverage, Margalef species richness (DMG), ammonium nitrogen (NH4+–N), and total N (TN) were the main factors of soil total Cd, and BD, TN, nitrate nitrogen (NO3−–N), mean crown diameter (MCD), and Shannon–Wiener index (H′) were the main factors of soil available Cd. The results indicate that PMC provides a plant community structure configuration decisions of a scalable, site-adaptable strategy for durable Cd stabilization and soil conservation in thin, carbonate-rich karst soils.