The Soil pH and Micronutrients Drive Ageratina Adenophora Invasion in Areas with Acidic and Nutrient-Poor Soils

Ageratina adenophora poses significant threats to agricultural and forestry production and biodiversity conservation worldwide. However, its expansion mechanism in highly acidic environments has not been studied in depth. To address this issue, we investigated the impacts of A. adenophora invasion on soil nutrient content and enzyme activity levels across 24 samples from Yunnan Province, China. The sampling sites were categorized into four invasion levels, namely, noninvaded (C), lightly invaded (L), moderately invaded (M), and severely invaded (S). Our findings revealed that the soil pH in severely invaded areas was 8.37% greater than that in noninvaded areas, with pH values ranging from 4.8 to 5.3. Notably, severely invaded soils demonstrated relatively high levels of soil organic carbon (SOC), available potassium (AK), aluminum (Al), available iron (Fe), available zinc (Zn), available copper (Cu), available manganese (Mn), exchangeable calcium (Ca), and exchangeable magnesium (Mg). However, the levels of available Al, boron (B), and phosphorus were significantly lower in these areas. Additionally, variations in the total nitrogen (TN), total potassium (TK), sucrase, and nitrate reductase activity levels were observed across the areas with different invasion levels. Correlation analysis underscored the pivotal role of pH in regulating soil nutrient availability and microbial activity levels. The random forest model (RFM) and structural equation modeling (SEM) results indicated that available Mn, AK, and available Zn are the dominant factors in noninvaded areas (p < 0.05), while Mg, B, and available Cu were the main factors in severely invaded areas (p < 0.05). These findings collectively demonstrate that A. adenophora invasion establishes favorable habitat conditions by altering soil pH, nutrient cycling, and enzyme activity levels, thereby suppressing the growth of native plants and ultimately displacing them from their ecological niches. This study provides a sound foundation for the formulation of stage-specific control strategies against A. adenophora invasion.