Soil biota are susceptible to abiotic stresses such as nitrogen deposition and climate change, but growing evidence also suggests that biological stressors are important. Non-native invasive species can have major impacts on soil biota, often changing microbial community composition and altering ecosystem function. Effective ecosystem management and the mediation of plant invasion to restore native habitats therefore requires a better understanding of the soil biotic response to interacting abiotic and biotic stressors. Soil fungi, which play a key role in carbon and nitrogen cycles through decomposition of organic matter (saprotrophic fungi) and nutrient exchange with native plants (mycorrhizae), are key elements of the soil biota that are especially sensitive to environmental stressors. Our work examines how the soil fungal community responds to garlic mustard (Alliaria petiolata) invasion and its eradication. This research was conducted at eight deciduous forests in the northeastern US where we collected soil samples from uninvaded (control), invaded, and eradicated plots. We characterized fungal community structure using ITS metabarcoding, along with a suite of edaphic traits, including soil pH, C and N concentrations, amino acid and inorganic N availability, and bacterial and fungal biomass. We examined relationships among garlic mustard cover, fungal community structure, and soil properties.
We found that garlic mustard invasion was associated with lower fungal beta diversity, higher fungal richness, and a consistent increase in the abundance of common and rare saprotrophic and white rot fungal genera across the invaded landscape. Invasion was also associated with higher soil pH, lower organic C content and C:N ratio, and increased nitrate availability. In particular, the ratio of saprotrophic to ectomycorrhizal fungi was negatively correlated with C:N ratio, suggesting that increased saprotrophic fungal relative abundance may reduce soil C content in invaded soil. Our findings suggest that garlic mustard may have a profound impact on fungal communities and edaphic traits despite site level differences in mineralogy, soil texture, and garlic mustard abundance.