Factors controlling microbial responses to Alliaria petiolata (garlic mustard) invasion in northeastern mixed forests

Background/Question/Methods

The non-native, non-mycorrhizal invasive plant Alliaria petiolata (garlic mustard) is an allelopathic forest Brassica that inhibits growth and root colonization of arbuscular mycorrhizal fungi (AMF). AMF responses to invasion are not consistent over biogeographic space and time, and AMF in midwestern USA appear more sensitive than AMF in northeastern USA. This may be related to the naiveté of A. petiolata-microbe interactions, but it is unclear if climatic or soil edaphic properties, like soil fertility, contribute to resilient or susceptible AMF communities. Nutrient availability is influenced by decomposers, and it is unknown if A. petiolata affects saprobes, in addition to climatic regimes, such as MAT, MAP and rates of nutrient deposition. We analyzed soils from eight northeastern forests distributed along a nitrogen deposition gradient, which captured variation in MAT, MAP and edaphic properties. We asked the following questions: 1) does A. petiolata influence microbial biomass and microbial community composition? What climatic, nutrient or edaphic variables influence microbial responses to invasion? We analyzed biomarkers (fatty acid methyl esters) associated with specific microbial groups, and bioavailable N pools and potential production. We used partial least squares (PLSR) and step-wise regression to understand the predictor variables influencing microbial responses to A. petiolata invasion.

Results/Conclusions

We observed significant microbial responses associated with A. petiolata invasion. Microbial decomposers exhibited mostly positive responses to invasion. Ratio of fungi:bacteria were significantly influenced by amino acid concentrations and potential proteolytic rates. Specifically, fungi responded positively to invasion when ammonium and amino acid concentrations were high, while bacteria were most controlled by nitrate concentrations. AMF responsiveness was controlled predominantly through ammonium dynamics, and AMF were only suppressed by A. petiolata in association with the highest inorganic N concentrations. Nutrient effects, including N-deposition rates, were most explanatory of microbial community composition in invaded soils, while MAP, MAT and soil pH were most important in uninvaded soils.  Results of a mantel test indicated that microbial community composition was significantly positively correlated with N-fertility only in the presence of A. petiolata, and soil nitrate was significantly elevated in association with invasion. Microbial community composition was significantly different between invaded and uninvaded organic horizon soils, but not the mineral horizon. N dynamics were largely associated with microbial responses to invasion, positively for decomposing and negatively for AM fungi. Rates of N-deposition in the midwest are roughly two times higher than the northeast, which may be important to understand when managing A. petiolata invasion.