The role of ammonium oxidizing communities in mediating effects of an invasive plant on soil nitrification

• Microstegium vimineum increases the abundance of ammonia-oxidizing archaea.
• M. vimineum alters the community composition of ammonia oxidizers.
• M. vimineum's impacts are seasonal, and have strong annual variation.
• These impacts gradually declined when M. vimineum naturally declined at a site.
• These impacts are therefore not likely to result in soil legacies.

Invasive plants often benefit from changes that they impose on soil microbes via positive plant–soil feedback, but the mechanisms that underlie these changes, and the legacy of their effects, remain poorly quantified. We investigated the impacts of an invasive annual grass, Microstegium vimineum, on the structure and functioning of soil microbial communities in a multi-year, field-based common garden experiment. Given previous reports that M. vimineum can both elevate nitrification rates in soil and benefit from enhanced nitrate availability, we sought to answer the following questions: 1) Does M. vimineum alter the abundance or composition of soil nitrifying microbial communities (ammonia oxidizing archaea and bacteria, AOA and AOB, respectively)? 2) Are such effects reversible or do soil legacy effects persist after M. vimineum is no longer present? After three years, invaded plots had greater AOA abundances than uninvaded native dominated plots, as well as different AOA community structure. However, after seven years, and following a period of M. vimineum replacement by native plants in the invaded plots, AOA abundances and nitrification rates declined toward levels found in uninvaded plots. Collectively, our results suggest that while the impacts of M. vimineum invasions on nitrogen cycling likely relate to their association with AOA, these effects may not persist if M. vimineum declines over time and native plants and their associated microbes are able to re-establish.