Comparing the impacts of an invasive grass on nitrogen cycling and ammonia-oxidizing prokaryotes in high-nitrogen forests, open fields, and wetlands

Numerous invasive plant species can increase soil nitrate (NO3−) by altering the nitrification process through plant-soil microbe interactions with ammonia-oxidizing archaea (AOA) and ammonia-oxidizing bacteria (AOB). We evaluated how the invasive species Microstegium vimineum influenced physico-chemical soil properties, inorganic nitrogen (N) cycling, and AOA and AOB abundances under various environmental conditions. We paired 75 M. vimineum-invaded plots with 75 neighboring reference plots across forests, open fields, and forested wetlands within a state park in the Mid-Atlantic United States that has received high levels of N deposition. Soils were sampled for physico-chemical properties, NO3− and ammonium (NH4+) pools and availability, and AOA and AOB abundances. There were multiple soil impacts associated with M. vimineum across all ecosystems, most consistent were increased soil pH and increased NO3− pools. For other impacts, the directionality and effect sizes varied among ecosystems (e.g. NH4+ pools were 34% lower in forests, 35% lower in open fields, and 90% higher in wetlands relative to reference plots). Finally, forests had nearly all of impacts predicted by a pH-mediated nitrification plant-soil feedback. This study highlights the ability of an invasive grass to alter N cycling and soil properties in forests, open fields, and wetlands that have received high N deposition. We also show how invader-mediated impacts to N cycling may be dependent on the context of the ecosystem being invaded, including its hydrology, ambient soil conditions, and substrate-availability.