Microstegium vimineum is associated with changes in nitrogen availability and fluxes across a broad landscape
Plant species affect ecosystem processes through their demand, use, and release of nutrients as well as through effects on microbial processes and other fauna. Invasive plants, in particular, are associated with higher nitrification rates and differential N partitioning in the systems that they invade—a pattern generally found through single-site studies under controlled conditions. We asked whether changes observed at the local scale are consistent across heterogeneous landscapes that more accurately capture a species’ realized niche. We conducted an observational study of Microstegium vimineum (Poaceae) in 33 paired invaded and uninvaded sites at the Coweeta LTER and at two other locations within 100 km, all of which encompass a high degree of environmental variation. We measured nitrogen mineralization rates and inorganic nitrogen availability across these plots and assessed whether greater differences could be seen due to site-specific variation or due to the presence of M. vimineum. We used ion-exchange resins to measure nitrate and ammonium availability, and we estimated potential net mineralization and net nitrification rates through a month-long incubation of soil obtained at these sites. In 16 of the 33 paired sites, we used qPCR to quantify the presence of ammonium-oxidizing bacteria and archaea.
We found that M. vimineum presence was a predictor of nitrification rates and nitrate availability across our sites, though its effect differed between the three locations examined. We also found a significantly higher abundance of ammonium-oxidizing bacteria and archaea in invaded plots, suggesting higher nitrification rates in these areas. Additionally, we found that one of the strongest predictors of N pools and fluxes was pH. Due to a substantial literature showing that M. vimineum increases pH—and because we found correlations between M. vimineum presence and pH at our sites as well—we suggest that pH may be an indicator for M. vimineum’s influence on N processes. Collectively, we find strong associations between M. vimineum invasions, nitrifying microorganisms, and changes in N cycling.