Ecosystem impacts of Microstegium vimineum vary across a regional gradient

Background/Question/Methods

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 in 33 paired invaded and uninvaded sites in the southeastern U.S. that 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 microorganisms (AOM).

Results/Conclusions

We found that AOM are 2.9 times more abundant in the invaded relative to uninvaded plots, indicating higher nitrification rates in these areas; however, we also found that net nitrification rates measured through the incubation are significantly lower in the invaded plots (-0.04 vs. 0.17 ug NO₃^--N g^-1 soil). We therefore suggest that changes in cycling rates from nitrifying microorganisms may be accompanied by greater immobilization. When we examined inorganic nitrogen available in situ, we did not find significant differences between invaded and uninvaded plots, though we did find changes in inorganic nitrogen availability due to pH. Thus, we cannot attribute changes in inorganic availability directly to M. vimineum, though we suggest that its effects on pH may be a pathway by which it affects nitrogen cycling: in addition to a substantial literature showing that M. vimineum increases pH, we find that pattern across our sites as well. Collectively, our results suggest that while M. vimineum is associated with some changes in nitrification, extrapolating these results across landscapes may not be warranted given potential microbial community responses to ecosystem changes as well as site-specific environmental conditions that more directly affect inorganic nitrogen availability.