What is a better indicator of an invader’s N cycling impact, its abundance or degree of dissimilarity?

Background/Question/Methods

Invasive species can alter critical ecosystem services such as nitrogen (N) retention and loss, but factors that mediate the magnitude of those impacts have received less attention. Two non-mutually exclusive hypotheses can explain variation in impact magnitudes: (1) the mass-ratio hypothesis (MRH): the impact of a species is proportional to its abundance and (2) the novel traits hypothesis (NTH): the impact of a species is greater in systems where it is most dissimilar from co-occurring species. Studies that test the relative importance of these hypotheses are lacking.

To better understand how these hypotheses mediate the impact of an invasive species on soil N cycling, we studied Microstegium vimineum, which is a highly invasive grass that tends to increase soil nitrate pools and N mineralization rates. We evaluated M. vimineum’s abundance and impact on soil N cycling in 18 plots, each composed of a native and invaded subplot, in the Duke Forest, NC. The nutrient economies of the forested plots were characterized along an inorganic to organic nutrient economy gradient based on native subplot soil N pools and fluxes and tree mycorrhizal status. M. vimineum’s impacts on soil N cycling were calculated based on the log response ratio of measured N pools and fluxes in paired plots. We expected to find higher M. vimineum abundance in plots that fall along the inorganic end of the nutrient economy gradient.  Moreover, if invader abundance (MRH) was a more important driver of impact magnitude than invader dissimilarity (NTH), then we expected M. vimineum to have larger impacts on soil N cycling along the inorganic end of the nutrient economy gradient.

Results/Conclusions

Preliminary results suggest that M. vimineum promotes greater increases in net N mineralization in plots that fall along the inorganic end of the nutrient economy gradient, where the M. vimineum is more abundant. However, M. vimineum’s impacts on soil N pools and net nitrification rate, showed no trend with respect to plot nutrient economy.  In fact, despite previous reports, we did not observe elevated nitrate or net nitrification rates across aggregated plots. These findings provide weak evidence that the MRH is a more important driver of impact magnitude than the NTH. Both MRH and NTH likely act in unison to mediate M. vimineum’s impacts along the nutrient economy gradient, so further studies are needed to disentangle them. These and future findings may help land managers make ecosystem-function-based decisions.