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<p>Species invasions are more prevalent than ever before. While the addition of a
species can dramatically change critical ecosystem processes, factors that mediate
the direction and magnitude of those impacts have received less attention. A better
understanding of the factors that mediate invasion impacts on ecosystem functioning
is needed in order to target which exotic species will be most harmful and which systems
are most vulnerable. The role of invasion on nitrogen (N) cycling is particularly
important since N cycling controls ecosystem services that provision human health,
e.g. nutrient retention and water quality.</p><p>We conducted a meta-analysis and
in-depth studies focused on the invasive grass species, Microstegium vimineum, to
better understand how (i) plant characteristics, (ii) invader abundance and neighbor
identity, and (iii) environmental conditions mediate the impacts of invasion on N
pools and fluxes. The results of our global meta-analysis support the concept that
invasive species and reference community traits such as leaf %N and leaf C:N are useful
for understanding invasion impacts on soil N cycling, but that trait dissimilarities
between invaded and reference communities are most informative. Regarding the in-depth
studies of Microstegium, we did not find evidence to suggest that invasion increases
net nitrification as other studies have shown. Instead, we found that an interaction
between its abundance and the neighboring plant identify were important for determining
soil nitrate concentrations and net nitrification rates in the greenhouse. In field,
we found that variability in environmental conditions mediated the impact of Microstegium
invasion on soil N pools and fluxes, primarily net ammonification, between sites through
direct, indirect, and interactive pathways. Notably, we detected a scenario in which
forest openness has a negative direct effect and indirect positive effect on ammonification
in sites with high soil moisture and organic matter. Collectively, our findings suggest
that dissimilarity in plant community traits, neighbor identity, and environmental
conditions can be important drivers of invasion impacts on ecosystem N cycling and
should be considered when evaluating the ecosystem impacts of invasive species across
heterogeneous landscapes.</p>
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