Biogeochemical effects of a forest understory plant invasion depend more on dissimilar nutrient economies than invader biomass

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There is increasing need to better understand how and why invasion impacts on biogeochemical cycling differ across heterogeneous landscapes. One hypothesis predicts invader impacts are greatest, where the invader is most abundant (the mass ratio hypothesis; MRH). Alternatively, invader impacts may be greatest in communities, where the nutrient acquisition strategies of the invader are most dissimilar from those of native species (the nutrient economy dissimilarity hypothesis; NEDH). We tested whether the impact of an invasive grass, Microstegium vimineum, on soil biogeochemistry could be better explained by MRH, NEDH, or both. Invaded and reference study plots were established at 3 locations (Indiana, North Carolina, and Georgia) that varied in the relative abundance of arbuscular mycorrhizal (AM) versus ectomycorrhizal (ECM) associated overstory trees (across a nutrient economy gradient), reflecting gradients in biotic nutrient acquisition strategies and edaphic factors. At 2 locations, we found a proxy for NEDH explained invader effects on soil conditions and the net effect of M. vimineum was to homogenize soil properties across the nutrient economy gradient toward conditions consistent with AM-dominated stands. At the third location, both ECM-dominance (NEDH) and invader biomass (MRH) predicted differences in soil moisture, pH, and nitrification rates with may be related to the high N availability and intermediate acidity at this location. Collectively, these results suggest the biogeochemical consequences of M. vimineum depend, in part, on preinvasion soil nutrient economies. Where preinvasion conditions are known, we provide a scalable and predictive approach to determine where impacts on biogeochemical cycling of C and N may be greatest.

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