Plant invasions can change the soil ecosystem, resulting in changes in the soil microbial community and nutrient cycling, but how quickly these changes develop is not certain.  The impact of invasion on the soil ecosystem may be assumed to be proportional to the time since invasion or to invasion density, but this assumption is untested and has little theoretical support.  Furthermore, if small amounts of invasion cause large changes in the ecosystem, assuming a linear response to invasion can have large costs in terms of future restoration efforts.  Using both litter and live-plant microcosms, we tested the hypothesis that structure and function of the microbial community changed in proportion to the density of Berberis thunbergii, an invasive shrub in the deciduous forests of the northeastern United States.  Microbial community structure was measured by extracting phospholipid fatty acids from the soil, while function was characterized in the live-plant microcosms by soil enzyme activity and nitrogen mineralization, and in the litter microcosms by the rate of litter decomposition.  

Results/Conclusions

The density of invasion strongly and linearly impacted decomposition rate in the litter microcosms (R² = 0.945), but microbial community structure displayed a non-linear response (R² = 0.513).  The bacteria:fungi ratio nearly doubled after the addition of only 2.5% Berberis litter to tree canopy litter, but further additions (5%, 10%, 25%, 50%, 75%, and 100%) did not result in any further increase in the bacteria:fungi ratio.  In live-plant microcosms, the responses of the soil microbial community were not linearly related to invasion density, and were varied between pots containing different plant species.  The results of both experiments suggest that the relationship between invasion density and the impact on the soil ecosystem is complex and not necessarily linear.  This result has implications for the management and control of invasive species, suggesting low-density invasions should not be assumed to have low impacts.  Future studies are needed to examine the impacts in low-density and early-stage invasions, and more work is needed to determine what conditions lead to either linear or nonlinear impacts of invasion.