Non-native nitrogen-fixing plants are a particular threat to ecosystems because of their capacity to alter soil nutrient cycles. Even after they have been removed, nitrogen pools and/or rates of N cycling may remain elevated – a “legacy” of invasion that outlives the plants’ presence. It remains unresolved how long such legacies last and what management strategies contribute to their relaxation. We quantified the magnitude of invasion legacies on soil N cycling rates in an inland pitch pine barren habitat invaded by the N-fixing black locust (Robinia pseudoacacia) tree. We used laboratory incubations to measure rates of net N mineralization and net nitrification in soils from 50 sites that included a range of land use (currently invaded, never invaded, and invaded-but-restored). Among restored sites, we took advantage of a chronosequence of time-since-restoration (0-16 years), as well as variation in fire history (0-4 fires). We hypothesized that, overall, N dynamics in restored sites would be intermediate between sites that are currently invaded and those that have never been invaded. We also hypothesized that, among the restored sites, N cycling rates would decline as time-since-restoration increased. Finally, we hypothesized that sites that experienced prescribed burns would have lower N cycling rates than those that have not burned.
Results/Conclusions
Currently-invaded sites had the highest rates of both net N mineralization and net nitrification by a factor of more than two. We observed a legacy of past invasion in restored sites in that rates of net N nitrification were significantly higher in restored sites than in never-invaded sites. Rates of net N mineralization did not differ between restored and never-invaded sites, though their rates of N cycling were all significantly lower than in locust-dominated sites. N cycling rates decreased significantly in restored sites as the number of years since restoration increased – but only in unburned plots. Fire had transient effects of reducing soil N cycling rates in the most-recently burned sites, but overall, N cycling rates in burned sites were not significantly lower compared to unburned ones. Our study demonstrates that, while the N-fixing black locust tree leaves a legacy of altered soil N dynamics even after it is removed, the magnitude of that legacy decreased with time even in the absence of other active management. These findings may inform management in the numerous other systems in which N-fixing invaders have transformed N cycling rates.