Soil Nitrogen Conditions Approach Preinvasion Levels following Restoration of Nitrogen-Fixing Black Locust (Robinia pseudoacacia) Stands in a Pine-Oak Ecosystem

Restoring native plant communities on sites formerly occupied by invasive nitrogen-fixing species poses unique problems due to elevated soil nitrogen availability. Mitigation practices that reduce available nitrogen may ameliorate this problem. We evaluated the effects of tree removal followed by soil preparation or mulching on native plant growth and soil nitrogen transformations in a pine-oak system formerly occupied by exotic nitrogen-fixing Black locust (Robinia pseudoacacia) trees. Greenhouse growth experiments with native grasses, Andropogon gerardii and Sorghastrum nutans, showed elevated relative growth rates in soils from Black locust compared with pine-oak stands. Field soil nutrient concentrations and rates of net nitrification and total net N-mineralization were compared 2 and 4 years since Black locust removal and in control sites. Although soil nitrogen concentrations and total net N-mineralization rates in the restored sites were reduced to levels that were similar to paired pine-oak stands after only 2 years, net nitrification rates remained 3-34 times higher in the restored sites. Other nutrient ion concentrations (Ca, Mg) and organic matter content were reduced, whereas phosphorus levels remained elevated in restored sites. Thus, 2-4 years following Black locust tree removal and soil horizon mixing achieved through site preparation, the concentrations of many soil nutrients returned to preinvasion levels. However, net nitrification rates remained elevated; cover cropping or carbon addition during restoration of sites invaded by nitrogen fixers could increase nitrogen immobilization and/or reduce nitrate availability, making sites more amenable to native plant establishment.