As non-native invasive plants spread and replace native trees and shrubs, the diversity, composition, and ecosystem function of forests is often changed and degraded. Many invasive species compromising forest health are non-native shrubs that suppress regeneration of native plants and alter soil-C and N cycles. Urban forests are particularly vulnerable, due to the increased invasion pressure and alterations of N cycling. Furthermore, high levels of soil-N may facilitate invasion of non-native shrubs (particularly Rosa multiflora), subsequently affecting N mineralization along a gradient of invasion. We selected three urban forest sites previously established as part of a long-term study, and classified them as experiencing low, moderate, or high invasion pressure based on the total number of invasive stems hectare-1. All woody plant stems were counted in both the herbaceous/seedling (height ≤ 1 m) and sapling layers (height > 1 m; DBH < 2.54 cm). Soil samples were collected during the 2017 summer season and analyzed for NO3--N and NH4+-N, both before and after a 30-day incubation period, to determine initial levels of soil-N and N-mineralization rates at these sites. Soil-N data were analyzed using one-way ANOVA to determine differences among sites.
Results/Conclusions
Our preliminary results suggest that mean N-mineralization rates differ based on invasion pressure (Fs=8.853, p=0.002). N-mineralization was significantly higher in the site with lowest invasion pressure (0.55 ± 0.07) compared to the moderate (0.31 ± 0.06) and high (0.07 ± 0.01 μg N produced g-1 soil day-1) invasion sites. Mean nitrification rate was significantly higher in the site with lowest invasion pressure (0.67 ± 0.08; Fs=16.807, p<0.001) compared to sites with moderate (0.14 ± 0.03) and invasion. Mean ammonification in the moderate invasion site (0.18 ± 0.05) was significantly higher than both low (-0.12 ± 0.03) and high (-0.09 ±0.01 μg NH4-N produced g-1 soil day-1) invasion sites (Fs=9.916, p=0.001). Thus, N-mineralization rates appear to be driven by ammonification under moderate invasion, and by nitrification under low invasion pressure. In spring and summer 2018, soils will be collected and analyzed to determine potential responses to several treatments, including shrub removals, native plant restoration, and soil amendment using mulched woody debris. Ultimately, we hope to understand whether soil N cycling can be altered to discourage or prevent the growth of invasive shrubs in susceptible urban forests.