Effect of simulated emerald ash borer infestation on nitrogen cycling in black ash (Fraxinus nigra) wetlands in northern Minnesota, USA

Black ash (Fraxinus nigra) wetlands are an important economic, cultural, and ecological resource in the northern Great Lake States, USA, and are threatened by the invasive insect, emerald ash borer (Agrilus planipennis Fairmmaire [EAB]). EAB-induced ash mortality can increase air temperatures, alter understory vegetation communities, and modify wetland hydrology by elevating the water table following canopy dieback, which may, in turn, alter nitrogen cycling in wetland soils and affect plant N uptake, N export to watersheds, and N emissions to the atmosphere. We sampled soils in replicated 1.6-ha plots six years after the following treatments were applied: girdling all ash greater than six cm diameter to simulate EAB-induced mortality, clearcutting with removal of all trees, and an unharvested control. In situ samples were collected from soil cores and from adjacent bulk areas influenced by vegetation to measure the concentration and rates of accumulation of inorganic nitrogen over the 2017 growing season. In the soil cores, the greatest nitrogen accumulation occurred in the unharvested control treatment and nitrate/nitrite were the dominant species of inorganic N. In contrast, in bulk soils, the clearcut treatment had greater overall nitrogen mineralization than both the control and girdle treatments, and ammonium had a greater relative abundance than nitrite/nitrate. Potential N mineralization (assessed with a lab incubation) was not affected by treatment, indicating that treatment effects observed in the field were likely due to changes in the microenvironment. Overall, the effect of simulated EAB on net nitrogen mineralization rates was minimal as mineralization was greatly constrained by ecosystem conditions compared to suitable, laboratory conditions, under which potential N mineralization was an order of magnitude greater than measured in the field. However, canopy treatment influenced water table levels, soil temperature, and vegetation communities, all of which influenced the relative abundance of inorganic N species. Our findings indicate limited indirect effects of EAB on net N mineralization in black ash wetlands.