Temperature and water-level effects on greenhouse gas fluxes from black ash (Fraxinus nigra) wetland soils in the Upper Great Lakes region, USA

Forested 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]). These wetlands are likely to experience higher water tables and warmer temperatures if they are impacted by large-scale ash mortality and other global change factors. Therefore, it is critical to understand how temperature, hydrology, and their interaction affect greenhouse gas fluxes in black ash wetland soils. In order to predict potential ecosystem changes, we sampled and incubated intact soil cores containing either mineral or organic (peat) soils from two black ash wetlands, monitored soil oxidation-reduction potential (Eh), and measured the efflux of carbon dioxide (CO₂), methane (CH₄), and nitrous oxide (N₂O) at two water-level treatments nested in three temperature treatments, 10 °C, 15 °C, or 20 °C. The water-level treatments were either saturated or drawdown, designed to mimic wetlands impacted or not impacted by EAB. Mean CO₂ fluxes increased with increasing temperature but did not vary significantly by soil type or water-level. Peat soil had 60 to 135 times significantly greater CH₄ flux in the saturated treatment and had minimal N₂O loss across all treatments, while mineral soils had 8 to 43 times significantly greater N₂O flux in the saturated treatment, and minimal CH₄ loss across all treatments. Gas fluxes generally increased and had greater variation with increasing temperature. The drawdown treatment resulted in significantly higher Eh during unsaturated periods in both soil types, but the response was more variable in the peat soil. Our findings demonstrate potential indirect effects of EAB in black ash wetlands, with implications for ecosystem functions associated with C and N cycling.