Effects of simulated emerald ash borer disturbance on carbon and nitrogen cycling in black ash wetland soils in the Great Lakes region, USA

Abstract

Forested black ash (Fraxinus nigra) wetlands are an important economic, cultural, and ecological resource in the northern Great Lake States, USA, which are threatened by the non-native invasive insect, emerald ash borer (Agrilus planipennis Fairmmaire (EAB)). EAB-induced ash mortality can modify wetland hydrology by elevating the water table, increase air temperature, and alter understory vegetation communities following canopy dieback. These changes may alter gas fluxes and nutrient cycling in wetland soils. In our first experiment, we incubated soil cores from two black ash wetlands with either mineral or peat soils, 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 (saturated or periodic drawdown) nested in three temperature treatments (10°C, 15°C, or 20°C). Findings indicate that elevated water tables in mineral soil black ash wetlands would result in greater N2O fluxes, while elevated water tables in peat soil black ash wetlands would result in greater CH4 fluxes and carbon release into the atmosphere. Increased soil temperature will lead to greater gaseous fluxes in both wetland ecosystems. In a second experiment, we sampled soils in plots that either simulated EAB disturbance or clearcut in black ash wetlands in northern Minnesota to assess effects on nitrogen mineralization rates. Soil cores installed in the treatment plots to isolate soil from the influence of vegetation were sampled over the 2017 growing season in addition to bulk soil samples collected from vegetated areas of the treatment plots. Soils in the clearcut treatment had greater ammonium accumulation than the control and girdle treatments, but when the influence of vegetation was removed, the isolated soil cores had greater rates of nitrification in the control treatment. Overall, the effect of simulated EAB was minimal and nitrogen mineralization rates were greatly constrained by ecosystem conditions compared to ideal, laboratory conditions. Our findings demonstrate potential indirect effects of EAB in black ash wetlands, with implications for ecosystem functions associated with C and N cycling.