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

Highlights

• Greenhouse gas fluxes increased as soil temperature increased.

• Elevated water-level in mineral soil cores led to increased nitrous oxide fluxes.

• Elevated water-level in peat cores led to greater methane fluxes.

• There was no effect of treatment on soil carbon dioxide fluxes in either soil.

• Methane and nitrous oxide fluxes were restricted to specific soil redox ranges.

Abstract

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.

Introduction

In the northern Great Lakes region (Upper Michigan, Wisconsin, and northern Minnesota), many forests have significant components of ash (Fraxinus spp.) in the overstory, which are especially vulnerable to changes in composition and succession sequences following emerald ash borer (EAB) infestation (MacFarlane and Meyer, 2005). Since its discovery in the United States in 2002, EAB has spread quickly across the country, killing ash forests within 6 years of infestation (Herms and McCullough, 2014; Knight et al., 2013; Smitley et al., 2008). The invasion of EAB and large-scale ash dieback can decrease litter input, increase amounts of large woody debris, and cause canopy gap formation; cascading effects that may lead to large-scale changes in ecosystem structure, food web interactions, and altered biogeochemical cycling (Kolka et al., 2018). Forested black ash (F. nigra) wetlands are especially prone to ecosystem-scale changes because ash makes up 40–100% of the canopy (Looney et al., 2015; Van Grinsven et al., 2017), they are already experiencing ash dieback (Palik et al., 2012, Palik et al., 2011), and transpiration by ash trees is a major control on hydrology (Telander et al., 2015). Ash mortality in these ecosystems causes the water table to rise, potentially transitioning a forested ecosystem to an open marsh (Slesak et al., 2014; Diamond et al., 2018). This groundwater sensitivity to transpiration makes forested ash wetlands especially vulnerable to altered ecosystem function following the invasion of EAB.

Changes in the canopy, understory vegetation communities, water-level, and soil temperature may lead to altered biogeochemical cycling and potential nutrient loss from black ash wetlands to the atmosphere and down-stream ecosystems following EAB invasion. The gaseous fluxes of carbon (as CO₂ and CH₄) and nitrogen (N₂O) as greenhouse gasses are measurable indicators of how changes in ecosystem dynamics will alter nutrient cycling and ecosystem function. Greenhouse gas fluxes are controlled by the abundance and productivity of microbial populations in the soil, which are regulated by several factors including temperature, organic matter, and the availability of electron donors and acceptors needed to complete the oxidation-reduction reactions of metabolic processes under aerobic and anaerobic conditions (Altshuler et al., 2019; Ebrahimi and Or, 2016; Hou et al., 2000). The availability of electron donors can also limit nutrient cycling reactions in the soil and therefore gas production (Wang et al., 1993). For example, denitrification requires a soluble carbon electron donor and nitrification uses ammonium as an electron donor. A noticeable shift in Eh and gas fluxes will only occur if soil microbes have an abundant food source (soil organic matter), abundant electron donors and acceptors to complete metabolic reactions, and an environment warm enough to be biologically active. Given the above, it is necessary to understand how soil temperature and degree of soil saturation affect gas fluxes in black ash wetlands with different hydroperiods and soil composition (i.e., mineral vs. organic soils).

Van Grinsven et al. (2018) measured soil CO₂ and CH₄ fluxes in black ash wetlands with simulated EAB disturbance in Upper Michigan. The disturbance resulted in significantly higher CO₂ fluxes, largely because the disturbed sites had elevated soil temperatures due to increased solar radiation with canopy loss. The average CH₄ fluxes in disturbed sites were not significantly different from the control sites, but larger CH₄ fluxes did occur more frequently in disturbed sites. Overall, changes in soil temperature and moisture regulated the soil carbon fluxes, producing significant treatment effects. In northern Minnesota, simulated EAB disturbance in black ash wetlands resulted in warmer air temperatures, but soil gas fluxes were not evaluated (Slesak et al., 2014). The differences in hydrology and soil composition between the black ash wetlands studied in Upper Michigan, and those in other regions of the Great Lakes states will likely result in different gaseous carbon flux responses following EAB induced ash dieback.

Field experiments simulating EAB-induced ash mortality in black ash wetlands in northern Minnesota and Upper Michigan have measured changes in water table, understory plant communities, soil temperature, and nutrient cycling (Davis et al., 2017; Looney et al., 2015; Slesak et al., 2014; Telander et al., 2015; Van Grinsven et al., 2017, Van Grinsven et al., 2018). Other studies and literature reviews suggest EAB will cause changes in ash foliar chemistry, invertebrate communities, and trophic interactions that will cascade through entire ecosystems (Chen et al., 2011; Nisbet et al., 2015; Perry and Herms, 2016; Youngquist et al., 2017). However, many hypothesized impacts of emerald ash borer on forested wetlands are still uncertain, and there is an urgent need to understand the cascading impacts of EAB on ecosystem processes such as greenhouse gaseous efflux. We used a soil core incubation experiment that mimicked potential future hydrologic and temperature regimes to investigate the interactions among water-level, soil temperature, soil oxidation-reduction potential (redox), and greenhouse gas (CO₂, CH₄, and N₂O) fluxes in black ash wetland soils in the Great Lakes region to determine if: (1) warmer soil temperatures will result in greater greenhouse gas fluxes, (2) higher water levels will increase greenhouse gas fluxes, and if so, (3) there is an interaction with soil temperature that amplifies the water table response, and (4) the response differs between mineral and peat soils. This study builds on previous research studying a critical system of feedbacks facing the imminent threat of emerald ash borer invasion and a cascade of ecosystem-scale changes (Davis et al., 2017; Looney et al., 2015; Slesak et al., 2014; Van Grinsven et al., 2018, Van Grinsven et al., 2017).