A microbial perspective on balancing trade-offs in ecosystem functions in a constructed stormwater wetland

Abstract

Green stormwater infrastructure, such as constructed wetlands (CWs), is a type of stormwater control measure that can decrease nutrient and pollutant loads from urban stormwater runoff. Wetland soil microorganisms provide nutrient and pollutant removal benefits which can also result in ecosystem disservices such as greenhouse gas (GHG) emissions and can inadvertently exacerbate climate change. Microbial respiration by facultative anaerobes in anoxic conditions is the primary pathway for nitrogen removal (benefit). Similar anoxic conditions that support denitrifying microorganisms can also support obligate anaerobes that produce methane (CH4) via methanogenesis (disservice). We examined nitrogen removal potential, GHG production, and microbial community structure within permanently flooded and shallow land or temporarily-flooded areas of a stormwater CW to identify zones for CW design optimization. Results indicate that permanently flooded zones compared to shallow land zones are greater sources of CH4 emissions (80.80 ± 118.31, 2.32 ± 9.33 mg CH4-C m−2 h−1, respectively) and emit more carbon to the atmosphere (7161.27 kg CO2, 93.20 kg CO2 equivalents, respectively). However, nitrogen removal potential rates were similar across both flooded and shallow land zones (24.45 ± 20.18, 20.29 ± 15.14 ng N2O-N hr−1 g−1 dry soil, respectively). At this particular CW, reduction of permanently flooded zones within the wetland could decrease GHG emissions (disservice) without limiting nitrogen removal (benefit) potential of the wetland. Holistic development and design of stormwater control measures, which account for microbial activity, provides the opportunity to maximize benefits (i.e., nutrient and pollutant removal) and reduce disservices (i.e., GHG emissions) of green stormwater infrastructure.