Tradeoffs between nutrient load reductions and greenhouse gas dynamics in constructed agricultural wetlands

Perceived tradeoffs between ecosystem services (ES) delivered by constructed ponds and wetlands in agricultural landscapes may limit their widespread uptake for environmental management. These nature-based solutions (NBS) can mitigate downstream eutrophying effects of agricultural nutrient runoff and contribute to carbon (C) storage. However, they can also be significant greenhouse gas (GHG) sources. Here, we report on water chemistry, dissolved GHG concentrations, total methane (CH 4 ) emissions, and sediment properties measured over three years at 40 Swedish free-surface constructed agricultural wetlands (ponds). Large temporal variations in inlet water chemistry highlighted the influence of seasonality and land management. Inlet phosphorus (P) concentrations were positively correlated with water column dissolved CH 4 and sediment P concentrations; a clear tradeoff in nutrient retention vs. climate impact. A more nuanced pattern occurred for nitrogen (N) where inlet concentrations correlated positively with dissolved nitrous oxide (N 2 O) concentrations. However, these wetlands mitigated downstream dissolved N levels as suggested by lower outflow concentrations. All waterbodies were supersaturated with CO 2 . Higher CO 2 and N 2 O concentrations occurred during fall/winter than summer. The opposite pattern occurred for CH 4 fluxes and dissolved CH 4 although relative contributions from ebullition were greater during fall/winter. Sediment C concentrations were unrelated to any measured parameters, suggesting it would be difficult to purposefully design wetlands for C sequestration. Although there are tradeoffs between mitigating downstream eutrophication and climate impacts, this should not preclude the use of constructed free surface wetlands as multifunctional NBS for ES delivery in agricultural landscapes.