Tradeoffs and synergies in nutrient retention and greenhouse gas production in constructed agricultural wetlands

Perceived tradeoffs between ecosystem services (ES) delivered by nature-based solutions (NBS) may limit their widespread use as a tool for environmental management. Small artificial waterbodies (constructed ponds and free surface wetlands) are one type of NBS that can help mitigate the downstream eutrophying effects of agricultural nutrient runoff and contribute to carbon (C) storage. However, these waterbodies can also be significant greenhouse gas (GHG) sources. Here, we report on water chemistry, dissolved GHG concentrations and sediment properties measured over three years at 40 Swedish constructed agricultural wetlands. We measured inlet and outlet water chemistry, water column dissolved GHG concentrations and sediment C and phosphorus (P) levels. All waterbodies were supersaturated with carbon dioxide (CO₂) and most were also supersaturated with nitrous oxide (N₂O). There were large temporal variations in inlet water chemistry, highlighting the importance of seasonality and land management. Inlet P concentrations were positively correlated with water column dissolved methane (CH₄) and sediment P concentrations; a clear tradeoff in nutrient retention vs. climate regulation. Inlet nitrogen (N) concentrations were positively correlated with N₂O concentrations, but these waterbodies were also more likely to mitigate downstream dissolved N levels as suggested by lower outflow N concentrations. Sediment C concentrations were unrelated to any measured parameters, suggesting that it would be difficult to purposefully design ponds and wetlands to sequester large amounts of carbon. Although there are tradeoffs between mitigating downstream eutrophication and climate impacts, this should not preclude the use of constructed wetlands and other types of NBS as tools for ES delivery in agricultural landscapes.