Instream constructed wetland capacity at controlling phosphorus outflow under a long-term nutrient loading scenario

Authors: Sohoulande, Clement; Szogi, Ariel; Novak, Jeffrey; Stone, Kenneth - Ken; Martin, Jerry; Watts, Donald - Don

Submitted to: Modeling Earth Systems and Environment
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/12/2023
Publication Date: 3/29/2023
Citation: Sohoulande Djebou, D.C., Szogi, A.A., Novak, J.M., Stone, K.C., Martin, J.H., Watts, D.W. 2023. Instream constructed wetland capacity at controlling phosphorus outflow under a long-term nutrient loading scenario. Modeling Earth Systems and Environment. https://doi.org/10.1007/s40808-023-01763-w.
DOI: https://doi.org/10.1007/s40808-023-01763-w

Interpretive Summary: Phosphorus fertilization is an essential component of agricultural production systems as it contributes to maintaining high crop yields and is often integrated with livestock manure nutrient management. However, the seasonal application of phosphorus on croplands causes unwanted losses into streams. Nevertheless, controlling phosphorus transport into streams could maintain its concentration below rates that negatively affect aquatic life. At the watershed level, there is a possibility to temporarily trap phosphorus transported from croplands into constructed wetlands. However, the long-term efficiency of these constructed wetlands in controlling phosphorus release or mitigating the impact of extreme precipitation and runoff events is less known. Hydrological models can be used to estimate the behavior of constructed wetlands and help better understand their long-term functionality under heavy precipitations. This study uses the soil and water assessment tool model to evaluate the capacity of a constructed wetland to control phosphorus outflow under a long-term nutrient loading scenario. The model was calibrated and validated using experimental data and the assumption of a continuous corn and soybean rotation across a small watershed. A multi-decadal simulation was used to evaluate long-term dissolved and total phosphorus outflow. Statistics show a significant relationship with the precipitation variability. Indeed, the long-term control of phosphorus release into the stream network seems effective under low to moderate precipitation conditions. However, the phosphorus release is likely out of the control of the constructed wetland during extreme precipitation events which cause phosphorus flushing from the bottom of the pond.

Technical Abstract: Phosphorus (P) fertilization is an essential component of agricultural production systems as it contributes to maintaining high crop yields and is often integrated with livestock manure nutrient management. However, the seasonal application of P on croplands causes unwanted P losses into aquatic systems. Nevertheless, controlling P transport into stream networks could maintain P concentration below rates that negatively affect aquatic life and the ecosystem in general. At the watershed level, there is a possibility to temporarily trap the P transported from agricultural landscapes into instream constructed wetlands (ICWs). However, the long-term efficiency of these ICWs in controlling P release into a stream network or mitigating the impact of extreme precipitation and runoff events is less known. Hydrological models can be used to portend ICWs behavior and help better understand their long-term functionality under exceptional climate conditions. This study uses the soil and water assessment tool (SWAT) model to evaluate the capacity of an ICW to control P outflow under a long-term nutrient loading scenario. The model was calibrated and validated for both stream and P flow using experimental ICW data and the assumption of a continuous corn and soybean rotation across a small agricultural watershed. A multi-decadal simulation was used to evaluate long-term dissolved P (DP) and total P (TP) outflow from the ICW under the spectrum of the climate condition in the watershed. Point biserial correlations of monthly balances of DP ('DP) and TP ('TP) in the ICW show a significant relationship with the precipitation variability. Indeed, the long-term ICW’s control of P release into the stream network seems effective under low to moderate precipitation conditions. However, the P release is likely out of the control of the ICW during extreme precipitation events which tends to trigger DP and TP flushing from the bottom of the ICW's pond.