Long-Term Nitrogen and Phosphorus Outflow from an Instream Constructed Wetland under Precipitation Variability

In many agricultural watersheds, surface runoff often causes unwanted nitrogen (N) and phosphorus (P) losses from croplands into stream networks. When this phenomenon is pronounced, it significantly changes N and P concentrations in streams affecting aquatic ecosystems. To protect stream water quality, the installation of instream-constructed wetlands (ICWs) for treating runoff water is often reported as a low-cost alternative to conventional water treatment systems. Indeed, ICWs have the capacity to collect and temporarily retain nutrients transported from agricultural landscapes and then slowly release them into downstream networks. However, the long-term hydrologic behavior of ICWs relative to N and P outflow control is still insufficiently reported. Especially in the context of climate change, it is relevant to investigate the effect of precipitation variability on ICWs N and P outflow. This study uses the soil and water assessment tool (SWAT) model to approximate the long-term hydrologic behavior of an experimental ICW installed in a small agricultural watershed. The model was set assuming a continuous corn and soybean rotation on croplands, then a multidecadal (period 2001–2020) simulation was used to evaluate the implication of precipitation variability on total nitrogen (TN), nitrate-N (NO3-N), total P (TP), and dissolved P (DP) outflows. Results show meaningful changes in the precipitation pattern with contrasting effects on N and P outflows. While analyses show significant trends in the maximum monthly precipitation, nutrient outflows during two consecutive decades, 2001–2010 and 2011–2020, show increases of 46% for TN, and 82% for TP. At the watershed scale, month-to- month TN and TP outflows range from 24 to 810 kg N and 26 to 1358 kg P during 2011–2020, compared with 42 to 398 kg N and 40 to 566 kg P during 2001–2010. The increase in nutrient outflow is particularly pronounced for TP and DP which show significant trends and high correlations (r > 0.70) with maximum monthly precipitation. An exception is nitrate-N outflow, which counts on average for less than 5% of TN outflow but appears more affected by the timing of N fertilization in the watershed.