Seasonal and Intra-Event Nutrient Levels in Farmed Prairie Potholes of the Des Moines Lobe

Abstract

HighlightsNitrogen, total phosphorus, and total suspended solids concentrations were higher in early season inundation.Nitrate concentrations in farmed potholes decreased with multiday ponding.Farmed potholes act as hotspots, contributing P to drainage through surface intakes.Abstract. The prairie pothole region ranges from central Iowa to the northwest into Montana and south-central Canada, totaling around 700,000 km2. This area contains millions of potholes, or enclosed topographical depressions, which often inundate with rainfall. Many are located in areas that have been converted to agricultural land through installation of artificial drainage. However, even with drainage, potholes pond or remain saturated during and after significant rain events. In this two-year study, surface water depth was collected hourly (typically from after planting through harvest) from eight farmed potholes (drained and under corn-soybean rotation) on the Des Moines Lobe in central Iowa. Nutrient data were collected daily and tested for nitrogen (N) and phosphorus (P) when inundation depth exceeded 10 cm. The data were analyzed in two ways. First, seasonal differences were investigated using samples from the first day of each inundation event. Surface water concentrations were higher in the early growing season than late season for total N (TN), NO3-N, NH3-N, total P (TP), and total suspended solids (TSS). Secondly, average event concentration changes were determined. Nitrate reductions occurred in 85% of multiday events, but these reductions were offset by increases in P. Total P and dissolved reactive P (DRP) had significant increases that averaged 0.51 and 0.46 mg L-1 per event, respectively, with event lengths of 2 to 19 days. This study demonstrates that inundated farmed potholes reduce NO3-N but serve as in-field hotspots, contributing elevated TP and DRP to drainage waters. When a surface intake directly connects inundated farmed potholes to drainage, new strategies, such as field management or engineered technologies, are needed to mitigate P export. This study is useful in informing policy regarding field management and conservation of farmed potholes. Keywords: Farmed wetland, Nitrogen, Nutrients, Phosphorus, Prairie pothole.