CROPPED POTHOLE DEPRESSIONS ARE HOTSPOTS OF NUTRIENT INFILTRATION

Leaching of nitrogen (N) and phosphorus (P) from soils in the Midwestern Corn Belt poses a major threat to human health and downstream ecosystems. Depressional wetlands are a common feature across the Midwest and constitute roughly 10% of the Des Moines Lobe. They concentrate overland and shallow subsurface flow and often contain standing water and coinciding nutrients. We hypothesize that these farmed wetlands play an outsize role in nutrient export compared to upland positions. To test this hypothesis, we measured infiltration fluxes and soil concentrations of N and P along a topographic gradient spanning a “ridge” to a depression under corn cultivation near Ames, IA. We found strong seasonal and spatial trends. January alone accounted for 61% of annual P infiltration following fall application of P fertilizer despite yielding only 7% of total water infiltration. Wetland soils had nearly three times higher water infiltration rates than surrounding uplands. This trend explains similarly elevated N infiltration (214%). These data suggest that management interventions that establish nutrient sinks (e.g. flood-tolerant perennial vegetation) in depressions could disproportionately mitigate nutrient losses to tile drains and downstream waters.