Groundwater nutrient concentrations during prairie reconstruction on an Iowa landscape

One anticipated benefit of ecosystem restoration is water quality improvement. This study evaluated NO3-N and phosphorus in subsurface waters during prairie establishment following decades of row-crop agriculture. A prairie seeding in late 2003 became established in 2006. Wells and suction cup samplers were monitored for NO3-N and phosphorus. Nitrate-N varied with time and landscape position. Non-detectable NO3-N concentrations became modal along ephemeral drainageways in 2006, when average concentrations in uplands first became <10 mg NO3-N L−1. This decline continued and upland groundwater averaged near 2 mg NO3-N L−1 after 2007. The longer time lag in NO3-N response in uplands was attributed to greater quantities of leachable N in upland subsoils. Spatial differences in vadose-zone travel times were less important, considering water table dynamics. Phosphorus showed a contrasting landscape pattern, without any obvious temporal trend. Phosphorus was greatest along and near ephemeral drainageways. Sediment accumulation from upland agricultural erosion provided a source of P along drainageways, where shallow, reductive groundwater increased P solubility. Phosphorus exceeded eutrophication risk thresholds in these lower areas, where saturation-excess runoff could readily transport P to surface waters. Legacy impacts of past agricultural erosion and sedimentation may include soluble phosphorus in shallow groundwater, at sites prone to saturation-excess runoff.

Research highlights▶ Prairie reconstruction in Iowa decreased NO3-N concentrations within 3–5 years. ▶ The rate at which nitrate decreased was greatest along waterways. ▶ Phosphorus contrasted nitrate, showing no temporal trend. ▶ Phosphorus greatest along waterways where groundwater could rise to become runoff. ▶ Along waterways, sediment accumulated due to past erosion is likely phosphorus source.