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Groundwater nutrient concentrations during prairie reconstruction on an Iowa landscape

Tomer, M.D., Schilling, K.E., Cambardella, C.A., Jacobson, P., Drobney, P.
Agriculture, ecosystems & environment 2010 v.139 no.1-2 pp. 206
prairies, land restoration, water quality, environmental impact, nitrate nitrogen, phosphorus, groundwater contamination, groundwater, agricultural land, field crops, temporal variation, spatial variation, drainage channels, leaching, losses from soil, vadose zone, groundwater flow, water table, solubility, eutrophication, soil erosion, Iowa
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 <10mg NO₃-NL⁻¹. This decline continued and upland groundwater averaged near 2mg NO₃-NL⁻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.