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Iowa City, IA, United States

Tomer M.D.,Ames Laboratory | Schilling K.E.,Iowa Geological Survey Bureau | Cambardella C.A.,Ames Laboratory | Jacobson P.,Grinnell College | Drobney P.,U.S. Fish and Wildlife Service
Agriculture, Ecosystems and Environment | Year: 2010

One anticipated benefit of ecosystem restoration is water quality improvement. This study evaluated NO 3-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 NO 3-N and phosphorus. Nitrate-N varied with time and landscape position. Non-detectable NO 3-N concentrations became modal along ephemeral drainageways in 2006, when average concentrations in uplands first became <10mg NO 3-NL -1. This decline continued and upland groundwater averaged near 2mg NO 3-NL -1 after 2007. The longer time lag in NO 3-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. © 2010. Source

Jha M.K.,North Carolina A&T State University | Schilling K.E.,Iowa Geological Survey Bureau | Gassman P.W.,Iowa State University | Wolter C.F.,Geographical Information System Analyst
Journal of Soil and Water Conservation | Year: 2010

The research was conducted as part of the USDA's Conservation Effects Assessment Project. The objective of the project was to evaluate the environmental effects of land-use changes, with a focus on understanding how the spatial distribution throughout a watershed influences their effectiveness.The Soil and Water AssessmentTool (SWAT) water quality model was applied to the Squaw Creek watershed, which covers 4,730 ha (11,683 ac) of prime agriculture land in southern Iowa. The model was calibrated (2000 to 2004) and validated (1996 to 1999) for overall watershed hydrology and for streamflow and nitrate loadings at the watershed outlet on an annual and monthly basis. Four scenarios for land-use change were evaluated including one scenario consistent with recent land-use changes and three scenarios focused on land-use change on highly erodible land areas, upper basin areas, and floodplain areas. Results for the Squaw Creek watershed suggested that nitrate losses were sensitive to land-use change. If land-use patterns were restored to 1990 conditions, nitrate loads may be reduced 7% to 47% in the watershed and subbasins, whereas converting row crops to grass in highly erodible land, upper basin, and floodplain areas would reduce nitrate loads by 47%, 16%, and 8%, respectively. These SWAT model simulations can provide guidance on how to begin targeting land-use change for nitrate load reductions in agricultural watersheds. Source

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