WATER QUALITY IMPROVEMENT FROM MANAGEMENT PRACTICES IN AGRICULTURAL WATERSHEDS
Location: Agroecosystems Management Research Unit
Title: Groundwater nutrient concentrations during prairie reconstruction on an Iowa landscape
Submitted to: Agriculture, Ecosystems and Environment
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: August 3, 2010
Publication Date: October 5, 2010
Citation: Tomer, M.D., Schilling, K.E., Cambardella, C.A., Jacobson, P., Drobney, P. 2010. Groundwater nutrient concentrations during prairie reconstruction on an Iowa landscape. Agriculture, Ecosystems and Environment. 139:206-213.
Interpretive Summary: Concerns about agricultural water quality lead us to ask how water quality would change if agricultural land were converted back to native prairie. Opportunities to answer this question rarely occur. We tracked nitrate and phosphorus (P) in groundwater during establishment of tall-grass prairie vegetation at a central Iowa site. We found nitrate and P contrasted one another. Nitrate showed its first decreases within three years, when the prairie first became well established. One year later, nitrate was seldom being detected in groundwater beneath drainageways (grassed waterways when cropped). Groundwater nitrate in upland positions also first declined below 10 ppm nitrate N after three years, and two years later nitrate decreased to an average 2 ppm nitrate-N. The differences resulted from greater amounts of leachable N in upland subsoils. Phosphorus showed a different story. While nitrate concentrations changed under prairie, P did not. The largest P concentrations in groundwater occurred in low-lying positions, along the drainageways. Along and near the drainageways, sediments have been deposited resulting from upslope soil erosion under tillage. These transported agricultural topsoils provide a significant source of P in areas where they accumulate as sediment. Shallow groundwater conditions in these areas help make this P soluble and mobile. We found P concentrations in these drainageways that indicated risk to surface water quality, should that shallow groundwater rise to the surface to become surface runoff. These results point out a legacy impact of agricultural erosion if eroded topsoils are transported and deposited downslope in areas where water tables are high. These results point out a long-term risk of soil erosion, and are of interest to stakeholders with interests in agricultural conservation and ecosystem restoration.
One anticipated benefit of ecosystem restoration efforts is improvement of water quality. This study evaluated NO3-N and phosphorus in subsurface waters during establishment of native prairie vegetation after decades of row-crop agriculture. Prairie seeding in late 2003 resulted in a good prairie cover by 2006. Groundwater monitoring wells were installed prior to seeding, and suction-cup samplers were installed in 2004. Nitrate-N and phosphorus concentrations were monitored. Nitrate-N varied with time and landscape position. In groundwater, non-detectable NO3-N concentrations became modal along ephemeral drainageways in 2006, when average concentrations in upland wells first dropped below 10 mg NO3-N L-1. This decline continued and NO3-N concentrations in upland groundwater averaged near 2 mg NO3-N L-1 after 2007. A toposequence of wells showed a lag in NO3-N decreases shifting upslope with time; the greater lag in uplands was attributed to lower carbon stocks and C:N ratios in upland sub-soils more than vadose zone travel times, given water table dynamics observed at the site. Phosphorus showed a contrasting landscape pattern, but without any temporal trend. Dissolved P concentrations were greatest in shallow groundwater along ephemeral drainageways. Accumulation of sediment eroded from uplands under agriculture provides a source of P along drainageways, where P can be solublized in shallow, reductive groundwater. P concentrations exceeded eutrophication-risk thresholds only along drainageways and at footslopes, where saturation-excess overland flows would most readily transport P to surface waters. Legacy impacts of past agricultural erosion can include soluble phosphorus in shallow groundwater, at sites most prone to saturation-excess overland flow.