Floodplain restoration enhances denitrification and reach‐scale nitrogen removal in an agricultural stream

Abstract

Abstract. Streams of the agricultural Midwest, USA, export large quantities of nitrogen, which impairs downstream water quality, most notably in the Gulf of Mexico. The two-stage ditch is a novel restoration practice, in which floodplains are constructed alongside channelized ditches. During high flows, water flows across the floodplains, increasing benthic surface area and stream water residence time, as well as the potential for nitrogen removal via denitrification. To determine two-stage ditch nitrogen removal efficacy, we measured denitrification rates in the channel and on the floodplains of a two-stage ditch in northcentral Indiana for one year before and two years after restoration. We found that instream rates were similar before and after the restoration, and they were influenced by surface water NO 3 À concentration and sediment organic matter content. Denitrification rates were lower on the constructed floodplains and were predicted by soil exchangeable NO 3 À concentration. Using storm flow simulations, we found that two-stage ditch restoration contributed significantly to NO 3 À removal during storm events, but because of the high NO 3 À loads at our study site, ,10% of the NO 3 À load was removed under all storm flow scenarios. The highest percentage of NO 3 À removal occurred at the lowest loads; therefore, the two-stage ditch's effectiveness at reducing downstream N loading will be maximized when the practice is coupled with efforts to reduce N inputs from adjacent fields.

Nutrient Retention in Restored Streams and Rivers: A Global Review and Synthesis

“…After decades of trying to quickly move runoff away from urban landscapes, perceptions regarding managing urban runoff have changed. There can be important water quality and flood safety benefits associated with reducing the velocity of runoff and retaining water on the landscape [56].…”

Combining precision conservation technologies into a flexible framework to facilitate agricultural watershed planning

“…Proponents suggest the two-stage design should be self-maintaining in terms of sediment storage and transport. The vegetated bench provides an environment for enhanced biodiversity, nutrient uptake, and denitrification (Roley et al 2012). Sites suited to the practice can potentially be mapped based on bank height, which can be mapped using LiDAR.…”

Assessment of beyond-the-field nutrient management practices for agricultural crop systems with subsurface drainage

“…Retaining vegetated benches increases the surface area of ditches and retention time during high flows. In Ohio and Indiana, denitrification rates have been found to be greater in sediments on naturally formed ditch benches than in sediments from side slopes of maintained trapezoidal ditches (Powell and Bouchard 2010), and having benches may influence in-stream denitrification rates (Roley et al 2012a(Roley et al , 2012b. Managing flow regimes in ditches can reduce N and moderate downstream P losses through greater capacity for sorption Strock et al 2007Strock et al , 2010Smith 2009).…”

“…A simulation study found that if 1% of a watershed was converted to benches within ditches, the increased storage could remove up to 20% of NO 3 -N loading in the system (Kallio 2010). There is extensive research on quantifying N removal rates and sediment dynamics in several ditches in Indiana, Michigan, and Ohio (Roley et al 2012a(Roley et al , 2012bRoley et al 2014;Davis et al 2015). At a constructed two-stage ditch in the Tippecanoe River watershed in Indiana, monitoring began one year prior to two-stage construction in both an upstream control and a downstream treatment reach and has continued for several years postconstruction.…”