Two-stage ditch restoration: A potential strategy for nitrogen removal from agricultural streams
Sarah S. Roley1, Jennifer L. Tank1, Laura T. Johnson1, Jake J. Beaulieu2, and Mia L. Stephen1. (1) Department of Biological Sciences, University of Notre Dame, Galvin Life Sciences, Notre Dame, IN 46556, (2) US EPA, 26 W. Martin Luther King Drive, Cincinnati, OH 45268
Two-stage ditch construction is a novel restoration technique intended to reduce nitrogen and sediment export from agricultural streams by recreating floodplain benches in formerly ditched channels. Increased floodplain surface area increases water-benthos contact time during storm flows, which may promote biological processing and nitrogen removal via denitrification. Two-stage benches were constructed in a northern Indiana stream in November 2007. We quantified in-stream and bench sediment denitrification rates every other month, beginning >1yr prior to the two-stage restoration. Pre-restoration, in-stream denitrification rates ranged from 5.0 µgN/gAFDM/h in October to 32.8 µgN/gAFDM/h in August and could be explained by stream nitrate concentration. Mean pre-restoration bench denitrification rates averaged 2.5 g N/m2/day, resulting in the removal of 1,870 g N/day in the 620 m reach. Restoration increased floodplain area from 756 m2 to 3,780 m2, increasing the average daily reach-scale nitrogen removal rate to 9,360 g/day. In-stream benthic area did not change as a result of restoration, and average in-stream N removal was 1,932 g N/day. Bench denitrification rates increased significantly when inundated, indicating that denitrification rates are maximized when the benches are saturated (e.g. during storms). Thus, restored floodplains via two-stage restoration have the potential to decrease N export from agricultural streams.