Conservation practices are needed to reduce the loss of nitrate via subsurface tile drainage systems and in this study we evaluated nitrate retention in a reconstructed oxbow in central Iowa that was engineered to receive inputs from two drainage tiles. Our objectives were to evaluate the hydrogeology and nitrate loading patterns and quantify the average and seasonal nitrate retention efficiency in the reconstructed oxbow. Over a two-year period, water and nitrate concentrations and loads into the oxbow were dominated by tile drainage inputs compared to groundwater seepage. Nitrate concentrations were highest in tile drainage water (9 to 17 mg/l), similar in upgradient groundwater and in the oxbow itself (4-8 mg/l) and lowest in downgradient groundwater (0.2 mg/l). Using N:Cl ratios, we estimated nitrate retention efficiency from May to September to range from 44% to 47% in 2014 and 2015, respectively, and found that on a monthly basis, greater retention efficiencies were measured in late summer and early fall. The nitrate retention efficiency was similar to other practices such as bioreactors, wetlands and saturated buffers. Given ecosystem benefits of oxbows and similar costs compared to bioreactors, we believe that reconstructing oxbows to receive tile drainage water should be considered a viable practice for tile drainage treatment in agricultural areas.

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N utrient export from the agricultural US Midwest influences streams and rivers and contributes to the development of hypoxia in the Gulf of Mexico (Turner et al., 2008). Gulf hypoxia is caused by nitrate exported from row crop agricultural fields via the Mississippi River. One path for nitrate export is leaching from soil and transport to streams through groundwater discharge and subsurface tile drainage systems. Watersheds with tile systems are highly susceptible to increased nitrate losses to streams and rivers, and conservation practices are needed mitigate these losses (Jaynes et al., 2001; Schilling et al., 2012; Tomer et al., 2013). Oxbows are natural waterbodies formed when a river cuts off a meander loop as it migrates within its floodplain (Wohlman and Leopold, 1957). While natural oxbows are among the most biologically diverse aquatic systems in the world (Ward, 1998), in agricultural regions, practices such as stream channelization and removal of riparian vegetation, along with increased drainage from tiles and ditches, profoundly changed natural stream hydrology (e.g., Schumm et al., 1984). Oxbows in agricultural areas tend to be isolated from the main channel and rapidly accumulate sediment and organic material from overbank flooding, as they transition to terrestrial habitat (Constantine et al., 2010). Oxbow restoration reverses this process by removing the fill material and restoring hydrologic connection and aquatic habitat (Kenney, 2018). Wetlands are effective in reducing downstream export of nutrients such as nitrate from agricultural lands (Crumpton et al., 2008). Due to their proximity to streams, oxbows, whether naturally occurring or restored, are a type of wetland that is ideally suited to process nitrate-rich water exiting agricultural fields via natural pathways or artificial tile systems. Thus, restoration of former oxbows (Zambory et al., 2019), particularly where they can intercept