Nitrate-N load reduction measured in a central Iowa restored oxbow

This short communication presents new data collected from a tile-fed oxbow site in north-central Iowa and reports on the quantification of NO3-N load reductions using a mass balance approach. Using improved monitoring equipment deployed at the site in 2017, including a continuously-reading nitrate sensor, a NO3-N budget for the oxbow was developed to estimate annual and monthly load reductions occurring in the reconstructed oxbow. Daily NO3-N input was primarily from two drainage tiles and concentrations in the oxbow ranged from <0.2 to 3.5 mg/l. Based on daily mass balance, the oxbow retained 42.3 kg of NO3-N, or 0.21 g N m−2 d−1, and the NO3-N retention efficiency was 35.4%. Removal efficiencies in early spring and late summer and fall were much higher than late spring and early summer. Based on mass load reductions, the effectiveness of oxbows for N load reductions is greatest when oxbows receive greater N loads from tile drainage compared to N loads delivered from flood pulses.