Mitigating Sources of Indirect Nitrous Oxide Emissions From Tile Drain by On-Site Wood-Chip Bioreactors.

Indirect nitrous oxide (N₂O) emissions originating from nitrate-laden agricultural drainage waters represent a substantial fraction of total N₂O emissions in the USA. Typical strategies to mitigate indirect N₂O emissions are either improving fertilization methods or on-site treatment of drainage water. Recently, wood-chip bioreactors have been shown to reduce nitrate in drainage waters. However, no field studies have evaluated balances between nitrate removal and release of dissolved N₂O by on-site wood-chip bioreactors. We measured nitrate-N, dissolved N₂O-N, and flow rates of bioreactors in two locations (Dundas, and Claremont, Minnesota, USA) during a growing season. Drainage waters contained an average of 24 and 16 mg N L^-1 of nitrate in Dundas and Claremont, respectively. Bioreactors reduced nitrate-N concentration in both locations (22-99% in Dundas, 6-77% in Claremont). Drainage waters contained an average of 21 and 9 µg N L^-1 of N₂O-N in Dundas and Claremont, respectively. Ratios of dissolved N₂O-N to NO₃-N in inflow of bioreactors were 0.000653 for Dundas and 0.000414 for Claremont, which were consistent with published data (0.0003-0.06). Bioreactor treatments reduced dissolved N₂O concentration in Dundas (average 64%), but increased in Claremont (average 21 times). Using indirect N₂O emission factors (EF5) by Intergovernmental Panel on Climate Change, the bioreactor in Dundas was estimated to mitigate 14 g N ha^-1 yr^-1 of indirect N₂O emissions. In contrast, the bioreactor in Claremont was estimated to increase 80 g N ha^-1 yr^-1 of indirect N₂O emissions. Results indicated that on-site wood-chip bioreactors could mitigate indirect N₂O emissions if hydraulic residence time is long enough, and denitrification process goes to completion, i.e., from NO₃ to N₂.