Nitrate removal and nitrous oxide production from hothouse effluent draining to a pipe bioreactor

Agricultural hothouses are intensive food production systems relying on high fertilization and irrigation. Runoff from hothouses can contain high levels of nitrogen (N) and drain into streams. Here, we investigate the effectiveness of buried, inline woodchip bioreactors constructed using PVC pipes in removing nitrate (NO3--N) and the possibility of pollution swapping from nitrate to the greenhouse gas nitrous oxide (N2O). Mean dissolved NO3--N removal and N2O gas production were 6.0 +/- 5.2 g N m(-3) h(-1) (0.9-12.3 g N m(-3) h(-1)), and 35.1 +/- 31.4 mg N L-1 h(-1) (14-83 mg N L-1 h(-1)) respectively, across five surveys. NO3--N removal and N2O-N production occurred primarily in hypoxic to anoxic conditions. Overall, these inline pipe bioreactors achieved nitrate removal efficiencies (NRE) of 14.5 +/- 6.8% (8.2%-25.0%) and N2O production equivalent to 0.7 +/- 0.6% (0.3-1.4%) of nitrate removal. Comparisons to the literature indicate that our bioreactors have a low NRE but a very high nitrate removal rate (NRR) on a woodchip volume basis. The bioreactor was operating at 21% water capacity. Therefore, increasing the bioreactor water height would be needed to maximize nitrate removal. Our results imply that these bioreactors can efficiently remove NO3-N without major N2O release to the atmosphere.