Search

Nitrogen loss via overland flow from agricultural land use is a global threat to waterways. On-farm denitrifying woodchip bioreactors can mitigate NO 3 - exports by increasing denitrification capacity. However, denitrification in sub-optimal conditions releases the greenhouse gas nitrous oxide (N 2 O), swapping the pollution from aquatic to atmospheric reservoirs. Here, we assess NO 3 - -N removal and N 2 O emissions from a new edge-of-field surface-flow bioreactor during ten rain events on intensive farming land. Nitrate removal rates (NRR) varied between 5.4 and 76.2 g NO 3 - -N m -3 wetted woodchip d -1 with a mean of 30.3 ± 7.3 g NO 3 - -N m -3 . The nitrate removal efficiency (NRE) was ∼73% in ideal hydrological conditions and ∼18% in non-ideal conditions. The fraction of NO 3 - -N converted to N 2 O (rN 2 O) in the bioreactor was ∼3.3 fold lower than the expected 0.75% IPCC emission factor. We update the global bioreactor estimated Q 10 (NRR increase every 10 °C) from a recent meta-analysis with previously unavailable data to >20 °C, yielding a new global Q 10 factor of 3.1. Mean N 2 O CO 2 -eq emissions (431.9 ± 125.4 g CO 2 -eq emissions day -1 ) indicate that the bioreactor was not significantly swapping aquatic NO 3 - for N 2 O pollution. Our estimated NO 3 - -N removal from the bioreactor (9.9 kg NO 3 - -N ha -1 yr -1 ) costs US$13.14 per kg NO 3 - -N removed and represents ∼30% NO 3 - -N removal when incorporating all flow and overflow events. Overall, edge-of-field surface-flow bioreactors seem to be a cost-effective solution to reduce NO 3 - -N runoff with minor pollution swapping to N 2 O., (Copyright © 2022. Published by Elsevier Ltd.)

Nitrogen runoff from fertiliser intensive land uses has become an issue worldwide, contributing to algal blooms, hypoxic waters and aquatic biodiversity losses. This study assessed potential nutrient pollution from blueberry farms in subtropical Australia and examines whether nutrient loads were driven by groundwater discharge and/or surface water runoff. Streams downstream of eight blueberry farms were compared to eight nearby control sites without any blueberry activity. In the 90 day sample period, there were three rain events >90 mm day -1 that produced runoff sufficient to create flooding. Overall, the results revealed a clear link between blueberry farming and nitrogen runoff in headwater streams. While NO X (nitrate + nitrite) was the dominant nitrogen species downstream of blueberry farms, dissolved organic nitrogen (DON) was the dominant species in control sites. The concentrations and loads of NO x were one order of magnitude lower in the eight non-blueberry (6.3 ± 2.0 μmol L -1 ; 1.6 ± 1.2 kg N-NO X ha -1 yr -1 ) than the eight blueberry (56.9 ± 14.2 μmol L -1 ; 21.8 ± 8.0 kg N-NO X ha -1 yr -1 ) sites. NO X concentrations and loads were highest following rain events. Radon ( 222 Rn, a natural groundwater tracer) observations and low nitrogen concentration in groundwater samples further suggest that surface runoff dominates the delivery of nitrogen to the creeks investigated. NO X concentrations and loads in creeks correlated with blueberry farm density. At >15% of blueberry land use in a catchment, there was a detectable influence in NO X concentrations and loads in the headwater streams. Assuming that our load estimates can be up-scaled to annual nitrogen creek exports, and that local farmers use the recommended amount of fertiliser (121 kg N ha -1 yr -1 ), between 18 and 25% of the used fertiliser was lost to the creeks. This implies that there are opportunities for decreasing the use of fertilisers in this catchment and managing any nitrogen that escapes to the creeks., (Copyright © 2018 Elsevier Ltd. All rights reserved.)