Nitrogen leaching from agricultural land use is known to impact Coffs Harbour stream water
quality. On-farm denitrifying woodchip bioreactors can potentially mitigate nitrogen,
particularly nitrate (NO3-) pollution by maximising denitrification capacity in between farms
and creeks. However, denitrification may release the powerful greenhouse gas nitrous oxide
(N2O), swapping from aquatic (NO3-) to atmospheric (N2O) pollution.
Here, we assess nitrate nitrogen (NO3--N) removal and N2O emissions from a new edge-of field
surface-flow bioreactor design during ten rain events on intensive farming land.
Our nitrate removal rates (NRR) varied between 5.4 and 76.2 g NO3--N m-3 wetted woodchip
d-1 with a mean of 30.3±7.3 g NO3--N m-3 d-1. The nitrate removal efficiency (NRE) ranged
from ~73% in ideal conditions to ~18% in non-ideal conditions. Overall, 9.9 kg NO3
--N ha-1 yr-1 were removed via the bioreactor, representing an overall 30% efficiency when
incorporating all flow and overflow events. However, inflows and treated outflows from the
bioreactor are ~254 and ~138 fold higher than ANZECC guideline values, respectively,
indicating that there is still a significant risk to local waterways from treated effluent.
The bioreactor did not contribute higher N2O emissions than what naturally occurs if the
bioreactor was absent, implying minor swapping from aquatic to atmospheric pollution. NO3
--N that was removed in the bioreactor and converted to N2O (rN2O) was ~3.3 fold lower than
the expected 0.75% IPCC emission factor.
Our modelled NO3--N removal from the bioreactor would cost AUS$17.8 per kg NO3
--N removed. Whilst off-farm NO3- losses are expected, even under best management practice,
the removal cost using this bioreactor is ~5 fold greater than the estimated cost of nitrogen
fertiliser application. Reducing on-farm NO3- use to lower environmental losses may be more
cost-effective than treating effluents; however, the combination of both management
techniques is likely necessary to protect environmental assets.
With minor design modifications, the bioreactor's efficiency will likely increase. Overall,
edge-of-field surface-flow bioreactors can be a useful tool to reduce NO3--N runoff in eastern
Australian intensive horticulture catchments and play an integral role in the suite of NO3
--N management solutions. This bioreactor represents a proof-of-concept and a new tool to
protect vital aquatic habitats such as the Solitary Islands Marine Park.