Nitrogen removal and nitrous oxide emissions in woodchips biofilters treating agricultural drainage waters

Eutrophication of aquatic ecosystems provoked by excess nitrogen (N) concentration is still a major concern worldwide with severe consequences such as hypoxia, biodiversity loss, and degradation of drinking water quality. To face these challenges, a novel N mitigation measure has emerged in the last decades consisting of biofilters made of woodchips. Drainage water from agricultural areas infiltrate through a layer of woodchips before it discharges to an aquatic recipient such as a ditch or a stream. The goal with this technique is to provide optimal conditions for denitrification i.e. an easy degradable carbon source (the woodchips) and an anaerobic environment. There is, however, some concerns regarding the emissions of the greenhouse gas nitrous oxide (N₂O) which can be a by-product of denitrification.

Here, we present results on N removal and N₂O emissions from 9 biofilters differing in age (1–8 years) and representing a total of 18 years of monitoring. The biofilters were all located in agricultural catchments in Denmark (temperate climate conditions). Nitrogen removal in the biofilters was estimated using a mass balance approach measuring N species dissolved in the water (total N, nitrate, nitrite, ammonium) using time proportional automated samplers placed at inlet and outlet of the biofilters. Nitrous oxide emissions were measured every third week both as gaseous form at the surface of the biofilters (closed chamber technique and gas chromatography) and in dissolved form in the water phase at inlet and outlet of the biofilters (headspace technique and gas chromatography). We take advantage of this unique dataset to identify the factors enabling to maximize N removal while minimizing N₂O emissions. Furthermore, we make a first assessment of the potential impact of the increasing number of biofilters on N₂O emissions in agricultural landscapes.