Nitrogen removal and nitrous oxide emissions from woodchip bioreactors treating agricultural drainage waters

Microbiome Structure and Function in Woodchip Bioreactors for Nitrate Removal in Agricultural Drainage Water

“…The relative abundance of functional denitrification genes showed a similar pattern across and within the bioreactors (Figure 4), and indicated relatively low nir/nosZ ratios, consistent with a low risk of N 2 O emissions (Conthe et al, 2019). This was in agreement with field measurements of dissolved and gaseous N 2 O fluxes from the three mature woodchip bioreactors, which showed a mean N 2 O-N emission factor of 0.6% of NO 3 -N removal (Audet et al, 2021). In studies of lab-scale woodchip bioreactors, ratios of nir/nosZ were initially high (> 10), but decreased after 6 months (Hellman et al, 2021), which supported that the genetic potential for N 2 O reduction develops toward generally low emissions from denitrifying bioreactors (Aalto et al, 2020).…”

“…Woodchips were sampled from three woodchip bioreactors in Denmark (2019-11-20), designed conceptually as shown in Figure 1. The annual mean NO 3 − concentrations at the inlet of BR1, BR2 and BR3 in 2019 were 49, 52 and 61 mg NO 3 − L −1 , respectively, and annual NO 3 − removal efficiencies were typically 26-56% at BR1 and BR2 and 40-70% at BR3 (Carstensen et al, 2019;Audet et al, 2021). The bioreactors had horizontal flow design (Figure 1) and received subsurface drainage water from agricultural fields mainly with annual cropping systems.…”

“…Sampling and analyses of environmental metadata and water chemistry at the bioreactor inlets and outlets were conducted as described in Audet et al (2021). The bioreactors were continuously monitored and the environmental metadata retrieved for this study represented a period from within 10 days before to 1 day after the time of woodchip sampling for metagenomic analyses.…”

“…Here we used metagenomics to obtain a more complete overview of the taxonomic composition and functional potential of woodchip bioreactor microbiomes. We report the microbiome composition at a given time point in three stable operating fullscale woodchip bioreactors (BR1-BR3), where NO 3 − removal efficiencies were previously monitored (Bruun et al, 2016;Carstensen et al, 2019;Audet et al, 2021) and where concurrent metadata on water chemistry were available. The aim was to determine if the microbiomes differed among the bioreactors due to different design parameters and location in different agricultural settings.…”

“…The aim was to determine if the microbiomes differed among the bioreactors due to different design parameters and location in different agricultural settings. Because of a close physical location and similarity in design, the microbiomes of BR1 and BR2 were hypothesized to be more similar to each other than to BR3, which had a smaller ratio of bioreactor area to drained area and achieved greater NO 3 − removal efficiency (Audet et al, 2021). Our second aim was to address how the microbiome and functional potential differed along the flow path within the bioreactors due to gradual NO 3 − depletion, which may regulate the potential of anaerobic respiration based on other terminal electron acceptors, such as sulfate (SO 4 2− ) and carbon dioxide (CO 2 ) with resulting environmental losses of hydrogen sulfide (H 2 S) and methane (CH 4 ), respectively.…”