Suitability of three wood species for denitrification bioreactor fill

As a relatively new technology to mitigate nitrate (NO₃^-) losses in agricultural drainage water, denitrification bioreactors can take advantage of locally available carbon sources, otherwise considered waste, to address a water quality problem. This work compared the potential of three widespread, on-farm tree species in New Zealand to serve as woodchip denitrification bioreactor fill to treat pastoral drainage waters. A batch experiment tested woodchips made from three tree species, willow (Salix matsudana), pine (Pinus radiataj and cypress (Cupressus lusitanica), each with or without leaves/needles and either fresh or field-aged, and further column testing was done with two particle size ranges of pine and willow chips at four retention times. All species were able to remove NO₃^- in both tests with removal rates averaging 1.5-6.6 g N/day/m3 during column testing. However, willow was not recommended due to high and more sustained releases of ammonium. Woodchip carbon materials with a relatively large proportion of fines (< 2 mm particle size) and low C:N should be avoided as bioreactor fill due to poorer NO₃^- removal and potential for nitrogen loss. Although inclusion of green foliage materials in this short-term work did not consistently effect NO₃^- removal or ammonium losses, treatments that included needles had poor particle size uniformity. This would potentially allow greater compaction and altered hydraulic conductivity over time as well as having lower C:N than aged media, which may exacerbate nitrogen leaching concerns. Using field-aged woody media can help avoid these issues. This work illustrates the potential of denitrification bioreactors for NO₃^- reduction in pastoral drainage waters, though longer-term and larger-scale studies are needed.