Reducing nitrogen contamination from agricultural subsurface drainage with denitrification bioreactors and controlled drainage

Reactive nitrogen leaving agricultural fields through subsurface drainage systems enters aquatic systems and contributes to eutrophication, habitat degradation and loss of biodiversity. Denitrification bioreactors, in combination with controlled drainage, are proposed as a means of reducing nitrogen emitted through subsurface agricultural drainage systems, but their suitability in colder climates where soils and drainage systems freeze during winter is poorly understood. This study presents the first field-scale evaluation of denitrification bioreactors under cold climate conditions during a three-year period in Quebec, Canada. Under a three-year crop rotation, about 55% of the total annual subsurface drainage water passed through bioreactors, which significantly lowered the total-nitrogen (72%) and nitrate-nitrogen (99%) median concentrations in the subsurface drainage outflows. Loadings of nitrate-nitrogen from the test fields to surface drainage ditches were reduced by 99%, equivalent to about 11 kg nitrate-nitrogen ha−1 year−1 removal in the test area and approximately 7 g nitrate-nitrogen removal m−3 bioreactor volume d−1. Aquatic environmental criteria non-compliance events declined by 96% for nitrate-nitrogen and by 50% for total-nitrogen during the three-year study. This study demonstrates that denitrification bioreactors, combined with controlled drainage, are an effective in-field technology for nitrogen removal from agricultural subsurface drainage water that will improve water quality under cold climate conditions.