Biochar fails to enhance nutrient removal in woodchip bioreactor columns following saturation

Denitrifying bioreactors are edge-of-field structures that remove excess nitrogen (N) from intercepted agricultural drainage by supporting the activity of denitrifying microorganisms with a saturated organic carbon substrate. Although these bioreactors successfully mitigate N export, the typical woodchip systems have little effect on phosphorus (P), which is also often present in environmentally harmful quantities in drainage waters. Currently, the evidence that amending woodchip bioreactors with biochar enhances both N and P removal rates is mixed, but more work is required to test this hypothesis under controlled conditions. To determine the effect of biochar amendment on nitrate (NO₃-N) and phosphate (PO₄-P) removal in woodchip bioreactors, three media types-aged woodchips (W), 10% (B₁₀) and 30% (B₃₀) biochar by volume-were tested under different operational conditions during five-day laboratory trials with horizontal, flow-through columns. Nutrient removal was observed under different flow rates yielding hydraulic residence times of 3, 6, and 12 hours with four formulations of simulated agricultural drainage, all combination of 16.1 or 4.5 mg L^-1 NO₃-N and 1.9 or 0.6 mg L^-1 PO₄-P. Each unique treatment with respect to media type, HRT, and influent formulation was tested in triplicate using independent columns. All treatments successfully removed NO₃-N, but PO₄-P removal was inconsistent. Cumulative NO₃-N removal efficiencies ranged 15-98% with an average removal rate of 11.0 g m^-3 d^-1; biochar amendment enhanced removal only in response to sufficiently high loading rates. Cumulative PO₄-P removal efficiencies ranged from 66% removal to 170% export of the influent load; biochar addition was associated with increased export. These results indicate that pine-feedstock biochar poses a substantial increase to PO₄-P leaching risk and only modestly enhances NO₃-N removal given sufficiently high loading.