Intensive farming, fertilizer application, and the mineralization of soil organic matter in the Upper Midwestern United States, coupled with an extensive subsurface tile drainage, has contributed to excessive nutrient loading to surface waters. Woodchip bioreactors are a cost effective and minimally invasive edge-of-field practice designed to remove nitrate (NO3-N) from subsurface drainage. The NO3-N removal efficiency of woodchip bioreactors can be impacted by several factors, including hydraulic residence time (HRT) and the type of carbon source used.

The first study (Chapter 3) examined the impact of three HRTs, 2 hours, 8 hours, and 16 hours, on denitrification in a set of nine highly controlled pilot-scale woodchip bioreactors in Central Iowa. The 2 h HRT had the highest NO3-N mass removal rate (MRR) at 9.0 g m-3 day-1, followed by the 8 h at 8.5 g m-3 day-1, and the 16 h at 7.4 g m-3 day-1, all of which were statistically different (p < 0.05). When accounting for bypass flow, the 2 h HRT still removed more NO3-N by mass than the other HRTs. NO3-N concentration reduction was significant for all HRTs from the inlet to the outlet (p < 0.05). The 16 h HRT removed the most NO3-N by concentration (7.5 mg L-1) and had the highest percent mass removal rate (PMRR) at 53.8%. The 8 h HRT removed an average of 5.5 mg L-1 NO3-N with a PMRR of 32.1%. The 2 h HRT removed an average of 1.3 mg L-1 NO3-N with a PMRR of 9.0%. Significant explanatory variables for PMRR were HRT (p < 0.001) and influent NO3-N concentration (p < 0.001) (r = 0.80).

The second study (Chapter 4) examined the potential to expand the nutrients removed by bioreactors to include P by adding biochar as an amendment to the carbon substrate. Six biomasses [Red Oak (Quercus rubra), Ash (Fraxinus spp.), Mixed Pine (Pinnus spp.), Loblolly Pine (Pinus taeda), Switchgrass (Panicum virgatum), and Corn Stover (Zea mays)] were pyrolyzed into biochar at three temperatures (400°C, 600°C, and 800°C). Experiments were conducted at the batch scale to test for removal of NO3-N and phosphate (PO43-) by woodchips mixed with biochar at a ratio by weight of 12:1 (8.3% application rate). Each batch contained 3 L of nutrient solution with 30 mg L-1 NO3-N and 10 mg L-1 PO43- and were sampled at 0, 4, 8, 12, and 24 hours. NO3-N removal was not correlated with particle size, but was positively correlated with pyrolysis temperature (r = 0.58, p < 0.05). None of the biochars removed significantly more NO3-N than the woodchip control. The 800°C biochar and herbaceous biochars removed significantly more PO43- than the control, but the woodchips leached PO43- into solution, meaning significance may not be the same after the initial leaching period (p < 0.05). Results indicate that biochar is not suited to be an amendment to enhance either NO3-N or PO43- removal in woodchip bioreactors.