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Highlights Novel three-bed, cascading-inlet bioreactor treated agricultural drainage from a 249-ha catchment. Nitrate removal rates and load reduction efficiencies were similar to those of traditional single-field bioreactors. Sedimentation problems reduced bed life; a sediment sensing and exclusion system solved them. This scale provides opportunities for centralized management and nutrient reduction verification. Abstract. Denitrifying bioreactors, a structural practice deployed at the field scale to meet water quality goals, have been underutilized and require additional evaluation at the small catchment scale. The objective of this study was to quantify the performance of a large, multi-bed denitrifying bioreactor system sized to treat agricultural drainage runoff (combined drainage discharge and surface runoff) from a 249-ha catchment. Three woodchip bioreactor beds, 7.6 m wide by 41 m long by 1.5 m deep, with cascading inlets, were constructed in 2016 in southern Minnesota, U.S. The beds received runoff for one water year from a catchment area that is 91% tile-drained row crops, primarily maize and soybeans. Initial woodchip quality differed among the three beds, affecting flow and nitrate removal rates. Bioreactor flow was unimpeded by sediment for twelve events from September 2016 to July 2017, during which time 55% of the discharge from the catchment was treated in the bioreactor beds. Average daily nitrate removal rates ranged from 2.5 to 6.5 g-N m-3 d-1 for the three bioreactor beds, with nitrate-N load removal of flow through the beds between 19% and 27%. When accounting for untreated by-pass flow, the overall nitrate-N removal of the multi-bed system was 12.5% (713 kg N). During high-flow events, incoming sediment clogged the reactor beds, decreasing their performance. There was 4,520 kg of sediment trapped in one bed, and evidence suggests the other two trapped a similar load. To solve this problem and prolong the bioreactor’s lifespan, we installed a shutoff gate that activated when inflow turbidity exceeded a threshold value. Finally, the findings indicate that catchment-scale denitrifying bioreactors can successfully remove nitrate load from agricultural runoff, but sediment-prevention measures may be required to extend the bioreactor's lifespan. Keywords: Bioreactor, Denitrification, Nitrate removal, Sedimentation, Subsurface drainage.

Improving water quantity and quality impacts of corn ( L.)- and soybean [ (L.) Merr.]-based cropping systems is a key challenge for agriculture in the US Midwest. Long-term field experiments are important for documenting those effects and exploring possible solutions. This study examines differences in soil water dynamics and nitrate-nitrogen (N) leaching among cropping systems and N fertilizer sources in a long-term experiment in southeastern Minnesota. Drainage and leachate concentrations were measured for 4 yr using automated equilibrium tension lysimeters installed below the root zone in replicated, large plots on a well-drained silt loam soil. Soil water storage was monitored using water content reflectometers. Corn–soybean and continuous corn cropping systems exhibited similar soil water dynamics, drainage rates (145–202 mm yr), leachate nitrate N concentrations (21.3–25.6 mg L), and nitrate N leaching loads (30–75 kg ha yr). Nitrate-N concentrations in the leachate were similar whether N was added as urea (21.2 mg L) or anhydrous ammonia (25.7 mg L). A perennial kura clover ( M. Bieb)-based cropping system with no N fertilizer significantly altered soil water dynamics and resulted in lower ( < 0.10) drainage rates (53 mm yr), nitrate N concentrations (7.1 mg L), and nitrate N leaching loads (2–5 kg ha yr) compared with corn–soybean or continuous corn, but also reduced corn grain yields. These impacts are generally consistent with a growing body of literature showing substantial environmental benefits of a kura clover living mulch system for corn production, but the economic viability of such a system is not yet proven.

Kura clover (Trifolium ambiguum M. Bieb.) living mulch has potential to improve the environmental impact of corn (Zea mays L.) production, especially in the context of corn silage or stover harvest. Our objective was to determine the effects ofkura clover living mulch on the water balance and nitrate leaching under corn near Arlington, WI. Treatments in the 2.5-yr experiment (April 2006-November 2008) were N-fertilized no-till corn following killed kura clover as the control and no-till corn in living mulch with fertilizer rates of 0 and 90 kg N ha -1 yr -1 . Soil water storage was 37 to 50 mm lower under the living mulch than the control in the spring, while the control experienced 29 to 36 mm greater soil water depletion than the living mulch in the summer. Evapotranspiration was similar across treatments, except in May when it was greater under the living mulch by 11 to 41 mm. The living mulch did not appreciably reduce drainage. Nitrate-N storage in the soil profile and nitrate-N concentrations in the soil solution at 1-m depth were significantly reduced (p = 0.10) under both living mulch treatments relative to the control. Flow-weighted nitrate-N concentrations were 23 mg L -1 for the control, 17 mg L -1 for the living mulch with 90 kg N ha -1 yr -1 , and 6 mg L -1 for the living mulch with 0 kg N ha - 1 yr -1 . Total nitrate-N leached was reduced 31 and 74% relative to the control under the living mulch with 90 and 0 kg N ha -1 yr -1 . respectively.