Biochemical processes within a two-stage agricultural drainage ditch in Mower County, MN: Methods for estimating nitrogen removal rates and efficiencies

Drainage ditch design has historically focused on providing adequate water conveyance. More recently, greater attention has been placed on alternative designs that assimilate excess nutrients. However, due to the hydrologic complexities, accurately calculating nutrient removal in these systems presents a challenge. In 2009, 1.89 km of a conventional drainage ditch in Mower County, MN, was converted to a two-stage design. The objective was to evaluate three different methods for calculating nitrogen removal in this system. Continuous data from the growing season of 2010 was used to produce average monthly removal efficiencies by comparing influent and effluent concentrations, which ranged from 19.5 % in May to 12.9 % in September. Three dates were used in 2013, 2011, and 2010 to produce mass-balance relationships for in-channel denitrification using isotopic tracers. Removal efficiencies were estimated at 21 % (2013), 32 % (2011), and ∼20 % (2010) using the mass-balance approach. Nitrous oxide production was measured from soil samples taken for one date in 2013 to estimate potential soil denitrification using the acetylene inhabitation assay, which varied greatly among habitat zones from 0.08 to 1.85 µg N₂O-N g DW⁻¹ g h⁻¹. Potential habitat-weighted soil denitrification ranged from 19 % to 42 % compared to 1–3 % estimated for a hypothetical conventional drainage ditch. Although denitrification rates and removal efficiencies are difficult to quantify, comparing the results from multiple dates using an array of methods can validate and add robustness to studies of two-stage ditch denitrification, providing further support for alternative drainage ditch designs as an effective method for addressing nutrient pollution to our natural waterways.