Biochemical Processes within a Two-Stage Agricultural Drainage Ditch in Mower County, Mn: Methods for Estimating Nitrogen Removal Rates and Efficiencies

Abstract

Drainage ditch design has historically focused on providing adequate water conveyance. More recently, greater attention has been placed on alternative designs that are inherently more stable, assimilate nutrients, and create vibrant ecosystems. In 2009, 1.89 km of a conventional drainage ditch in Mower County, MN, was converted to a two-stage design. Three different methods were used to calculate nitrogen removal: (1) comparison of the average influent and effluent concentrations (2) mass-balance relationships for in-channel denitrification using isotopic tracers, (3) potential soil denitrification using the acetylene inhibition assay. Three different dates were used in the mass-balance approach: 2013 as a 84-m test section as well as 2011 and 2010 as estimates for the entire ditch reach. Continuous data from 2010 was also used to produce average monthly removal efficiencies for the growing season by comparing influent and effluent concentrations. The removal efficiency was estimated at 21% for the 2013 data, 32% for the 2011 data, and ~20% for the 2010 data. Average monthly removal efficiencies ranged from 19.5% in May to 12.9% in September 2010. Nitrous oxide production varied greatly among habitat zones, ranging 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% to 3% estimated for a conventional drainage ditch. Although denitrification rates and removal efficiencies are difficult to quantify, all removal methods produced results that were similar to one another as well as to results found in the literature for other two-stage ditches.

Keywords: agricultural landscapes, best management practice, Nutrient management, agricultural drainage ditch, nature-based solutions