Physical and chemical characterization of agricultural drainage ditches managed for nutrient removal

Andry Z. Ranaivoson, Southwest Research & Outreach Center, University of Minnesota, Lamberton, MN and Jeffrey S. Strock, Southwest Research & Outreach Center, University of Minnesota, Lamberton

Background/Question/Methods
Ditches are common in drained agricultural landscapes. Managed ditches are a promising strategy for mitigating nutrient loss from row crop production. Optimization of drainage ditches using minimally invasive low-grade weirs has been shown to increase hydraulic residence time and reduce nutrient load and nutrient concentration. Our objective was to identify abiotic and biotic drivers underlying nutrient reductions. We estimated denitrification rate, hydraulic loading rate, and hydraulic residence time along with measurements of water temperature, pH, redox potential, and dissolved oxygen as indicators of nutrient transformation and removal in experimental drainage ditches in Minnesota, USA.
Results/Conclusions
Data were collected over the period of May-November from 2018 to 2019 and from April-August in 2020. The three-year mean denitrification rate, hydraulic loading rate, and hydraulic residence time were 3.7 kg N d-1 (range: 2.2 to 5.5 kg N d-1), 0.89 m/d (range: 0.31 to 1.79 m d-1), and 23.5 hr. (range: 3.1 to 38.6 hr). Pooled results from the middle and outlet sampling locations of the treatment and control ditch for annual water temperature were 17.5 oC (s.d. 3.06 oC) and 18.3 oC (s.d. 3.29 oC), pH 7.57 (s.d. 0.47) and 7.62 (s.d. 0.46), redox potential 82.6 mVolts (s.d. 48.2 mVolts) and 83.2 mVolts (s.d. 44.7 mVolts), and dissolved oxygen 8.08 mg/L (s.d. 1.81 mg/L) and 8.33 mg/L (s.d. 3.02 mg/L), respectively. Agricultural drainage ditches provide many ecosystem services such as water quality maintenance, carbon storage, flood abatement, and biodiversity support.