Managing natural processes in drainage ditches for nonpoint source nitrogen control

Abstract:

In watersheds dominated by agriculture, artificial drainage systems can efficiently and quickly transport excess water from agricultural soils. The application of more nitrogen (N) than a crop uses creates a surplus in the soil and increases the risk of N loss to the environment. We examine issues associated with agricultural N use, N transfer from artificially drained agricultural land to drainage ditches, N cycling within ditches, and options for management. Watercourses in agricultural watersheds often have high concentrations of N and are effectively N saturated. Numerous processes are involved in N cycling dynamics and transport pathways in aquatic ecosystems including N mineralization, nitrification, and denitrification. Flow control structures can lower N losses related to artificial drainage by increasing water retention time and allowing greater N removal. An ongoing study in Minnesota compares the impact of flow control structures on N losses from paired ditches with and without flow control. During the first year of observation, results were mixed, with lower N concentrations in nonstorm event samples from the ditch with the flow control structure, but no significant difference in annual total N load between the two ditches. Appropriate management of drainage ditches represents a potential opportunity to remove biologically available forms of N from drainage water through a combination of physical and biogeochemical processes.