Managing natural processes in drainage for non-point source nitrogen control

Authors: STROCK, J - UNIV OF MINNESOTA; Dell, Curtis; Schmidt, John

Submitted to: Journal of Soil and Water Conservation
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/20/2007
Publication Date: 7/20/2007
Citation: Strock, J.S., Dell, C.J., Schmidt, J.P. 2007. Managing natural processes in drainage ditches for non-point source nitrogen control. Journal of Soil and Water Conservation. 62:188-196.

Interpretive Summary: In many areas of the U.S., artificial drainage has been added to allow access to fields for timely planting of crops and to prevent crop losses from flooding. The types of drainage systems established vary from region to region, depending on factors such as topography and soil properties. In some areas, adequate drainage is provided strictly with the construction of a series of open ditches that connect fields with natural waterways. In other parts of the U.S., a combination of subsurface tiles and ditches are used. While artificial drainage has greatly increased crop production, it has also increased the quantities of nitrogen (N) and other nutrients that reach water bodies. Nitrogen is essential for the growth of plants and microorganisms, but excess N in water can lead to excess growth of algae and microorganisms and adversely impact fish and aquatic plants. In this paper, 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. Nitrogen is readily cycled between chemical forms. Nitrate is generally the form of nitrogen of the greatest environmental concern, because it is found in aqueous solution and is readily transported with moving water. Nitrate can be removed from water by plant uptake or denitrification (conversion of nitrate to N gases under anaerobic conditions). Management options that have the potential to decrease N losses from artificially drained agricultural systems include controlled drainage, denitrification curtains, and vegetated buffers. With controlled drainage, flow control structures are installed to increase the retention time of water in ditches and surrounding soil and allow more nitrate to be removed through denitrification. Denitrification curtains are trenches filled with organic materials, which creates conditions favorable for denitrification, and result in the removal of nitrate from shallow groundwater as it flows into the drainage ditch. Vegetated buffers are placed adjacent to ditches so nitrate can be taken up by plant roots to also remove N from groundwater as it flow into a ditch. A case study of ongoing research in Minnesota comparing the impact of flow control structures on N losses from paired ditches, with and without flow control, is presented. During the first year of observation, results were mixed, with less N in non-storm event samples from the ditch with the flow control structure, but no difference in annual export of N from the two ditches. Flow control has been in place for only a short time, but greater evidence of the positive impacts of flow control on reduce N losses is anticipated as the Minnesota experiment proceeds. Appropriate management of drainage ditches represents a potential opportunity to remove nitrate from drainage water through a combination of physical and biogeochemical processes.

Technical 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 non-storm 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.