Nutrient Reduction Capabilities of Agricultural Drainage Ditch Wetlands: Creation and Policy Implications (pp. 185-201)
Authors: (Usborne, E., Littlejohn K.A., Pierce, S.C., Kröger, R., Department of Wildlife, Fisheries and Aquaculture, Mississippi State University, Mississippi State, MS)
Abstract: Wetlands in agricultural landscapes are considered beleaguered systems due to significant hydrological modifications. Drainage ditches can be identified as agricultural wetlands since they are systems that display hydrological fluctuations with vegetative and substrate properties similar to wetlands. Management of these systems involves encouragement of vegetation establishment as well as creating controlled drainage to improve hydraulic residence time and enhance nutrient reduction. Runoff from agricultural areas delivers phosphorus (P), either as dissolved P or particulate P, and nitrogen (N) as highly mobile nitrate (NO3ˉ). Reducing the excess N and P reaching adjacent water bodies has been a primary concern of best management practices (BMPs). Reducing flow velocity through vegetated drainage ditches allows sediment and particulate bound P to settle out of the water column, and subjects NO3ˉ and dissolved P to biogeochemical processes. The implementation of low-grade weirs has shown to significantly increase sediment accumulation (and therefore decrease particulate P loads), and decrease NO3- concentration between 32 and 52%. These results suggest weirs have the ability to decrease nutrient concentrations by potentially altering hydraulic residence time as well as biogeochemical processes within ditches. Understanding the mechanisms behind how these systems respond to the addition of weirs is important in the efforts to reduce excess bioavailable N and P contributions to downstream eutrophication. Because of the potential benefits, drainage ditches are being advocated as BMPs to reduce nonpoint source pollution from entering downstream aquatic receiving systems. Policy surrounding wetland creation in agricultural landscapes should be carefully examined to understand, that even without federal oversight, they could perform valuable functions for wetland mitigation in landscapes of extensive agriculture.