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Examining the role of organic carbon amendments as a possible best management practice to improve nitrogen removal in agricultural drainage ditches

Robert Kröger

Graduate Student(s):  Derek Faust

This research is funded by Mississippi State University's Forest & Wildlife Research Center (FWRC) and Mississippi Agricultural & Forestry Experiment Station (MAFES) and aims to continue our research efforts regarding best management practices (BMPs) to increase nitrogen removal in agricultural drainage ditch systems. Several BMPs are already in place throughout the Mississippi Delta region and the rest of the Lower Mississippi Alluvial Valley. Several of the BMPs in place take advantage of and enhance biological and physical wetland characteristics of drainage ditches to increase nitrogen removal. The primary role of these BMPs is to reduce nutrient and sediment loading to the Gulf of Mexico and thus aid in reducing the size of the hypoxic zone. With particular regard to nitrogen removal, three factors have consistently been implicated in affecting denitrification (a mechanism of nitrogen removal by conversion of nitrate to atmospheric nitrogen N2): 1) availability of anaerobic conditions, 2) nitrate availability, and 3) organic carbon availability. Low-grade weirs can create areas of anaerobic sediment conditions in drainage ditches. Additionally, nitrate is often not limiting in drainage ditches due to runoff of fertilizers from adjacent fields, contributing to nutrient loading in the Gulf of Mexico. Organic carbon limitation as related to nitrogen removal has not been extensively studied or addressed through BMP implementation in drainage ditches.

As such, this research will investigate organic carbon limitation on nitrogen removal by making organic carbon amendments to drainage ditch systems. The link between amounts of both nitrate and organic carbon will be coupled using carbon-to-nitrogen ratios to determine an optimal ratio for nitrogen removal. Three studies with increasing spatial scales will be used to evaluate how organic carbon amendments may increase nitrogen removal in agricultural drainage ditches. In the first study, dissolved or particulate organic carbon will be added to drainage ditch sediments in laboratory microcosms to determine nitrogen removal over several periods of time. In the second study, we will scale up to experimental drainage ditches containing low-grade weirs to evaluate the effects of organic carbon amendments and hydraulic residence time on nitrogen removal. Finally, in the third study, sediment and water samples will be obtained from at least 100 drainage ditches throughout the Lower Mississippi Alluvial Valley to examine the relationships between nitrogen and organic carbon contents. This research will serve as the first steps in investigating if organic carbon amendments may be used as a BMP and how such a practice would be developed and implemented in agricultural drainage ditches. Development of organic carbon amendments as a BMP would add to the other BMPs already available and in place for preventing nutrient contamination and impairment from agricultural runoff to receiving waters.

Gulf of Mexico Research

Completed Projects