Understanding nitrogen and organic carbon contents of agricultural drainage ditches in the Lower Mississippi Alluvial Valley

Author(s): Faust, D.; Kroger, R.; Rush, S.

Agricultural fertilizer applications have resulted in excessive nitrogen loading to agricultural drainage ditches, contributing to the Gulf of Mexico hypoxic zone. The purpose of this study was to assess relationships between organic carbon and nitrogen content of drainage ditches and evaluate the spatial scope in which organic carbon amendments may be used in remediating nutrient loading throughout the Lower Mississippi Alluvial Valley. Water and sediment samples were obtained from agricultural drainage ditches in Missouri, Arkansas, Mississippi, and Louisiana. Nitrate, nitrite, ammonia, and total nitrogen concentrations were determined in overlying and pore water, along with characterizing dissolved organic carbon aromaticity (spectral absorbance at 254 nm) and molecular weight (ratio of spectral absorbance at 254:365 nm). Concentrations of ammonia and nitrate nitrogen and total organic carbon in overlying and pore waters were variable, with ranges of 0.0117 to 20.4 mg L-1, 0.05 to 17.0 mg L-1, and 0.0 to 17.0 mg L-1. However, concentrations of nitrogen species and dissolved organic carbon were generally higher in the pore water compared to those in overlying water. Pore waters generally had lower molecular weight character of dissolved organic carbon than overlying water, although this trend was dependent on the state and site from which the sample was collected. The results of this study show that there is spatial variability in nitrogen species and organic carbon throughout the Lower Mississippi Alluvial Valley and demonstrate the importance of evaluating where organic carbon may be limiting nitrogen removal in agricultural drainage ditches.
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