Nitrogen and organic carbon contents of agricultural drainage ditches of the Lower Mississippi Alluvial Valley

Abstract

Use of agricultural fertilizers as a means of increasing production has resulted in excessive nutrient loading to agricultural drainage ditches, contributing to the Gulf of Mexico hypoxic zone. Drainage ditches can have wetland characteristics and functionality, including the capacity to remediate nutrient loading, which can be promoted using management practices. The purpose of this study was to assess relationships between organic carbon (C) and nitrogen (N) contents of drainage ditch sediment and waters, and to evaluate the spatial scope in which organic C is potentially limiting N removal in drainage ditches throughout the Lower Mississippi Alluvial Valley (LMAV). Water and sediment samples were obtained from agricultural drainage ditches in Missouri, Arkansas, Mississippi, and Louisiana (<i>n</i> = 89). Nitrate (NO₃⁻), nitrite (NO₂⁻), ammonium (NH₄⁺), and total N concentrations were determined in overlying and pore water, along with characterizing dissolved organic C aromaticity and molecular weight using absorbance analyses. A significant correlation was observed between sediment percentage N and percentage total organic C (<i>r</i> = 0.944, <i>p</i> &lt; 0.001). Sediment organic C ranged from 0.253% to 6.04%, well below values observed in restored and natural wetlands. Results of this study show there is spatial variability in N species and organic C content of ditch water and sediment throughout the LMAV. This study expands on the knowledge available on the organic C content of agricultural drainage ditches in the LMAV. The results of this study indicate organic C may be limiting denitrification in agricultural drainage ditches, and suggest these ditches would benefit from organic C amendments.