Effects of organic carbon amendments on nitrogen removal in agricultural drainage ditch sediments

Author(s): Faust, D.; Kröger, R.

Agricultural fertilizer applications have resulted in loading of nitrogen nutrients to agricultural drainage ditches in the Lower Mississippi Alluvial Valley, contributing to the Gulf of Mexico hypoxic zone. Previous studies have observed that nitrogen loading decreases with implementation of best management practices within and in proximity to aquatic ecosystems in agricultural settings. The purpose of this study was to determine effects of organic carbon amendments on nitrogen removal in agricultural drainage ditch water at various lengths of time and carbon-to-nitrogen (C:N) ratios. In one experiment, control, dissolved organic carbon (DOC), and particulate organic carbon (POC) amendment groups were prepared in laboratory microcosms for time treatments of 3, 7, 14, and 28 days with six replicates per treatment. In a second experiment, control, DOC, and POC amendment groups were prepared in microcosms at C:N ratios of 5, 10, 15, and 20 with six replicates per treatment. A permutational multivariate analysis of variance was used to detect statistically significant differences in nitrogen nutrient among treatments (F11,60=19.0, P=0.001 and F8,45=23.2, P<0.001). Mean increases of 3.27 ± 0.52 and 19.2 ± 4.5 mg N L-1 of total nitrogen were observed in overlying water of all treatments of experiments one and two, respectively. However, 60-100% removal of nitrate-nitrogen in overlying water was observed in all treatments with removal occurring in DOC and POC treatments. These results indicate that amendments of organic carbon made to drainage ditch sediments increase nitrate-nitrogen removal, particularly over longer periods of time. Any amendments of DOC increase N removal, regardless of C:N ratio, while amendments of POC at a C:N ratio of five are optimal for N removal. This study provides support for using organic carbon amendments as a best management practice in agricultural drainage ditches.

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