Nitrate removal from agricultural drainage ditch sediments with amendments of organic carbon: Potential for an innovative best management practice

Agricultural fertilizer applications have resulted in loading of nutrients to agricultural drainage ditches in the Lower Mississippi Alluvial Valley. The purpose of this study was to determine effects of dissolved organic carbon (DOC) and particulate organic carbon (POC) amendments on nitrate-nitrogen (NO₃⁻-N) removal from overlying water, pore water, and sediment of an agricultural drainage ditch. Two experiments were conducted. In experiment 1, control (i.e., no amendment), DOC, and POC treatments were applied in laboratory microcosms for time intervals of 3, 7, 14, and 28 days. In experiment 2, control, DOC, and POC treatments were applied in microcosms at C/N ratios of 5:1, 10:1, 15:1, and 20:1. There were statistically significant effects of organic carbon amendments in experiment 1 (F_2,71 = 27.1, P < 0.001) and experiment 2 (F_2,53 = 39.1, P < 0.001), time (F_1,71 = 14.5, P < 0.001) in experiment 1, and C/N ratio (F_1,53 = 36.5, P < 0.001) in experiment 2. NO₃⁻-N removal varied from 60 to 100 % in overlying water among all treatments. The lowest NO₃⁻-N removals in experiment 1 were observed in the control at 14 and 28 days, which were significantly less than in DOC and POC 14- and 28-day treatments. In experiment 2, significantly less NO₃⁻-N was removed in overlying water of the control compared to DOC and POC treatments at all C/N ratios. Amendments of DOC and POC made to drainage ditch sediment: (1) increased NO₃⁻-N removal, especially over longer time intervals (14 to 28 days); (2) increased NO₃⁻-N removal, regardless of C/N ratio; and (3) NO₃⁻-N removal was best at a 5:1 C/N ratio. This study provides support for continued investigation on the use of organic carbon amendments as a best management practice for NO₃⁻-N removal in agricultural drainage ditches.