Investigating the role of organic carbon amendments and microbial denitrification gene abundance in nitrogen removal from experimental agricultural drainage ditches with low-grade weirs.

Low-grade weirs placed within agricultural drainage ditches in the Lower Mississippi Alluvial Valley can be used as a management practice to enhance nitrogen removal. The addition of organic carbon amendments within ditches that contain weirs could further increase nitrogen removal. Through repeated trials, changes in NO3--N concentration between inflow and outflow were variable in the ditch without weirs, while only decreases in concentration were observed in ditches with weirs. Significant differences in NO3--N concentrations were observed between treatments, with greater removal of NO3--N observed in dissolved organic carbon treatments compared to control and particulate organic carbon treatments. At medium- and high-flow rates, respectively, dissolved organic carbon treatments resulted in greater NO3--N concentration decreases of 31.6% and 27.1% compared to 19% and 11.6% in particulate organic carbon treatments and 18.6% and 17.2% in control treatments. Significant effects of weirs and sampling date on nirS, nirK, nosZ, and 16S rRNA gene abundances were observed. Observed increases in NO3--N removal with organic carbon amendments, provides support for continued investigation on improving the efficacy of organic carbon amendments as a best management practice for NO3--N removal in agricultural drainage ditches. Practitioner points Dissolved organic carbon amendments increased nitrate-nitrogen removal. Only decreases in nitrate-nitrogen concentration were observed in ditches with weirs. Increasing flow rate did not affect nitrate-nitrogen removal. Abundance of denitrification-performing microbes likely did not affect N removal. Lack of anaerobic soil conditions and short residence time reduced nitrate-N removal.