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

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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 ‐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 ‐N concentrations were observed between treatments, with greater removal of ‐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 ‐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 ‐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 ‐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.