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. Use of low‐grade weirs and organic carbon amendments in agricultural drainage ditches were evaluated for nitrogen removal efficacy. Only decreases in nitrate‐nitrogen concentration from ditch inflow to outflow were observed in ditches with weirs, while changes in concentration were variable in the ditch without weirs. Dissolved organic carbon treatments had greater nitrate‐nitrogen removal compared to particulate organic carbon and control treatments. Presence of weirs and sampling date also affected microbial denitrification gene abundance. Increases in nitrate‐nitrogen removal observed with weirs and organic carbon amendments provide support for further investigation to improve efficacy of these best management practices in drainage ditches.