Remediation of excess nitrogen (N) in agricultural runoff can be enhanced by establishing wetland vegetation, but the role of denitrification in N removal is not well understood in drainage ditches. We quantified differences in N retention during experimental runoff events followed by stagnant periods in mesocosms planted in three different vegetation treatments: unvegetated, cutgrass [ (L.) Sw.], and common cattail ( L.). We also quantified denitrification rates using membrane inlet mass spectrometry from intact cores extracted from each mesocosm treatment. All treatments retained 60% or more of NO-N loads during the 6-h experimental runoff event, but mesocosms planted with cutgrass had significantly higher (68%) retention than the cattail (60%) or unvegetated (61%) treatments. After the runoff event, mesocosms planted in cattail reduced NO-N concentrations by >95% within 24 h and cutgrass achieved similar reductions within 48 h, whereas reductions in the unvegetated mesocosms were significantly less (65%). Cores from cutgrass mesocosms had significantly higher average denitrification rates (5.93 mg m h), accounting for as much as 56% of the immobilized NO-N within 48 h, whereas denitrification rates were minimal in cores from the unvegetated (-0.19 mg m h) and cattail (0.2 mg m h) mesocosms. Our findings have implications for mitigating excess NO-N in agricultural runoff. While vegetated treatments removed excess NO-N from the water column at similar and significantly higher rates than unvegetated treatments, the high denitrification rates observed for cutgrass highlight the potential for permanent removal of excess N from agricultural runoff in vegetated ditches and wetlands.
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