Enhancing ditch denitrification with rice cutgrass: experimental evidence for a simple nitrate runoff mitigation tool

Author(s): Taylor, J.; Speir, S.; Moore, M.; Scott, J.

Widespread implementation of best management practices (BMPs) that mitigate nitrogen (N) runoff are needed to reduce significant environmental impacts including eutrophication of fresh and coastal waters. Denitrification is a biologically-mediated mechanism that converts NO3-N to N2 gas and reduces N transport to downstream waterbodies. We investigated NO3-N mitigation and denitrification potential in ditch sediments vegetated with rice cutgrass (Leersia oryzoides). An initial study was conducted to quantify differences in N retention and denitrification during experimental runoff events between three different vegetation treatments: unvegetated, rice cutgrass, and common cattail (Typha latifolia L.). Vegetated mesocosms removed significantly more NO3-N from the water column than unvegetated systems. However, sediments planted with cutgrass had significantly higher average denitrification rates (5.93 mg m-2 h-1) than cattails and unvegetated sediments (0.2 mg and 0.19 m-2 h-1). Whole mesocosm mass balance indicated that denitrification accounted for as much as 56% of the immobilized nitrate over a 48 hr period. A follow up study examined the effects of nitrate availability on uptake and denitrification in sediments planted with cutgrass over four seasons. Michaelis-Menten models described the relationship between nitrate concentration and N2 flux rates for spring, summer, and fall seasons. Summer denitrification models exhibited the highest Vmax and K, with maximum N2 fluxes of approximately 20 mg m-2 h-1. Denitrification rates were strongly correlated with NO3-N uptake by vegetated sediments in spring and summer, but low uptake in fall and winter resulted in virtually no net denitrification during these seasons. Whole mesocosm 48 hr denitrification was estimated using models from study 2 applied to data from study 1. The predicted contribution of denitrification to N mitigation based on Michaelis-Menten kinetics was slightly higher, less variable, but within one standard error of original estimates based on applying average denitrification rates to mass balance estimates for the same data set (310.80 ± 5.03 vs 284.48 ± 29.69 mg). Our results indicate that ditch sediments vegetated with cutgrass not only immobilize a significant fraction of nitrate, but also permanently remove significant amounts of immobilized nitrate through microbial denitrification. Ditches vegetated with cutgrass can provide an important tool for mitigating N runoff from agricultural landscapes, particularly during the growing season when ditches receive irrigation tailwater.

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