Submitted to: Journal of Environmental Quality
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/11/2017
Publication Date: 10/19/2017
Citation: Speir, S.L., Taylor, J.M., Scott, J.T. 2017. Seasonal differences in relationships between nitrate concentration and denitrification rates in ditch sediments vegetated with rice cutgrass (Leersia oryzoides). Journal of Environmental Quality. 46:1500–1509. https://doi.org/10.2134/jeq2016.11.0450.
DOI: https://doi.org/10.2134/jeq2016.11.0450

Interpretive Summary: Excess nitrogen in runoff from agricultural areas can negatively impact water quality in rivers, lakes, and coastal areas. Denitrification is a natural process that converts nitrogen in waters and soils to nitrogen gas, thereby returning excess nitrogen to the atmosphere. Scientists from ARS and the University of Arkansas investigated effects of nitrogen conditions and seasons on denitrification kinetics in ditch sediments planted in rice cutgrass. The amount of nitrogen dissolved in water that was converted to nitrogen gas increased with increasing nitrate concentration until the process became saturated at high concentrations in the spring and summer seasons. The highest observed denitrification rates occurred during the summer when temperature was high and cutgrass was growing with very little net conversion of nitrogen observed during the fall or winter. Results confirm a previous study finding, that sediments vegetated with rice cutgrass can remove substantial amounts of nitrate from runoff through the denitrification process. However, seasonal patterns in mitigation capacity need to be considered when developing vegetated ditch management strategies.

Technical Abstract: Increased application of nitrogen (N) fertilizers in agricultural systems contributes to significant environmental impacts, including eutrophication of fresh and coastal waters. Rice cutgrass (Leersia oryzoides) can significantly enhance denitrification potential in agricultural ditch sediments and potentially reduce N export from agricultural watersheds, but relationships with known drivers are not well understood. To address this, we examined effects of nitrate (NO3-) availability on dinitrogen gas (N2) and NO3- fluxes seasonally. Denitrification rates were measured as N2 flux from vegetated intact sediment cores using Membrane Inlet Mass Spectrometry (MIMS). We developed Michaelis-Menten models for N2 fluxes across NO3- gradients in the spring, summer, and fall seasons. Summer N2 models exhibited the highest Vmax and K, with maximum N2 fluxes of approximately 20 mg m-2 h-1. Maximum percent NO3- retention occurred at 1 mg L-1 NO3- in the overlying water in all seasons, except summer where maximum retention persisted from 1-5 mg L-1. Denitrification rates were strongly correlated with NO3- uptake by vegetated sediments in spring (r2 = 0.94; p < 0.0001) and summer (r2 = 0.97; p < 0.0001), but low NO3- uptake in fall and winter resulted in virtually no net denitrification during these seasons. Our results indicate that vegetated ditch sediments are likely effective NO3- sinks during the growing season. Ditch sediments vegetated with cutgrass not only immobilized a significant fraction of NO3- entering them, but also permanently removed as much as 30-40% of the immobilized NO3- through microbial denitrification.