Seasonal Differences in Relationships between Nitrate Concentration and Denitrification Rates in Ditch Sediments Vegetated with Rice Cutgrass. Issue 6 (1st November 2017)

• Record Type: Journal Article
• Authors: Speir, Shannon L.; Taylor, Jason M.; Scott, J. Thad
• Is Part Of: Journal of Environmental Quality. Volume 46:Issue 6(2017)
• Page Start: 1500
• Page End: 1509
• Publication Date: 2017-11-01
• Subjects: Agricultural ecology -- Periodicals; Environmental engineering -- Periodicals; Pollution -- Periodicals; 630
• DOI: 10.2134/jeq2016.11.0450
• Languages: English
• ISSNs: 0047-2425

Abstract

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 (L.) Sw.] 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. Net denitrification rates were measured as positive N2 fluxes 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 V max (maximum amount of net N2 flux) and K (concentration of NO3 − in the overlying water at which the net N2 flux is half of V max ), with a maximum production of N2 of ∼20 mg N2 m −2 h −1 . Maximum percentage NO3 − retention occurred at 1 mg NO3 − L −1 in the overlying water in all seasons, except summer where maximum retention persisted from 1 to 5 mg NO3 − L −1 . Denitrification rates were strongly correlated with NO3 − uptake by vegetated sediments in spring ( r 2 = 0.94, p < 0.0001) and summer ( r 2 = 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 may act as 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 to 40% of the immobilized NO3 − through microbial denitrification. Core Ideas: Dinitrogen gas fluxes exhibited Michaelis–Menten kinetics in the spring, summer, and fall. Denitrification was strongly correlated with nitrate uptake in spring and summer. Up to 40% of immobilized nitrate was removed via microbial denitrification. Cutgrass ditch sediments are effective nitrate sinks during the growing season.