Seasonal Differences in Relationships between Nitrate Concentration and Denitrification Rates in Ditch Sediments Vegetated with Rice Cutgrass

Abstract

Increased application of nitrogen (N) fertilizers in agricultural systems contributes to significant environmental impacts, including eutrophication of fresh and coastal waters. Rice cutgrass [ (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 (NO) availability on dinitrogen gas (N) and NO fluxes seasonally. Net denitrification rates were measured as positive N fluxes from vegetated intact sediment cores using membrane inlet mass spectrometry (MIMS). We developed Michaelis-Menten models for N fluxes across NO gradients in the spring, summer, and fall seasons. Summer N models exhibited the highest (maximum amount of net N flux) and (concentration of NO in the overlying water at which the net N flux is half of ), with a maximum production of N of ∼20 mg N m h. Maximum percentage NO retention occurred at 1 mg NO L in the overlying water in all seasons, except summer where maximum retention persisted from 1 to 5 mg NO L. Denitrification rates were strongly correlated with NO uptake by vegetated sediments in spring ( = 0.94, < 0.0001) and summer ( = 0.97, < 0.0001), but low NO 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 NO sinks during the growing season. Ditch sediments vegetated with cutgrass not only immobilized a significant fraction of NO entering them but also permanently removed as much as 30 to 40% of the immobilized NO through microbial denitrification.

Nitrate availability affects denitrification in Phragmites australis sediments

“…The V max values measured in P. australis vegetated sediments during summer (∼10,000 mg N m −2 d −1 ) were 10-60 times greater than the values measured for a variety of emergent plants incubated under similar water temperature conditions (Table 2). Wintertime V max values measured in the present experiment also were markedly greater than those reported by Messer, Burchell, Birgand, Broome, & Chescheir (2017) and Speir et al (2017) for Schoenoplectus tabernaemontani (C.C. Gmel.)…”

“…Wintertime V max values measured in the present experiment also were markedly greater than those reported by Messer, Burchell, Birgand, Broome, & Chescheir () and Speir et al. () for Schoenoplectus tabernaemontani (C.C. Gmel.)…”

“…Conversely, P. australis demonstrated a weaker affinity for NO 3 − compared with other macrophytes such as Carex appressa R. Br. (Payne et al., ) and Leersia oryzoides (Speir et al., ). By affecting denitrification drivers (i.e.…”

“…Recently, we parameterized factors that regulate denitrification rates associated with P. australis , including the role of water velocity and biofilm presence (Castaldelli et al., ; Soana, Gavioli, et al., ). In addition, others have started to investigate the role of NO 3 − availability on plant‐mediated denitrification for a variety of species (Messer, Burchell, Birgand, Broome, & Chescheir, 2017; Speir, Taylor, & Scott, ), but to our knowledge, there is no literature describing relationships between denitrification rates and NO 3 − availability in sediments vegetated with P. australis .…”

Recognizing both denitrification and nitrogen consumption improves performance of stream diel N2 flux models

“…Hanrahan et al () used sacrificial microcosm incubations and membrane‐inlet mass spectrometry (MIMS) to compare rates from different habitat zones within two‐stage ditches. Alternatively, continuous flow‐through incubations of intact sediment cores rely on core removal from ditch habitats and incubations in flow‐through laboratory settings to measure gas fluxes (Taylor et al ; Speir et al ). All of these methods rely on incubations in the dark at a constant temperature.…”