Quantity and quality of organic matter (detritus) drives N₂ effluxes (net denitrification) across seasons, benthic habitats, and estuaries

Abstract

[1] N 2 flux rates (net denitrification) were measured over a diel cycle, seasonally, in 12 benthic habitats across three warm temperate Australian coastal systems. Dark N 2 -N fluxes were strongly controlled by sediment oxygen demand (SOD) across the 3 estuaries, 4 seasons, and 12 benthic habitats (r 2 = 0.743; p < 0.001; n = 142; slope = 0.0170). However, some of the slopes differed significantly between seasons and among estuaries and habitats, and all of the slopes were correlated with the δ 13 C values and C:N ratios of sediment organic matter. Ternary mixing diagrams with the contribution of algal, seagrass, and terrestrial/ mangrove material to sediment organic matter showed that habitats, seasons, and estuaries dominated by a mixture of seagrass and algal material had the lowest slopes, and slopes increase as habitats, seasons, and estuaries have an increasing contribution from terrestrial/ mangrove material. Overall, the slopes of dark N 2 fluxes versus SOD were low compared to previous studies, most likely due to either, or a combination of, the C:N ratio of the organic matter, the mixture of C:N ratios making up the organic matter, the structure of the organic matter, and/or the SOD rates. This study demonstrated that it is not only the quantity but also the type (quality), and maybe the mixture, of organic matter that is an important control on denitrification. As such, rapid global changes to detrital sources to coastal systems due to losses of mangrove, seagrasses, and saltmarshes, and associated increases in algae and macrophytes, are also expected to impact system level losses of nitrogen via denitrification.Citation: Eyre, B. D., D. T. Maher, and P. Squire (2013), Quantity and quality of organic matter (detritus) drives N 2 effluxes (net denitrification) across seasons, benthic habitats, and estuaries, Global Biogeochem.

Denitrification in an oligotrophic estuary: a delayed sink for riverine nitrate

“…In contrast, the organic content differed sig-nificantly between the sediment types. Similar to the sediment O 2 dynamics, the same denitrification rates at different organic contents imply that not all of the organic matter in the sediment could be utilized by the denitrifying bacteria and that not only organic matter quantity but also quality needs to be considered (Hietanen & Kuparinen 2008, Eyre et al 2013, Bonaglia et al 2017). This was also indicated by the lack of correlation of LOI with total denitrification, Dn or Dw.…”

Sediment Nitrogen Fixation: a Call for Re-evaluating Coastal N Budgets

“…Denitrification is influenced by oxygen concentration, as well as carbon and nitrate availability (Cornwell et al 1999;Ward et al 2009). Field and mesocosm studies (Fulweiler et al 2007(Fulweiler et al , 2013Eyre et al 2013) in Narragansett Bay, RI suggest that both the quality and quantity of available organic carbon may control the balance between these processes. However, rates of these processes have not been measured separately and simultaneously, and heterotrophic N-fixation has not been proven to occur in sediments beyond the observation of N 2 uptake.…”

Push‐pull incubation method reveals the importance of denitrification and dissimilatory nitrate reduction to ammonium in seagrass root zone

“…Isotope additions of either 15 or 15 are commonly used; the isotope pairing technique (IPT), developed by Nielsen (1992) uses 15 additions to differentiate between denitrification of nitrate derived from the water column and from coupled nitrification in the surface sediments. Measurements of net N 2 fluxes without isotope additions are also used with both cores and chambers (Ferguson et al ; Smyth et al ; Eyre et al ). Each method has its own strengths and weaknesses ( see Cornwell et al and Steingruber et al for more complete comparisons); however, none of these techniques both preserve conditions in the field and directly account for subsurface rates.…”