Vertical Distribution of Denitrification in an Estuarine Sediment: Integrating Sediment Flowthrough Reactor Experiments and Microprofiling via Reactive Transport Modeling

Abstract

Denitrifying activity in a sediment from the freshwater part of a polluted estuary in northwest Europe was quantified using two independent approaches. High-resolution N 2 O microprofiles were recorded in sediment cores to which acetylene was added to the overlying water and injected laterally into the sediment. The vertical distribution of the rate of denitrification supported by nitrate uptake from the overlying water was then derived from the time series N 2 O concentration profiles. The rates obtained for the core incubations were compared to the rates predicted by a forward reactive transport model, which included rate expression for denitrification calibrated with potential rate measurements obtained in flowthrough reactors containing undisturbed, 1-cmthick sediment slices. The two approaches yielded comparable rate profiles, with a near-surface, 2-to 3-mm narrow zone of denitrification and maximum in situ rates on the order of 200 to 300 nmol cm ؊3 h ؊1 . The maximum in situ rates were about twofold lower than the maximum potential rate for the 0-to 1-cm depth interval of the sediment, indicating that in situ denitrification was nitrate limited. The experimentally and model-derived rates of denitrification implied that there was nitrate uptake by the sediment at a rate that was on the order of 50 (؎ 10) nmol cm ؊2 h ؊1 , which agreed well with direct nitrate flux measurements for core incubations. Reactive transport model calculations showed that benthic uptake of nitrate at the site is particularly sensitive to the nitrate concentration in the overlying water and the maximum potential rate of denitrification in the sediment.Anthropogenic inputs of nitrogen are a major cause of eutrophication in aquatic environments (39). Denitrification, the bacterial dissimilatory reduction of nitrate to gaseous products, counteracts eutrophication by removing inorganic nitrogen and releasing it to the atmosphere and by decomposing organic matter (34). The emissions resulting from denitrification, however, include nitrous oxide, a powerful greenhouse gas (8). Because of its environmental importance, denitrification has been the subject of numerous studies (for reviews, see references 12, 16, and 34). Oxygen and nitrate concentrations, temperature, pH, and the availability of suitable electron donor substrates (mainly organic carbon compounds) are considered the key factors controlling the occurrence and rate of denitrification (6,22,38).In estuarine and coastal environments, the most intense denitrification often occurs in sediments, where reduced solute transport rates enable the establishment of anaerobic conditions. A number of methods have been developed to quantify denitrification and related nitrogen transformations in sediments; these methods include, among others, benthic flux measurement (17), isotope pairing techniques (32), and microprofiling using nitrous oxide (31) or nitrate microsensors (9,20). The available data show that there are large spatial and temporal variations in denitrification rates in nearsho...

Impacts of vegetation, tidal process, and depth on the activities, abundances, and community compositions of denitrifiers in mangrove sediment

“…Our results are supported by previous studies showing that denitrification activities and denitrifier abundances were always higher in the surface soils and sediments and declined with depth (Boz et al 2013;Fischer et al 2013;Papaspyrou et al 2014). Possible explanations might be the variations for the dissolved organic carbon, oxygen, and microbial biomass (Laverman et al 2007;Boz et al 2013;Fischer et al 2013). In this study, it is clear that changes of sediment properties were more evident in the surface rather than the deeper sediments (Table 1).…”

“…Moreover, not all the possible denitrifiers in the sediment were functionally active. Some nitrifiers also carry the denitrifying genes since they can drive nitrifier denitrification under the sub-aerobic conditions which often occurs in the surface layer (Wrage et al 2001;Laverman et al 2007). This may explain the observation of the enhancement of activities and abundances in the 0-5-cm sediments but no variation of (nirK+ nirS)/nosZ and N 2 O/(N 2 O+N 2 ) ratios.…”

Gas geochemical characteristics of hydrothermal plumes at the HAKUREI and JADE vent sites, the Izena Cauldron, Okinawa Trough

“…Excess N 2 O was also observed in hydrothermal fluid observed at the Galapagos Spreading Centre and thought to result from microbial denitrification at hydrothermal ecosystem (Lilley et al, 1982). Moreover, even when hydrothermalism is absent, microbial denitrification in the sediment produce N 2 O (e.g., Usui et al, 1998;Laverman et al, 2007) and could result in positive N 2 O concentration anomaly in bottom water. These facts suggest that biological N 2 O production likely account for the N 2 O excess observed at Stn.…”

Combined Gel Probe and Isotope Labeling Technique for Measuring Dissimilatory Nitrate Reduction to Ammonium in Sediments at Millimeter-Level Resolution

“…Conventional methods to investigate the relative importance of different pathways of dissimilatory NO 3 Ϫ reduction in sediments include slurry incubations (4,12,30), methods that use sealed sediment cores (7,47), and methods that use flowthrough sediment cores (16,27,37). Only one study used reconstituted sediment cores in which DNRA activity was measured at centimeter-level resolution (39).…”

“…Until now, the influence of chemical factors on DNRA and denitrification in sediments has been assessed by slurry incubations (4,12,30), by flux measurements with sealed sediment cores (7, 47) or flowthrough sediment cores (16,27,37), and in one case, in reconstituted sediment cores sliced at centimeter-level resolution (39). Here, we present a new method, the combined gel probe and isotope labeling technique, to determine the vertical distribution of the net rates of DNRA in sediments.…”