Denitrification In Sediments Research Articles

Interlake variation and environmental controls of denitrification across different geographical scales

AME Aquatic Microbial Ecology Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsSpecials AME 69:1-16 (2013) - DOI: https://doi.org/10.3354/ame01619 FEATURE ARTICLE Interlake variation and environmental controls of denitrification across different geographical scales Antti Juhani Rissanen1,2,*, Marja Tiirola1, Susanna Hietanen3, Anne Ojala4 1Department of Biological and Environmental Science, University of Jyväskylä, Survontie 9, 40500 Jyväskylä, Finland 2Lammi Biological Station, University of Helsinki, Pääjärventie 320, 16900 Lammi, Finland 3Department of Environmental Sciences, University of Helsinki, PO Box 65 (Viikinkaari 1), 00014 University of Helsinki, Finland 4Department of Environmental Sciences, University of Helsinki, Niemenkatu 73, 15140 Lahti, Finland *Email: antti.j.rissanen@jyu.fi ABSTRACT: Denitrification in lakes significantly reduces the nitrogen (N) load from land to oceans, but the factors controlling it remain poorly understood. Therefore, interlake variation of denitrification in sediments of small to medium-sized lakes (from 0.03 to 17.8 km2) was studied across different geographical scales. At the local scale, the denitrification rates and sediment microbial communities were studied in 4 boreal lakes in close proximity (within 20 km) using the isotope pairing technique (IPT) and molecular methods. These local scale data were combined with previously published data on denitrification rates from 10 other European lakes to extend the analysis to the regional (boreal area) and continental (boreal and temperate areas) scales. Denitrification varied considerably among lakes, ranging from ~50 to ~600, ~0 to ~600 and ~0 to ~12900 μmol N m-2 d-1 at the local, regional and continental scales, respectively. This variation was primarily due to nitrate availability. The structure of the denitrifier community studied at the local scale was independent of the denitrification rates but varied among lakes correlating with nitrate availability and sediment organic content. Removal of nitrate and total N load by denitrification was less efficient in boreal than in temperate lakes. In addition, a meta-analysis of published N mass balance data indicates that the total N retention (denitrification + N sedimentation) is less efficient in boreal than in temperate lakes. Anaerobic ammonium oxidation (anammox) was studied at the local scale but was not detected, although (based on molecular markers) several anammox genera were present in the sediments. KEY WORDS: Denitrification · Lake · Sediment · nirK Full text in pdf format Information about this Feature Article NextCite this article as: Rissanen AJ, Tiirola M, Hietanen S, Ojala A (2013) Interlake variation and environmental controls of denitrification across different geographical scales. Aquat Microb Ecol 69:1-16. https://doi.org/10.3354/ame01619 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in AME Vol. 69, No. 1. Online publication date: April 03, 2013 Print ISSN: 0948-3055; Online ISSN: 1616-1564 Copyright © 2013 Inter-Research.

Deep‐sea nutrient loss inferred from the marine dissolved N2/Ar ratio

AbstractSome estimates of the budget of bioavailable nitrogen (fixed‐N) suggest that the oceanic nitrogen cycle is grossly out of balance. We use observations of dissolved N2/Ar ratios along abyssal flow paths in the ocean to investigate fixed‐N loss by benthic denitrification, one of the largest uncertainties. Dissolved N2/Ar in the deep‐sea increases from the North Atlantic to the North Pacific. At depths >3500 m, this can be explained by the mixing of low N2/Ar source waters from the North Atlantic with high N2/Ar source waters from the Southern Ocean. Benthic denitrification in sediments bathed by these abyssal waters is below the detection limit. However, measureable increases in N2/Ar at depths of 2000–3000 m between the subtropical and subarctic North Pacific, regions that share the same source water, must be caused by benthic denitrification. The Cascadia Basin, with high denitrification rates and connection to the open North Pacific, is a likely source.

Denitrification, anammox and nitrate reduction in sediments of the southern Great Barrier Reef lagoon

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 478:57-70 (2013) - DOI: https://doi.org/10.3354/meps10040 Denitrification, anammox and nitrate reduction in sediments of the southern Great Barrier Reef lagoon Dirk V. Erler*,1, Lindsay A. Trott2, Daniel M. Alongi2, Bradley D. Eyre1 1Centre for Coastal Biogeochemistry, School of Environmental Science and Management, Southern Cross University, Lismore NSW 2480, Australia. 2Australian Institute of Marine Science, PMB 3, Townsville MC QLD 4810, Australia *Email: dirk.erler@scu.edu.au ABSTRACT: We provide the first reported estimates of anammox activity in tropical continental shelf sediments (southern section of the Great Barrier Reef lagoon; GBRL). The measured contribution of anammox to total N2 production was up to 70% but restricted to only 1 of the 4 (2 inshore and 2 offshore) sites assayed. Sediment characteristics (contents of total organic carbon [TOC] and manganese [Mn], C:N ratio) at this site appeared to favour anammox activity and the estimated maximum rate was 4.9 µmol m-2 h-1. Anammox bacteria may be a significant contributor to N2 production along the coastal zone of the GBRL. The availability of labile (low C:N) TOC seemed to drive denitrification to completion in the offshore sediments. However, rates of NO3- reduction to NH4+ at the offshore sites were comparable to or higher than denitrification rates. It was unclear whether dissimilatory or assimilatory processes were responsible for the observed reduction of NO3- to NH4+ at the offshore sites. At the 2 inshore sites, NO3- reduction to NH4+ was a larger sink for NO3- than denitrification. Anammox does exist in the tropical continental shelf sediments of the GBRL and should be studied further to determine its role in larger scale N cycling. The roles of assimilatory and dissimilatory nitrate reduction to ammonium also need to be assessed within the GBRL. KEY WORDS: Great Barrier Reef · Anammox · Dissimilatory/assimilatory nitrate reduction to ammonia · Denitrification Full text in pdf format PreviousNextCite this article as: Erler DV, Trott LA, Alongi DM, Eyre BD (2013) Denitrification, anammox and nitrate reduction in sediments of the southern Great Barrier Reef lagoon. Mar Ecol Prog Ser 478:57-70. https://doi.org/10.3354/meps10040 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 478. Online publication date: March 25, 2013 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2013 Inter-Research.

The role of sediments on the Bering Sea shelf N cycle: Insights from measurements of benthic denitrification and benthic DIN fluxes

Continental shelves are hotspots for sedimentary denitrification, and the loss of N through denitrification can limit primary production in ecosystems. Spatial and seasonal trends in sedimentary denitrification and benthic nutrient fluxes are poorly characterized in the highly productive Bering Sea shelf ecosystem. Through the Bering Sea Ecosystem Study (BEST) program, we measured benthic fluxes of N2 and dissolved inorganic nitrogen (DIN: NH4++NO2−+NO3−), the extent of coupled sedimentary nitrification/denitrification, and the water column DIN deficit relative to phosphate, as indicated by a modified N* parameter (N**), on the Bering Sea shelf in the spring and summer 2009–2010. We found that sedimentary denitrification is widespread over the shelf, it is fueled mostly through coupled nitrification/denitrification, the net balance of sedimentary DIN flux is near zero over the shelf, and that the water column DIN deficit varies widely according to season and year. In the summer, N** in the surface layer appeared to be strongly affected by non-Redfieldian uptake of inorganic nutrients by phytoplankton in the spring bloom; in the winter, N** was strongly affected by sedimentary denitrification. Our findings indicate that the estimate of total N loss in Bering Sea shelf sediments should be revised upwards by at least 50% to 5.2–6.2TgNy−1. In addition, sediments are not a significant source of remineralized N for primary production over the shelf; hence sedimentary denitrification exacerbates N-limitation of the ecosystem.

The role of highly sratified nutrient-rich small estuaries as a source of dissolved inorganic nitrogen to coastal seawater, the Qishon (SE Mediterranean) case

We studied the role of small, highly stratified, sulfate and nutrient enriched estuaries, as a source or sink of inorganic nitrogen species, using the Qishon estuary at the Mediterranean coast of Israel, as a case study. Measurements of nutrient concentrations, δ15N and δ18O of nitrate+nitrite, δ13CDIC and δ18OH2O were performed during 2008–2009 along the upper-fresh and lower-saline water masses, as well as sediment porewater depth-profiles. Such estuaries are characterized by relatively low removal flux of NO3- (via sedimentary denitrification) and enhanced (×3) upward flux of NH4+ (via sulfate reduction), attributed to the penetration of seawater of low NO3- and high dissolved oxygen and sulfate concentrations. The role of such small estuaries in releasing dissolved inorganic nitrogen, especially in sensitive oligotrophic areas as the Levantine basin and in the long-term, as a result of enhanced seawater penetration due to the expected sea level rise, has important environmental policy implications.

Sediment denitrification in waterways in a rice-paddy-dominated watershed in eastern China

Rice-paddy-dominated watersheds in eastern China are intensively cultivated, and lands with two crops receive as much as 550–600 kg ha–1 year–1 of nitrogen (N), mainly through the addition of N-based fertilizers. However, stream N concentrations have been found to be relatively low. Waterways in the watersheds are assumed to be effective “sinks” for N, minimizing its downstream movement. We directly measured net sediment denitrification rates in three types of waterways (ponds, streams/rivers, and a reservoir) and determined the key factors that control net sediment denitrification. Such information is essential for evaluating the impact of the agricultural N cycle on the quality of surface water. The pond–stream–reservoir continuum was sampled every 2 months at nine sites in an agricultural watershed between November 2010 and December 2011. Net sediment N2 fluxes/net sediment denitrification rates were determined by membrane inlet mass spectrometry and the N2/Ar technique. A suite of parameters known to influence denitrification were also measured. Net denitrification rates ranged between 28.2 ± 18.2 and 674.3 ± 314.5 μmol N2–N m–2 h–1 for the streams, 23.7 ± 23.9 and 121.2 ± 38.7 μmol N2–N m–2 h–1 for the ponds, and 41.8 ± 17.7 and 239.3 ± 49.8 μmol N2–N m–2 h–1 for the reservoir. The mean net denitrification rate of the stream sites (173.2 ± 248.4 μmol N2–N m–2 h–1) was significantly higher (p < 0.001) than that of the pond sites (48.3 ± 44.5 μmol N2–N m–2 h–1), and the three types of waterways all had significantly higher (p < 0.01) mean net denitrification rates in summer than in other seasons. Linear regression and linear mixed effect model analysis showed that nitrate (NO3 −–N) concentration in surface water was the primary controlling factor for net sediment denitrification, followed by water temperature. Using monitoring data on NO3 −–N concentrations and temperature of the surface water of waterways and an established linear mixed effect model, total N removed through net sediment denitrification in the pond–stream–reservoir continuum was estimated at 46.8 ± 24.0 t year–1 from July 2007 to June 2009, which was comparable with earlier estimates based on the mass balance method (34.3 ± 12.7 t year–1), and accounted for 83.4 % of the total aquatic N. However, the total aquatic N was only 4.4 % of the total N input to the watershed, and thus most of the surplus N in the watershed was likely to be either denitrified or stored in soil. High doses of N in a rice-paddy-dominated watershed did not lead to high stream N concentrations due to limited input of N into waterways and the high efficiency of waterways in removing N through denitrification.

Denitrification in the water column of the central Baltic Sea

Removal of fixed nitrogen in the water column of the eastern Gotland Basin, central Baltic Sea, was studied during two cruises in September 2008 and August 2010. The water column was stratified with anoxic sulfidic bottom water meeting oxic nitrate containing water at the oxic–anoxic interface. Anammox was never detected whereas denitrification was found in all incubations from anoxic depths and occurred immediately below the oxic–anoxic interface. Sulfide (H2S+HS−+S2−) was in most cases the only electron donor for denitrification but, in contrast to previous findings, denitrification was in some situations driven by organic matter alone. Nitrous oxide (N2O) became an increasingly important product of denitrification with increasing sulfide concentration and was >80% of the total N gas formation at 10μM sulfide. The potential rates of denitrification measured in incubations at elevated NO3- or sulfide concentrations were converted to in situ rates using the measured water column concentrations of NO3- and sulfide and the actual measured relations between NO3- and sulfide concentrations and denitrification rates. In situ denitrification ranged from 0.24 to 15.9nMN2h−1. Assuming that these rates were valid throughout the anoxic NO3- containing zone, depth integrated in situ denitrification rates of 0.06–2.11mmol Nm−2d−1 were estimated. The thickness of this zone was generally 3–6m, which is probably what can be maintained through regular turbulent mixing induced by internal waves at the oxic–anoxic interface. However, layers of up to 55m thickness with low O2 water (<10μM) were observed which was probably the result of larger scale mixing. In such a layer nitrification may produce NO3- and once the O2 has been depleted denitrification will follow resulting in enormous rates per unit area. Even with an active denitrification layer of 3–6m thickness the pelagic denitrification per unit area clearly exceeded sediment denitrification rates elsewhere in the Baltic Sea. When extrapolated to the entire Baltic Proper (BP) denitrification in the water column was in the range of 132–547ktonNyr−1 and was thus at least as important as sediment denitrification which has recently been estimated to 191ktonNyr−1. With a total external N-input of 773ktonNyr−1 it is clear that denitrification plays a significant role in the N-budget of the BP.

干潟堆積物における脱窒・アナモックス速度に影響を及ぼす要因

干潟堆積物における脱窒・アナモックス速度に影響を及ぼす要因

Anammox, denitrification and fixed-nitrogen removal in sediments from the Lower St. Lawrence Estuary

Abstract. Incubations of intact sediment cores and sediment slurries reveal that anammox is an important sink for fixed nitrogen in sediments from the Lower St. Lawrence Estuary (LSLE), where it occurs at a rate of 5.5 ± 1.7 µmol N m−2 h−1. Canonical denitrification occurs at a rate of 11.3 ± 1.1 µmol N m−2 h−1, and anammox is thus responsible for up to 33% of the total N2 production. Both anammox and denitrification are mostly (&gt; 95%) fueled by nitrate and nitrite produced in situ through benthic nitrification. Nitrification accounts for &gt; 15% of the benthic oxygen demand and may, therefore, contribute significantly to the development and maintenance of hypoxic conditions in the LSLE. The rate of dissimilatory nitrate reduction to ammonium is three orders of magnitude lower than denitrification and anammox, and it is insignificant to N-cycling. NH4+ oxidation by sedimentary Fe(III) and Mn(III/IV) in slurry incubations with N isotope labels did not occur at measurable rates; moreover, we found no evidence for NH4+ oxidation by added Mn(III)-pyrophosphate.

The kinetics of denitrification in permeable sediments

Permeable sediments comprise the majority of shelf sediments, yet the rates of denitrification remain highly uncertain in these environments. Computational models are increasingly being used to understand the dynamics of denitrification in permeable sediments, which are complex environments to study experimentally. The realistic implementation of such models requires reliable experimentally derived data on the kinetics of denitrification. Here we undertook measurements of denitrification kinetics as a function of nitrate concentration in carefully controlled flow through reactor experiments on sediments taken from six shallow coastal sites in Port Phillip Bay, Victoria, Australia. The results showed that denitrification commenced rapidly (within 30 min) after the onset of anoxia and the kinetics could be well described by Michaelis–Menten kinetics with half saturation constants (apparent Km) ranging between 1.5 and 19.8 μM, and maximum denitrification rate (Vmax) were in the range of 0.9–7.5 nmol mL−1 h−1. The production of N2 through anaerobic ammonium oxidation (anammox) was generally found to be less than 10 % of denitrification. Vmax were in the same range as previously reported in cohesive sediments despite organic carbon contents one order of magnitude lower for the sediments studied here. The ratio of sediment O2 consumption to Vmax was in the range of 0.02–0.09, and was on average much lower than the theoretical ratio of 0.8. As a consequence, models implemented with the theoretical ratio of 0.8 are likely to overestimate denitrification by a factor of ~3. The most likely explanation for this is that the microbial community is not able to instantaneously shift or optimally use a particular electron acceptor in the highly dynamic redox environment experienced in permeable sediments. In contrast to previous studies, we did not observe any significant rates of oxic denitrification.

Carbon availability limits potential denitrification in watercress farm sediment

Wetlands are widely used to improve water quality, in particular for removal of nutrients such as nitrates. Watercress farms are artificial wetlands that could potentially reduce transfers of nitrate to rivers; however, little is known about denitrification occurring in these particular wetlands. Watercress farming is a declining activity, which could have effects on nitrate removal in these wetlands. Therefore, we investigated the potential for denitrification in sediments from cultivated and abandoned watercress farms using flow-through reactors in the laboratory. Nitrate reduction rates ranged from 1.03 to 7.51μmol NO3−cm−3day−1 (mean±SD: 3.2±2.1μmol NO3−cm−3day−1). In general, sediments downstream of watercress beds exhibited higher nitrate reduction rates and contained more organic carbon than upstream sediments. The highest potential nitrate reduction rates were reached by sediments from cultivated watercress beds. As carbon was hypothesized to be a key factor in controlling nitrate reduction rates, the effects of different carbon sources on these rates were investigated by adding carbon to the sediment. Carbon was added either as labile organic acids (acetate or a mixture of lactate, fumarate, and succinate) or in the form of site-specific macrophytes (watercress [Nasturtium officinale] and reed [Phragmites australis]). Nitrate reduction rates increased significantly upon supply of organic acids or macrophytes. The addition of watercress resulted in the highest potential nitrate reduction rates. This study shows that denitrification in watercress farms is carbon-limited and suggests that the abandonment of watercress farms, with declines in macrophyte production and shifting macrophyte dominance, is likely to decrease the ability of these wetlands to remove nitrate by denitrification. Nitrate removal in watercress farms could be improved by supplying easily available organic C or by management of the vegetation, in particular in the upstream part of the beds.

Modeling pathways of riverine nitrogen and phosphorus in the Baltic Sea

A better understanding of the fate of nutrients entering the Baltic Sea ecosystem is an important issue with implications for environmental management. There are two sources of nitrogen and phosphorus: riverine input and atmospheric deposition. In the case of nitrogen, the fixation of dinitrogen by diazotrophic bacteria represents a third source. From an analysis of stable nitrogen isotope ratios it was suggested that most of the riverine nitrogen is sequestered in the coastal rim, specifically along the southern Baltic Sea coast with its coarse sediments, whereas nitrogen from fixation dominates the central basins. However, pathways of nutrients and timescales between the input of the nutrients and their arrival in different basins are difficult to obtain from direct measurements. To elucidate this problem, we use a source attribution technique in a three‐dimensional ecosystem model, ERGOM, to track nutrients originating from various rivers. An “age” variable is attributed to the marked elements to indicate their propagation speeds and residence times. In this paper, we specifically investigate the spreading of nitrogen and phosphorus from the riverine discharges of the Oder, Vistula, Neman and Daugava. We demonstrate which regions they are transported to and for how long they remain in the ecosystem. The model results show good agreement with source estimations from observedδ15N values in sediments. The model results suggest that 95% of nitrogen is lost by denitrification in sediments, after an average time of 1.4 years for riverine nitrogen. The residence time of riverine phosphorus is much longer and exceeds our simulated period of 35 years.

Simple transfer functions for calculating benthic fixed nitrogen losses and C:N:P regeneration ratios in global biogeochemical models

Empirical transfer functions are derived for predicting the total benthic nitrate loss (LNO3) and the net loss of dissolved inorganic nitrogen (LDIN) in marine sediments, equivalent to sedimentary denitrification. The functions are dynamic vertically integrated sediment models which require the rain rate of particulate organic carbon to the seafloor (RRPOC) and a proposed new variable (O2‐NO3)bw (bottom water O2 concentration minus NO3− concentration) as the only input parameters. Applied globally to maps of RRPOC and (O2‐NO3)bw on a 1° × 1° spatial resolution, the models predict a NO3− drawdown of 196 Tg yr−1 (LNO3) of which 153 – 155 Tg yr−1 is denitrified to N2 (LDIN). This is in good agreement with previous estimates using very different methods. Our approach implicitly accounts for fixed N loss via anammox, such that our findings do not support the idea that the relatively recent discovery of anammox in marine sediments might require current estimates of the global benthic marine N budget to be revised. The continental shelf (0 – 200 m) accounts for &gt;50% of global LNO3 and LDIN, with slope (200 – 2000 m) and deep‐sea (&gt;2000 m) sediments contributing ca. 30% and 20%, respectively. Denitrification in high‐nitrate/low‐oxygen regions such as oxygen minimum zones is significant (ca. 15 Tg N yr−1; 10% of global) despite covering only ∼1% of the seafloor. The data are used to estimate the net fluxes of nitrate (18 Tg N yr−1) and phosphate (27 Tg P yr−1) across the sediment‐water interface. The benthic fluxes strongly deviate from Redfield composition, with globally averaged N:P, N:C and C:P values of 8.3, 0.067 and 122, respectively, indicating world‐wide fixed N losses (by denitrification) relative to C and P. The transfer functions are designed to be coupled dynamically to general circulation models to better predict the feedback of sediments on pelagic nutrient cycling and dissolved O2 distributions.

Incorporating spatial variation of nitrification and denitrification rates into whole‐lake nitrogen dynamics

Despite dramatic increases in nitrogen (N) loading to fresh waters and growing scientific attention on the changing N cycle, measurements of nitrification and denitrification rates in lakes are lacking. In particular, we know little about how these processes vary spatially within a lake, and how this potential spatial variation contributes to a lake's N dynamics. We measured sediment nitrification and denitrification rates at 40 sites in Gull Lake, Michigan (USA), and found that the shallow edge sediments (&lt;2 m deep) of the lake were hot spots of N transformation. Nitrification rates were comparable in sediments at all depths, while sediment denitrification rates were highest in the shallow edge habitat, and lowest in the profundal sediments (&lt;2 m and &gt;10 m deep, respectively). We scaled‐up our sediment transformation rates across the lake to illustrate spatial variability in nitrification and denitrification. For whole‐lake nitrification, the contribution of shallow edge, littoral, and profundal sediments followed in proportion to lake surface area of each habitat. In contrast, the contribution of each of these areas to whole‐lake denitrification was not proportional to their respective surface areas, and instead was equal across the 3 habitat types, with each area contributing roughly 30% of the total N loss via denitrification. Spatially representative characterization of nitrification and denitrification in lentic ecosystems requires incorporation of the spatial variation in these transformations with a particular focus on littoral sediments, and this is often overlooked in studies of lentic N cycling. Furthermore, anthropogenic changes to lake shorelines that influence N cycling in littoral sediments may have a disproportionate effect on whole‐lake ecosystem function.

Role of sediment denitrification in water column oxygen dynamics: comparison of the North American East and West Coasts

Abstract. Low oxygen concentrations, either natural or anthropogenically driven, can severely affect coastal marine ecosystems. A deeper understanding of oxygen dynamics is required in order to improve numerical models, eventually to predict the timing and severity of hypoxia. In this study we investigate the effect of sediment denitrification on oxygen concentrations in bottom waters over the continental shelf. We used two coupled physical-biological models based on the Regional Ocean Modelling System (ROMS) to compare summer simulations with and without denitrification within the sediments for two North American shelves: the Middle Atlantic Bight (MAB) and the Vancouver Island Shelf (VIS). These regions belong to western and eastern boundary current systems, respectively, and are characterized by different physical and biological dynamics. Both models assume coupled nitrification-denitrification within the sediments. Denitrification represents a loss of bioavailable nitrogen through the production of dinitrogen gas, with the potential to affect biogeochemical cycles. In our MAB model, this loss of regenerated nutrients through denitrification within the sediments significantly affects primary production, since recycled nitrogen supports most of the primary production in that region. The diminished primary production and consequent decrease of organic matter flux to the seafloor leads to less sediment oxygen consumption and higher oxygen concentrations in bottom waters. However, changes in regenerated nitrogen on the VIS barely affect primary production due to the efficient supply of new nutrients through wind-driven upwelling during summer and the nutrient-rich coastal current. We recommend that modelling experiments focusing on oxygen dynamics (as well as oxygen budget calculations) should include sediment denitrification in coastal regions where regenerated primary production dominates productivity.

Effects of macrophyte-associated nitrogen cycling bacteria on denitrification in the sediments of the eutrophic Gonghu Bay, Taihu Lake

Integrated Elodea nuttallii-immobilized nitrogen cycling bacteria (INCB) technology was used for ecological restoration in the eutrophic Gonghu Bay, Taihu Lake. Sediment denitrification was investigated through microcosm incubations with four different treatments: bare sediment core as control without restoration, sediment + E. nuttallii, sediment + E. nuttallii + INCB, and sediment + INCB. The sediments with E. nuttallii-INCB assemblage (E-INCB) had the highest denitrification rates among all the treatments, and the E-INCB increased the denitrification rate by 162% in the sediments. The presence of macrophytes yielded a penetration depth of O2 to more than 20 mm below the sediment–water interface (SWI), while the depth was only 4 mm in the sediments without macrophytes. The quantity of denitrifier in E-INCB sediments (within ~2 cm below the SWI) showed a significant increasing trend during one-month incubation, which was one order of magnitudes higher than that in the sediments without INCB. Macrophytes caused deeper O2 penetration and increased oxic-anoxic interface, which could stimulate the coupled nitrification–denitrification. The high denitrification rate of the E-INCB treatment may result from the increased inorganic nitrogen content in the vicinity of the SWI, causing more nitrate to reach the anoxic denitrification zone. The results showed that E-INCB assemblage could increase benthic N removal by stimulating denitrification via combined O2 penetration and enhanced microbial N cycling processes. E-INCB might be used as a potential restoration method for controlling fresh water system eutrophication.

Denitrification rates and controlling factors in two agriculturally influenced temporary Mediterranean saline streams

In this study, we tested the hypothesis that agriculture, through its influence on water NO3−-N availability, would control denitrification in agriculturally influenced temporary saline streams, and that water salinity would not affect this process. We also tested the effect of summer drought on the denitrification process. We approached these objectives by estimating sediment denitrification (using the acetylene inhibition technique) in two temporary Mediterranean streams following an increased natural water salinity and agricultural gradient under pre- and post-drought conditions. During the pre-drought conditions, the water NO3−-N concentration was the main predictor of denitrification rates. Together with the water NO3−-N concentration, sediment redox conditions and water salinity appeared to be significant predictors, the latter showing a negative effect. During the post-drought, denitrification rates dropped significantly in both streams and no abiotic factors seemed to significantly influence this process. Our results suggest that high water salinity and drought affected negatively the stream-denitrifying capacity. This study highlights that stressors such as water salinity and hydrological intermittency should be considered in future stream management plans in order to preserve the role of streams on controlling the NO3−-N export, especially in the context of a warmer and drier climate.

Quantifying denitrification in rippled permeable sands through combined flume experiments and modeling

We measured denitrification in permeable sediments in a sealed flume tank with environmentally representative fluid flow and solute transport behavior using novel measurements. Numerical flow and reactive transport models representing the flume experiments were implemented to provide mechanistic insight into the coupled hydrodynamic and biogeochemical processes. There was broad agreement between the model results and experimental data. The model showed that the coupling between nitrification and denitrification was relatively weak in comparison to that in cohesive sediments. This was due to the direct advective transport between anoxic pore water and the overlying water column, and little interaction between the mostly oxic advective region and the underlying anoxic region. Denitrification was therefore mainly fueled by nitrate supplied from the water column. This suggests that the capacity of permeable sediments with nonmigratory ripples to remove bioavailable nitrogen from coastal ecosystems is lower than that of cohesive sediments. We conclude that while experimental measurements provide a good starting point for constraining key parameters, reactive transport models with realistic kinetic and transport parameters provide critical insight into biogeochemical processes in permeable sediment that are difficult to experimentally evaluate.

Directly measured net denitrification rates in offshore New England sediments

Continental shelf sediments are important locations for denitrification in the marine environment. Here, we report a robust, directly measured set of sediment net denitrification rates from five sites off the coast of southern Rhode Island (USA). Using the N2/Ar technique, we sampled three of these sites at an almost bimonthly rate from October 2009 to January 2011, and two additional sites during July 2010. Net denitrification ranged from approximately 20 to 75 N2–N μmolm−2h−1 with a mean of 44±4 N2–N μmolm−2h−1. These values are similar to other studies that report direct-measurements of sediment net N2 fluxes located along the East coast of the United States. Net sediment denitrification rates were not significantly different between sites, nor did the rates change over the sampled temperature range (5–17°C). Net denitrification rates were weakly correlated, but exhibited significant relationships with sediment oxygen demand. Positive and negative fluxes of nitrate along the sediment–water interface suggest that both direct and coupled denitrification are occurring. Sediment molar C:N ratios were consistent temporally and spatially, indicating that sediment organic matter quality may be responsible for the similar rates of net denitrification at our sites. If we apply our rates to the 360,000km2 North East United States shelf area, we calculate that 0.14Tmol of N per year are removed, accounting for just over 5% of the total N entering the North Atlantic shelf region. Additionally, we predict a historical denitrification rate based on primary production estimates, which indicate that denitrification rates may have been three times higher in the past than our current measurements. Thus, as seen in a nearby coastal system, the nitrogen cycle in offshore New England regions could have changed during the last century.

Partitioning whole‐lake denitrification using in situ dinitrogen gas accumulation and intact sediment core experiments

We measured in situ hypolimnion and metalimnion dinitrogen (N2) accumulation rates and N2 production from sediments in contact with the reservoir epilimnia for 3 small (&lt; 1 km2) and shallow reservoirs with inherently low stratification stability. Hypolimnion areal denitrification rates ranged from 98.6 to 168 µmol N2‐N m−2 h−1, while metalimnion rates were up to two times greater. A laboratory experiment simulating the temporary breakdown and reestablishment of the oxycline confirmed that short‐term metalimnetic denitrification could be important in reactive nitrogen (N) loss in these reservoirs. Net denitrification occurred in sediments in contact with reservoir epilimnia during seasonal mixing, but net N2 fixation occurred in these sediments during stratification. The observed shift between net denitrification and net N2 fixation in epilimnetic sediments corresponded with an annual cycle of nitrate availability in the epilimnia. Annual whole‐reservoir denitrification rates ranged from 12 to 23 g N m−2 yr−1. The hypolimnion contribution to annual denitrification was 16‐25%. Sediments in contact with epilimnia were the greatest N2 source in all of the reservoirs. However, denitrification in the anoxic metalimnion was of similar magnitude to denitrification in epilimnetic sediments at one reservoir, and variability in metalimnetic denitrification rates was related to the strength of stratification stability. Integrating habitat‐specific measurements into whole‐lake denitrification rates showed that hydrologically dynamic habitats represent important denitrification hotspots, and that periodic disturbances fuel hot moments for denitrification.