Sediment Denitrification Rates Research Articles

Remediation of Coastal Sediments by Addition of Calcium Nitrate and Presence of Benthos in a Muddy Tidal Flat

This study investigated the change in denitrification rate in muddy tidal flats sediment by chemical and biological remediation. Column experiments were conducted to determine the dependence of denitrification rate on the redox potential. Filtered seawater was fed continuously to the pipe filled with sediment from the muddy tidal flats. An increase in the redox potential from -198 mV to 21 mV with bioturbation led to increases in the denitrification rate from 24.2 mg/m2/day to 29.8 mg/m2/day. The increase in denitrification was explained by the acceleration of mass transfer in the sediment. Redox-potential increased from -400 mV to -200 mV by the addition of 10 g calcium nitrate, acid volatile sulfide was below the detection limit to 20 cm depth, and denitrification rate was increased from 21.3 mg/m2/day to 36.4 mg/m2/day. It is considered that the increased nitrogen concentration led to an increase in microbial activity.

Assessing Nitrogen Transformations in a Flooded Agroecosystem Using the Isotope Pairing Technique and Nitrogen Functional Gene Abundances

Predicting N losses in flooded agricultural systems is complex because of the large number of potential N transformation pathways. Although coupled nitrification/denitrification at the flooded sediment near-surface oxic/anoxic interface is a recognized pathway of fertilizer N loss, the role of the newly discovered anaerobic ammonium oxidation (anammox) pathway in agricultural systems is poorly understood. A survey study was implemented to characterize sediment denitrification rates, anammox activity, and fluxes of ammonium and nitrate, and the vertical distribution of N functional genes were measured in Hawaiian taro (Colocasia esculenta) fields under three fertilizer management regimens (conventional, organic, and hybrid) as a model for flooded agroecosystems. Potential denitrification rates and anammox activity were measured using a slurry-based isotope pairing technique, and fluxes were determined by porewater modeling. Although significant numbers of anammox 16S rRNA genes were detected, only negligible anammox activity was found, illustrating the need to quantify not only the presence but also the activity of these bacteria. Quantitative polymerase chain reaction was performed for bacterial amoA, nirS, nosZ, and the 16S rRNA gene at 1-cm soil depth increments. Slurry isotope pairing technique-based potential denitrification rates (4.3-12.3 mmol N 2 m -2 day -1 ) were on the high end of previous anaerobic incubation studies. Flux-derived denitrification rates were up to three orders of magnitude lower than slurry isotope pairing technique rates, suggesting strong nitrification regulation. Organically managed fields exhibited significantly lower slurry and flux-based denitrification rates than conventional and hybrid systems. This study supports the use of alternative fertilizer management techniques in flooded agroecosystems by the mitigation of N losses through denitrification and decreased overall N flux rates.

Factors Controlling Sediment Denitrification Rates in Grassland and Forest Streams

Factors Controlling Sediment Denitrification Rates in Grassland and Forest Streams

Direct and Indirect Effects of Dissolved Organic Matter Source and Concentration on Denitrification in Northern Florida Rivers

Using a natural gradient of dissolved organic carbon (DOC) source and concentration in rivers of northern Florida, we investigated how terrestrially-derived DOC affects denitrification rates in river sediments. Specifically, we examined if the higher concentrations of DOC in blackwater rivers stimulate denitrification, or whether such terrestrially-derived DOC supports lower denitrification rates because (1) it is less labile than DOC from aquatic primary production; whether (2) terrestrial DOC directly inhibits denitrification via biochemical mechanisms; and/or whether (3) terrestrial DOC indirectly inhibits denitrification via reduced light availability to—and thus DOC exudation by—aquatic primary producers. We differentiated among these mechanisms using laboratory denitrification assays that subjected river sediments to factorial amendments of NO3− and dextrose, humic acid dosing, and cross-incubations of sediments and water from different river sources. DOC from terrestrial sources neither depressed nor stimulated denitrification rates, indicating low lability of this DOC but no direct inhibition; humic acid additions similarly did not affect denitrification rates. However, responses to addition of labile C increased with long-term average DOC concentration, which supports the hypothesis that terrestrial DOC indirectly inhibits denitrification via decreased autochthonous production. Observed and future changes in DOC concentration may therefore reduce the ability of inland waterways to remove reactive nitrogen.

Benthic denitrification, nitrification, and nitrate reduction rates in a range of grass carp polyculture enclosures

We measured denitrification,nitrification,and ammonification rates in the pond sediment of several systems for the polyculture of grass carp(Ctenopharyngodon idellus) using AIT(acetylene inhibition).The rates of denitrification,nitrification,and ammonification ranged from 0?734.15 μmol/(m2?d),0?1209.20 mmol/(m2?d),and 0?41.25 mmol/(m2?d),respectively.The denitrification rates were relatively high,and were similar to those measured in estuaries and pools.The nitrification rates were either very low or undetectable during the mid-period of culture.The ammonification rates increased each month in all systems,but were highest in the triple polyculture system.A higher ratio of shrimp(Litopenaeus vannamei) to grass carp was associated with an earlier increase in denitrification.However,the system was unable to maintain this level of nitrification.Conversely,a higher abun-dance of grass carp resulted in a delayed increase in denitrification,but a more rapid,and sustained increase dur-ing the culture period.In general,cultures with fewer species had lower rates of nitrification and cultures with a higher proportion of silver carp(Hypophthalmichthys molitrix) had higher rates of nitrification.The polyculture group(shrimp,grass carp,and silver carp) outperformed the monoculture triple group(shrimp only) and the binary groups(grass carp and shrimp or grass carp and silver carp).We tested two stocking densities of the polyculture group,but it is unclear which of these groups performed best.The objective of the present study is to access the effects of different poly culture systems of grass carp on the dynamic changes of N at the ponds sediment-water interface,and to provide a certain basis for the further optimize of polyculture models.

Assessment of denitrification process in lower Ishikari river system, Japan

Sediment denitrification rate and its role in removal of dissolved nitrate load in lower Ishikari river system were examined. Denitrification rate were measured using acetylene inhibition technique on the sediment samples collected during August 2009–July 2010. The denitrification rate varied from 0.001 to 1.9μgNg−1DMh−1 with an average value of 0.21μgNg−1DMh−1 in lower Ishikari river system. Denitrification rate showed positive correlation with dissolved nitrate concentration in the river basin, indicating overlying water column supplied nitrate for the sediment denitrification processes. Nutrient enrichment experiments result showed that denitrification rate increased significantly with addition of nitrate in case of samples collected from Barato Lake however no such increase was observed in the samples collected from Ishikari river main channel and its major tributaries indicating that factors other than substrate concentration such as population of denitrifier and hydrological properties of stream channel including channel depth and flow velocity may affects the denitrification rate in lower Ishikari river system. Denitrification rate showed no significant increase with the addition of labile carbon (glucose), indicating that sediment samples had sufficient organic matter to sustain denitrification activity. The result of nutrient spiraling model indicates that in- stream denitrification process removes on an average 5%d−1 of dissolve nitrate load in Ishikari river. This study was carried out to fill the gap present in the availability of riverine denitrification rate measurement and its role in nitrogen budget from Japanese rivers characterize by small river length and high flow rate.

Evidence and a conceptual model for the co‑occurrence of nitrogen fixation and denitrification in heterotrophic marine sediments

Marine waters are often nitrogen (N) limited. Denitrification, the microbial conver- sion of nitrate to dinitrogen (N2) gas, is responsible for significant N removal from the coastal ocean. In contrast, nitrogen fixation, the microbial transformation of N2 to ammonium, is typically regarded as an inconsequential N source. The imbalance between these 2 processes is responsi- ble, at least in part, for N limitation in the coastal ocean. Organic matter quality and quantity has been shown to determine rates of these critical N cycling processes. We hypothesized that the tim- ing of organic matter deposition to the benthos might also be important in determining which pro- cess dominates. We tested this hypothesis using a coupled biogeochemical-molecular approach. We report directly measured net sediment denitrification rates and corresponding expression of nirS, a gene in the denitrification pathway, with the simultaneous expression of nifH, a gene asso- ciated with nitrogen fixation. The timing of organic matter deposition determined the magnitude of the net sediment N2 fluxes. Highest rates of denitrification occurred soon after deposition, and the lowest rates occurred over 200 d after the last deposition event concomitant with increased nifH expression. Phylogenetic evidence suggests that sulfur and sulfate reducers are responsible for the nitrogen fixation. Globally, warming water temperatures, changes in light, and reduced nutrient loads through management intervention have been linked to decreases and/or altered phenology of water column productivity. Based on a conceptual model developed here, we sug- gest that in these systems, heterotrophic sediment nitrogen fixation may become an important component of the nitrogen budget.

Influence of Chironomid Larvae on oxygen and nitrogen fluxes across the sediment-water interface (Lake Taihu, China)

The microscale distribution of oxygen, the nitrogen flux and the denitrification rates in sediment inhabited by chironomid larvae (Tanypus chinensis) were measured in eutrophic Lake Taihu, China. The presence of the chironomids in the sediment increased the oxygen diffusional flux from 10.4 ± 1.4 to 12.7 ± 2.5 mmol O2/(m2·day). The burrows of the larvae represented “hot spots” and strongly influenced the nitrogen cycles and diagenetic activity in the sediment. The results indicate that the bioturbation effects of Tanypus chinensis chironomid larvae increased the capacity of the sediment as a sink for nitrate and a source for ammonium. Nitrate influx and ammonium outflux were increased 8.8 and 1.7 times, respectively. Under bioturbation, the amount of nitrate consumed was greater than the amount of ammonium released. The total denitrification rate was also enhanced from 0.76 ± 0.34 to 5.50 ± 1.30 mmol N/(m2·day). The net effect was that the bioturbated sediments acted as a net sink for inorganic nitrogen under direct and indirect bioturbation effects compared to the control.

Marine denitrification rates determined from a global 3-D inverse model

Abstract. A major impediment to understanding long-term changes in the marine nitrogen (N) cycle is the persistent uncertainty about the rates, distribution, and sensitivity of its largest fluxes in the modern ocean. We use a global ocean circulation model to obtain the first 3-D estimate of marine denitrification rates that is maximally consistent with available observations of nitrate deficits and the nitrogen isotopic ratio of oceanic nitrate. We find a global rate of marine denitrification in suboxic waters and sediments of 120–240 Tg N yr−1, which is lower than many other recent estimates. The difference stems from the ability to represent the 3-D spatial structure of suboxic zones, where denitrification rates of 50–77 Tg N yr−1 result in up to 50% depletion of nitrate. This depletion reduces the effect of local isotopic enrichment on the rest of the ocean, allowing the N isotope ratio of oceanic nitrate to be achieved with a sedimentary denitrification rate about 1.3–2.3 times that of suboxic zones. This balance of N losses between sediments and suboxic zones is shown to obey a simple relationship between isotope fractionation and the degree of nitrate consumption in the core of the suboxic zones. The global denitrification rates derived here suggest that the marine nitrogen budget is likely close to balanced.

Ammonification and denitrification rates in coastal Louisiana bayou sediment and marsh soil: Implications for Mississippi river diversion management

The Caernarvon Diversion directs Mississippi River water into coastal marshes in the Breton Sound Estuary. Elevated nitrogen levels in the Mississippi River water result in nutrient loading to these coastal marsh systems and consequently to the coastal ocean. The goal of this study is to determine the potential nitrate removal rates for two different substrates. Bayou sediments represent the low flow conditions, when the water is constrained within the canals with high potential for transport to the coastal ocean. The marsh soil represents the high flow diversion events when flood water inundates up into the marshes. We sought to remove the plant effect by using cores containing bayou sediment and marsh soil, removing all roots and flooding with a water column containing 2mg NO3-NL−1. Water column nitrate and ammonium concentration were monitored over 9d. Net nitrate loss in bayou sediments was 9.5±1.5mg Nm−2d−1 while the nitrate loss was significantly less at 7.2±0.9mg Nm−2d−1 in the marsh soil. A comparison of nitrate reduction rates in vegetated and non-vegetated marsh soils indicated that the rate of denitrification increased tenfold in vegetated soils. This increase could be attributed to the “plant effect”. Our results suggest that operating diversions on the high flow end of the spectrum would promote nitrate delivery over the vegetated marsh rather than flowing only through canals. Flooding of the vegetated marsh maximizes the potential for removal of riverine nitrate and limits delivery of nitrate to the coastal ocean, thereby mitigating expressions of eutrophication including algal blooms and hypoxia.

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

Benthic primary production and bacterial denitrification in a Mediterranean eutrophic coastal lagoon

Microphytobenthos and macroalgal mats are simultaneously present in eutrophic lagoons and are expected to have different direct and indirect effects on nitrogen related processes through uptake and inhibition or stimulation of microbial activity. To assess the relative contribution of different primary producer communities and heterotrophic processes to benthic nitrogen cycling, we studied nitrogen uptake and bacterial denitrification in the eutrophic Sacca di Goro lagoon (northern Italy). Benthic fluxes of oxygen and dissolved inorganic nitrogen (DIN), and rates of nitrification-coupled (Dn) and water–nitrate (Dw) denitrification were measured every 30–45days for one year at two shallow sites. Station Giralda is close to the main freshwater inlet, has turbid waters and muddy-organic and bioturbated sediments with microphytobenthos (MPB). Station Gorino is brackish with muddy-sand sediments which are covered by macroalgal mats of the genus Ulva. Here, sediment patches with and without macroalgae (MA) were simultaneously studied.Sediments with MPB were net heterotrophic and regenerated large amounts of ammonium to the water column. At Gorino, sediments with MA were net autotrophic through the year, and DIN fluxes were mainly controlled by macroalgal uptake. On an annual basis, denitrification rates were three fold higher at Giralda (2.27±0.06molNm−2yr−1) than at Gorino (0.83±0.01molNm−2yr−1), due to higher nitrate in the water column and nitrification in surface sediments. At Gorino, denitrification was one order of magnitude lower than DIN uptake by macroalgae (10.39±1.30molNm−2yr−1). Nevertheless, the differences between denitrification rates in sediments with and without MA were unexpectedly negligible, showing that the denitrification capacity was not suppressed by macroalgal competition. Results from this study suggest that in eutrophic lagoons nitrogen cycling seems less affected by MPB compared to more oligotrophic coastal waters and that most of the available DIN flows through benthic macroalgae. However, Ulva is only a temporary N-sink and most of its nitrogen pool can be either rapidly recycled or exported by tidal currents to the open sea.

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.

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.

Denitrification: an ecosystem service provided by salt marshes

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 448:79-92 (2012) - DOI: https://doi.org/10.3354/meps09526 Denitrification: an ecosystem service provided by salt marshes Ana I. Sousa1,2,*, Ana I. Lillebø2, Nils Risgaard-Petersen3, Miguel A. Pardal4, Isabel Caçador1 1Centre of Oceanography (CO), Faculty of Sciences, University of Lisbon, Campo Grande, 1749-016 Lisbon, Portugal 2Department of Biology & CESAM, University of Aveiro, Campus de Santiago, 3810-193 Aveiro, Portugal 3Institute of Biological Science, Center for Geomicrobiology, Aarhus University, Ny Munkegade, Building 1540, 8000 Aarhus C, Denmark 4Centre for Functional Ecology (CFE), Department of Life Sciences, University of Coimbra, Apartado 3046, 3001-401 Coimbra, Portugal *Email: aisousa@fc.ul.pt ABSTRACT: We hypothesized that denitrification rates, as an N removal process, would be enhanced in salt marsh rhizosediments as compared to sediments without vegetation (bare mudflats). Denitrification rates (measured by the 15N-isotope pairing technique), potential nitrification, and nutrient fluxes were seasonally quantified in a Spartina maritima salt marsh and in adjacent bare mudflats. Potential nitrification rates were significantly higher in autumn and winter, but there were no significant differences between the bare mudflats and S. maritima vegetated sediment. Seasonally, denitrification rates in vegetated sediments under dark conditions were significantly higher in winter (676 ± 497 µmol N2 m−2 h−1, mean ± SD), whereas bare mudflats showed a maximum rate of 151 ± 24 µmol N2 m−2 h−1 in summer. The high denitrification rates recorded in winter may be due to many abiotic and biotic factors, namely higher potential nitrification and nitrate availability in the water column, lower competition for nitrogen within the sediment, and less competition between plants, microphytobenthos, and nitrifiers, especially in dark conditions. Hence, during winter, there was a higher contribution of S. maritima marshes to N removal through denitrification, highlighting the role of the marshes in this ecosystem service. As a whole, considering the seasonal variations of the studied processes, it cannot be concluded whether or not annual denitrification was significantly different between the vegetated sediment and the bare mudflats. KEY WORDS: Denitrification · Salt marshes · Ecosystem services · Eutrophication · Spartina maritima · 15N-isotope pairing technique Full text in pdf format PreviousNextCite this article as: Sousa AI, Lillebø AI, Risgaard-Petersen N, Pardal MA, Caçador I (2012) Denitrification: an ecosystem service provided by salt marshes. Mar Ecol Prog Ser 448:79-92. https://doi.org/10.3354/meps09526 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 448. Online publication date: February 23, 2012 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2012 Inter-Research.

Hypoxia and nitrogen processing in the Baltic Sea water column

Steep redoxclines form between oxic surface water and the stagnant, sulfidic hypolimnion in the eutrophied, brackish water Baltic Sea. Nitrification, denitrification, and anammox were measured at and below the redoxcline to quantify the role of water‐column nitrogen processes in the overall magnitude of nitrogen removal in the Baltic Sea. Rates of nitrification were very high (up to 85 nmol N L−1 d−1) at the oxic‐anoxic interface, but, surprisingly, nitrification was separated from the processes reducing nitrate to N2 (up to 810 nmol N L−1 d−1) by tens of meters in depth. N2 production was dominated by chemolithotrophic denitrification, with anammox playing only a negligible role, and limited to the water layers in which nitrite or nitrate coexisted with sulfide. The alternating oxygen concentrations in the basin induce irregular bursts of nitrogen removal. Nitrification takes place after mixing of ammonium‐rich deep water with oxic water, and denitrification uses the formed nitrite and nitrate once anoxic conditions re‐establish. Although removal rates can be high, conditions allowing such rates are likely short‐lived. While the sedimentary denitrification rates at the shallower, oxic areas are lower, they are more constant in time, highlighting the need to avoid hypoxia, which would prevent sedimentary denitrification.

Predictability of bacterial activity and denitrification in aquatic sediments with continuous measurements of redox potential

Redox potential has been adopted as a qualitative parameter for interpreting solubility changes of nutrients and contaminants and the biological activity within wetland systems for several decades. The majority of studies considering the redox geochemistry in sediments used measurements of bulked material and single point measurement of biogeochemical parameters for interpretation, yet it remains questionable whether this information is reliable for environments that are very dynamic, such as wetlands. In this study it is evaluated whether variations in redox potential reflect dynamics of denitrification and overall bacterial respiration using continuous measurements of redox potential in time-series experiments in laboratory microcosms, in which the biogeochemical variation was enhanced by bioturbation. The results presented here suggest that measurements of redox potential have predictive potential in approximating rates of denitrification and overall bacterial respiration in aquatic sediments. The data clearly suggest that, while sediment bulk measurements and measurements of single profiles of redox potential, denitrification and bacterial activity often fail to provide ecological relevant information in dynamic systems, measurements of spatial and temporal redox potential profiles provide a useful parameter that reflects biogeochemical processes and functioning of sediments.

Impact of copper on the diversity, abundance and transcription of nitrite and nitrous oxide reductase genes in an urban European estuary

In this study, the effects of increasing copper (Cu) concentrations on the denitrification pathway and on the diversity of a denitrifier community and gene expression were evaluated in intertidal sandy sediments of the Douro River estuary (north-west Portugal). The results demonstrated that sediment denitrification rates were highly affected by Cu; almost complete inhibition (85%) of the process was observed in sediments amended with 60 μg Cu g(-1) wet wt sediment. Moreover, the addition of Cu stimulated the accumulation of both N(2)O and NO(2)(-) and inhibited the rate of NO(3)(-) uptake. Further, the amendment with even the lowest Cu concentration (4 μg Cu g(-1) wet wt sediment) yielded a drastic decrease in the abundance of nirK, nirS and nosZ (between 79% and 81%) assessed by means of real-time PCR. In agreement, reverse transcription-PCR-denaturing gradient gel electrophoresis analysis of nirK, nirS and nosZ transcripts showed a progressive decrease in the diversity of the transcription products of these genes with an increase of the Cu concentration.