Nitrogen Cycling In Sediments Research Articles

Sediment nitrification and denitrification in a Lake Superior estuary

Inorganic nitrogen (N) transformations and removal in aquatic sediments are microbially mediated, and rates influence N-transport. In this study we related physicochemical properties of a large Great Lakes embayment, the St. Louis River Estuary (SLRE) of western Lake Superior, to sediment N-transformation rates. We tested for associations among rates and N-inputs, vegetation biomass, and temperature. We measured rates of nitrification (NIT), unamended base denitrification (DeNIT), and potential denitrification [denitrifying enzyme activity (DEA)] in 2011 and 2012 across spatial and depth zones. In vegetated habitats, NIT and DeNIT rates were highest in deep (ca. 2m) water (249 and 2111mgNm−2d−1, respectively) and in the upper and lower reaches of the SLRE (>126 and 274mgNm−2d−1, respectively). Rates of DEA were similar among zones. In 2012, NIT, DeNIT, and DEA rates were highest in July, May, and June, respectively. System-wide, we observed highest NIT (223 and 287mgNm−2d−1) and DeNIT (77 and 64mgNm−2d−1) rates in the harbor and from deep water, respectively. Amendment with NO3− enhanced DeNIT rates more than carbon amendment; however, DeNIT and NIT rates were inversely related, suggesting the two processes are decoupled in sediments. Average proportion of N2O released during DEA (23–54%) was greater than from DeNIT (0–41%). Nitrogen cycling rates were spatially and temporally variable, but we modeled how alterations to water depth and N-inputs may impact DeNIT rates. A large flood occurred in 2012 which temporarily altered water chemistry and sediment nitrogen cycling.

Elevated nitrate enriches microbial functional genes for potential bioremediation of complexly contaminated sediments

Nitrate is an important nutrient and electron acceptor for microorganisms, having a key role in nitrogen (N) cycling and electron transfer in anoxic sediments. High-nitrate inputs into sediments could have a significant effect on N cycling and its associated microbial processes. However, few studies have been focused on the effect of nitrate addition on the functional diversity, composition, structure and dynamics of sediment microbial communities in contaminated aquatic ecosystems with persistent organic pollutants (POPs). Here we analyzed sediment microbial communities from a field-scale in situ bioremediation site, a creek in Pearl River Delta containing a variety of contaminants including polybrominated diphenyl ethers (PBDEs) and polycyclic aromatic hydrocarbons (PAHs), before and after nitrate injection using a comprehensive functional gene array (GeoChip 4.0). Our results showed that the sediment microbial community functional composition and structure were markedly altered, and that functional genes involved in N-, carbon (C)-, sulfur (S)-and phosphorus (P)- cycling processes were highly enriched after nitrate injection, especially those microorganisms with diverse metabolic capabilities, leading to potential in situ bioremediation of the contaminated sediment, such as PBDE and PAH reduction/degradation. This study provides new insights into our understanding of sediment microbial community responses to nitrate addition, suggesting that indigenous microorganisms could be successfully stimulated for in situ bioremediation of POPs in contaminated sediments with nitrate addition.

Seasonality and depth distribution of the abundance and activity of ammonia oxidizing microorganisms in marine coastal sediments (North Sea).

Microbial processes such as nitrification and anaerobic ammonium oxidation (anammox) are important for nitrogen cycling in marine sediments. Seasonal variations of archaeal and bacterial ammonia oxidizers (AOA and AOB) and anammox bacteria, as well as the environmental factors affecting these groups, are not well studied. We have examined the seasonal and depth distribution of the abundance and potential activity of these microbial groups in coastal marine sediments of the southern North Sea. This was achieved by quantifying specific intact polar lipids as well as the abundance and gene expression of their 16S rRNA gene, the ammonia monooxygenase subunit A (amoA) gene of AOA and AOB, and the hydrazine synthase (hzsA) gene of anammox bacteria. AOA, AOB, and anammox bacteria were detected and transcriptionally active down to 12 cm sediment depth. In all seasons, the abundance of AOA was higher compared to the AOB abundance suggesting that AOA play a more dominant role in aerobic ammonia oxidation in these sediments. Anammox bacteria were abundant and active even in oxygenated and bioturbated parts of the sediment. The abundance of AOA and AOB was relatively stable with depth and over the seasonal cycle, while anammox bacteria abundance and transcriptional activity were highest in August. North Sea sediments thus seem to provide a common, stable, ecological niche for AOA, AOB, and anammox bacteria.

Response of an Arctic Sediment Nitrogen Cycling Community to Increased CO2

Ocean acidification influences sediment/water nitrogen fluxes, possibly by impacting on the microbial process of ammonia oxidation. To investigate this further, undisturbed sediment cores collected from Ny Alesund harbour (Svalbard) were incubated with seawater adjusted to CO2 concentrations of 380, 540, 760, 1,120 and 3,000 µatm. DNA and RNA were extracted from the sediment surface after 14 days' exposure and the abundance of bacterial and archaeal ammonia oxidising (amoA) genes and transcripts quantified using quantitative polymerase chain reaction. While there was no change to the abundance of bacterial amoA genes, an increase to 760 µatm pCO2 reduced the abundance of bacterial amoA transcripts by 65 %, and this was accompanied by a shift in the composition of the active community. In contrast, archaeal amoA gene and transcript abundance both doubled at 3,000 µatm, with an increase in species richness also apparent. This suggests that ammonia oxidising bacteria and archaea in marine sediments have different pH optima, and the impact of elevated CO2 on N cycling may be dependent on the relative abundances of these two major microbial groups. Further evidence of a shift in the balance of key N cycling groups was also evident: the abundance of nirS-type denitrifier transcripts decreased alongside bacterial amoA transcripts, indicating that NO3− produced by bacterial nitrification fuelled denitrification. An increase in the abundance of Planctomycete-specific 16S rRNA, the vast majority of which grouped with known anammox bacteria, was also apparent at 3,000 µatm pCO2. This could indicate a possible shift from coupled nitrification–denitrification to anammox activity at elevated CO2.

Balance of assimilative and dissimilative nitrogen processes in a diatom-rich tidal flat sediment

Abstract. Tidal flat sediments are subject to repetitive mixing and resuspension events. In a short-term (24 h) 15N-labelling experiment, we investigated reactive nitrogen cycling in a tidal flat sediment following an experimentally induced resuspension event. We focused on (a) the relative importance of assimilatory versus dissimilatory processes and (b) the role of benthic microalgae therein. 15N-labelled substrate was added to homogenized sediment, and 15N was subsequently traced into sediment and dissolved inorganic nitrogen (DIN) pools. Integration of results in a N-cycle model allowed us to quantify the proportion of major assimilatory and dissimilatory processes in the sediment. Upon sediment disturbance, rates of dissimilatory processes like nitrification and denitrification were very high, but declined rapidly towards a steady state. Once this was reached, the balance between assimilation and dissimilation in this tidal mudflat was mainly dependent on the nitrogen source: nitrate was utilized almost exclusively dissimilatory via denitrification, whereas ammonium was rapidly assimilated, with about a quarter of this assimilation due to benthic microalgae (BMA). Benthic microalgae significantly affected the nitrogen recycling balance in sediments, because in the absence of BMA activity the recovering sediment turned from a net ammonium sink to a net source. The driving mechanisms for assimilation or dissimilation accordingly appear to be ruled to a large extent by external physical forcing, with the entire system being capable of rapid shifts following environmental changes. Assimilatory pathways gain importance under stable conditions, with a substantial contribution of BMA to total assimilation.

The potential effect of sustained hypoxia on nitrogen cycling in sediment from the southern North Sea: a mesocosm experiment

The potential effect of sustained hypoxia (up to 70 days) on the production of N2 gas through denitrification and anammox, as well as sediment–water exchange of nitrite, nitrate and ammonia, oxygen consumption and penetration, were measured in mesocosms using sediment collected from the southern North Sea (north of Dogger Bank). As expected, both the penetration of oxygen into, and consumption of oxygen by, the sediment decreased by 42 and 46 %, respectively, once hypoxia was established. Importantly, the oxygen regime did not change significantly (P > 0.05) during the experiment, suggesting that organic carbon was not depleted. During the first 10 days, the exchange of NO3−, NO2− and NH4+ between the sediment and water was erratic but once a steady state was established the sediment acted as either a sink for fixed nitrogen under hypoxia or as a source in the controls. Over the course of the mesocosm experiment the rate of both anammox and denitrification increased, with anammox increasing disproportionately under hypoxia relative to the controls, whereas the rate of increase in denitrification was the same for both. Under sustained hypoxia the production of N2 gas increased by 72 % relative to the controls, with this increase in N2 production remaining constant regardless of the duration of hypoxia. Longer periods of stratification and oxygen depletion are predicted to occur more regularly in the bottom waters of shallow coastal seas as one manifestation of climate change. Under sustained hypoxia the potential for nitrogen removal by the production of N2 gas in this region of the southern North Sea was estimated to increase from 2.1 kt N 150 days−1 to 3.6 kt 150 days−1, while the efflux of dissolved inorganic nitrogen ceased altogether; both of which could down regulate the productivity of this region as a whole.

Measuring nitrification in sediments – comparison of two techniques and three 15NO measurement methods

Nitrification is a crucial process in sediment nitrogen cycling. We compared two 15N tracer-based nitrification measurement techniques (isotope pairing technique (IPT) combined with 15N nitrate pool dilution and 15N ammonium oxidation) and three different 15N analyses from bottom water nitrate (ammonia diffusion, denitrifier and SPINMAS) in a sediment mesocosm. The 15N nitrate pool dilution technique combined with IPT can be used to quantify the in situ nitrification, but the minimum detection limit for the total nitrification is higher than that in the 15N ammonium oxidation technique. The 15N ammonium oxidation technique, however, is not applicable for sediments that have high ammonium content. If nitrate concentration and the amount of 15N label in the sample are low, the 15N nitrate analysis should be done with the denitrifier method. In higher 15N concentrations, the less sensitive SPINMAS method can also be applied. The ammonia diffusion method is not suitable for bottom water 15N nitrate analyses.

The Effects of Hypoxia on Sediment Nitrogen Cycling in the Baltic Sea

Primary production in the eutrophic Baltic Sea is limited by nitrogen availability; hence denitrification (natural transformation of nitrate to gaseous N(2)) in the sediments is crucial in mitigating the effects of eutrophication. This study shows that dissimilatory nitrate reduction to ammonium (DNRA) process, where nitrogen is not removed but instead recycled in the system, dominates nitrate reduction in low oxygen conditions (O(2) <110μM), which have been persistent in the central Gulf of Finland during the past decade. The nitrogen removal rates measured in this study show that nitrogen removal has decreased in the Gulf of Finland compared to rates measured in mid-1990s and the decrease is most likely caused by the increased bottom water hypoxia.

Effects of detrital non-native and native macroalgae on the nitrogen and carbon cycling in intertidal sediments

The decay of non-native and native seaweed mixing may modify sediment biogeochemistry and organic matter transfers within benthic food webs according to their composition and biomass. The non-native species Sargassum muticum was deliberately added to the sediment of an intertidal sandflat at different biomass and mixed to the native species Ulva sp. and Fucus vesiculosus. The sediment porewater was then 13C and 15N enriched to test whether both detrital diversity and biomass influenced the transfer of porewater carbon and nitrogen to the sediment and to the macrofauna consumers. More 15N-nitrogen was mobilized to sediments and macrofauna when the 3-species detrital mixing was buried, probably because this mixing provided species-specific compounds such as polyphenols due to the presence of S. muticum and F. vesiculosus, as well as large amounts of nitrogen due to the presence of Ulva. Our study revealed the importance of detrital diversity and non-native seaweeds for the nitrogen cycling in the benthic food web.

Cultivation of Autotrophic Ammonia-Oxidizing Archaea from Marine Sediments in Coculture with Sulfur-Oxidizing Bacteria

The role of ammonia-oxidizing archaea (AOA) in nitrogen cycling in marine sediments remains poorly characterized. In this study, we enriched and characterized AOA from marine sediments. Group I.1a crenarchaea closely related to those identified in marine sediments and "Candidatus Nitrosopumilus maritimus" (99.1 and 94.9% 16S rRNA and amoA gene sequence identities to the latter, respectively) were substantially enriched by coculture with sulfur-oxidizing bacteria (SOB). The selective enrichment of AOA over ammonia-oxidizing bacteria (AOB) is likely due to the reduced oxygen levels caused by the rapid initial growth of SOB. After biweekly transfers for ca. 20 months, archaeal cells became the dominant prokaryotes (>80%), based on quantitative PCR and fluorescence in situ hybridization analysis. The increase of archaeal 16S rRNA gene copy numbers was coincident with the amount of ammonia oxidized, and expression of the archaeal amoA gene was observed during ammonia oxidation. Bacterial amoA genes were not detected in the enrichment culture. The affinities of these AOA to oxygen and ammonia were substantially higher than those of AOB. [(13)C]bicarbonate incorporation and the presence and activation of genes of the 3-hydroxypropionate/4-hydroxybutyrate cycle indicated autotrophy during ammonia oxidation. In the enrichment culture, ammonium was oxidized to nitrite by the AOA and subsequently to nitrate by Nitrospina-like bacteria. Our experiments suggest that AOA may be important nitrifiers in low-oxygen environments, such as oxygen-minimum zones and marine sediments.

Impact of Point-Source Pollution on Phosphorus and Nitrogen Cycling in Stream-Bed Sediments

Diffusive equilibration in thin films was used to study the cycling of phosphorus and nitrogen at the sediment-water interface in situ and with minimal disturbance to redox conditions. Soluble reactive phosphate (SRP), nitrate, nitrite, ammonium, sulfate, iron, and manganese profiles were measured in a rural stream, 12 m upstream, adjacent to, and 8 m downstream of a septic tank discharge. Sewage fungus adjacent to the discharge resulted in anoxic conditions directly above the sediment. SRP and ammonium increased with depth through the fungus layer to environmentally significant concentrations (440 and 1800 microM, respectively) due to release at the sediment surface. This compared to only 0.8 microM of SRP and 2.0 microM of ammonium in the water column upstream of the discharge. Concomitant removal of ammonium, nitrite and nitrate within 0.5 cm below the fungus-water interface provided evidence for anaerobic ammonium oxidation (anammox). "Hotspots" of porewater SRP (up to 350 microM) at the downstream site demonstrated potential in-stream storage of the elevated P concentrations from the effluent. These results provide direct in situ evidence of phosphorus and nitrogen release from river-bed sediments under anoxic conditions created by sewage-fungus, and highlight the wider importance of redox conditions and rural point sources on in-stream nutrient cycling.

Sensitivity analysis of nitrogen and carbon cycling in marine sediments

Biogeochemical cycles in coastal sediments encompass numerous interconnected processes and are sensitive to a high number of external forces. Usually a small subset of these factors is considered when developing state-of-the-art models of marine nutrient cycling. This study therefore aims to assess the degree of complexity required in the model to represent the dependency of major biogeochemical fluxes on both intrinsic as well as external factors. For this, a sensitivity analysis (SA) of the generic Integrated Sediment Model ( ISM) was performed comparing two different model setups: 1) a back barrier tidal flat in the German Wadden Sea and; 2) a deep sea site in the Argentine Basin. Both setups were first calibrated to fit pore water profiles of SO 4 2+, NH 4 + and CH 4. We then employed a new type of SA that evaluates parameter impact rather than targeting variable change. General structural stability of the model is demonstrated by similar sensitivity patterns of both setups regarding carbon and nitrogen cycling. Mean temperature, organic carbon bio-availability, bacterial adaptation and sediment texture emerge as the most influential parameters of ubiquitous importance. It appears that in coastal settings, transport and sediment mixing and the composition of suspended particles in the bottom water are especially important. The nitrogen cycle displays a high responsiveness to internal feedback mechanisms as well as interdependencies to carbon and metal cycling, which is statistically reflected by sensitivities to 79% of all parameters. In contrast, the carbon cycle appears to be mainly controlled by organic matter decay. The SA also pointed to unexpected responses of the sediment system, which are analyzed by further scenario calculations. These, for example, reveal a nonlinear response of nitrification, denitrification and benthic fluxes of NH 4 and NO 3 to changing bioturbation and bioirrigation due to the interactions of different metabolic pathways.

Influence of advective bio-irrigation on carbon and nitrogen cycling in sandy sediments

In sandy sediments, the burrow ventilation activity of benthic macrofauna can generate substantial advective flows within the sediment surrounding their burrows. Here we investigated the effects of such advective bio-irrigation on carbon and nitrogen cycling in sandy sediments. To this end, we combined a range of complementary experimental and modelling approaches in a microcosm study of the lugworm Arenicola marina (Polychaeta: Annelida). Bio-irrigation rates were determined using uranine as a tracer, while benthic fluxes of oxygen (O-2), total carbon dioxide (TCO2), dissolved inorganic nitrogen (NH4+, Sigma NO2-+NO3-) and dinitrogen (N-2) were measured in closed-core incubations containing lugworms acclimatized for a relatively short (2 d) and long (3 wk) duration. The fluxes induced by A. marina were compared to those induced by mechanical mimics that simulate the flow pattern induced by the lugworm. These mechanical mimics proved a useful tool to simulate the effect of l! ugworm irrigation on sediment biogeochemistry. Subsequently, reactive transport model simulations were performed to check the consistency of the measured fluxes and rates, and to construct closed mass balances for sedimentary nitrogen. This reactive transport model Successfully captured the essential features of the nitrogen cycling within the sediment. Advective irrigation by both lugworm and mechanical mimics significantly stimulated the sediments O-2 consumption, organic matter mineralization rate (TCO2 release), and denitrification rate (N-2 production). While sedimentary O-2 consumption was directly correlated to advective input of O-2, increasing irrigation rates increased the importance of coupled nitrification-denitrification over the external input of nitrate from the overlying water.

Insights into the Genome of Large Sulfur Bacteria Revealed by Analysis of Single Filaments

Marine sediments are frequently covered by mats of the filamentous Beggiatoa and other large nitrate-storing bacteria that oxidize hydrogen sulfide using either oxygen or nitrate, which they store in intracellular vacuoles. Despite their conspicuous metabolic properties and their biogeochemical importance, little is known about their genetic repertoire because of the lack of pure cultures. Here, we present a unique approach to access the genome of single filaments of Beggiatoa by combining whole genome amplification, pyrosequencing, and optical genome mapping. Sequence assemblies were incomplete and yielded average contig sizes of approximately 1 kb. Pathways for sulfur oxidation, nitrate and oxygen respiration, and CO2 fixation confirm the chemolithoautotrophic physiology of Beggiatoa. In addition, Beggiatoa potentially utilize inorganic sulfur compounds and dimethyl sulfoxide as electron acceptors. We propose a mechanism of vacuolar nitrate accumulation that is linked to proton translocation by vacuolar-type ATPases. Comparative genomics indicates substantial horizontal gene transfer of storage, metabolic, and gliding capabilities between Beggiatoa and cyanobacteria. These capabilities enable Beggiatoa to overcome non-overlapping availabilities of electron donors and acceptors while gliding between oxic and sulfidic zones. The first look into the genome of these filamentous sulfur-oxidizing bacteria substantially deepens the understanding of their evolution and their contribution to sulfur and nitrogen cycling in marine sediments.

Seasonal and short-term variation in denitrification and anammox at a coastal station on the Gulf of Finland, Baltic Sea

Benthic processes were measured at a coastal deposition area in the northern Baltic Sea, covering all seasons. The N2 production rates, 90–400 μmol N m−2 d−1, were highest in autumn-early winter and lowest in spring. Heterotrophic bacterial production peaked unexpectedly late in the year, indicating that in addition to the temperature, the availability of carbon compounds suitable for the heterotrophic bacteria also plays a major role in regulating the denitrification rate. Anaerobic ammonium oxidation (anammox) was measured in spring and autumn and contributed 10% and 15%, respectively, to the total N2 production. The low percentage did, however, result in a significant error in the total N2 production rate estimate, calculated using the isotope pairing technique. Anammox must be taken into account in the Gulf of Finland in future sediment nitrogen cycling research.

The effects of semi-lunar spring and neap tidal change on nitrification, denitrification and N 2O vertical distribution in the intertidal sediments of the Yangtze estuary, China

To explore the influences of semi-lunar spring and neap tidal changes on nitrogen cycling in intertidal sediments, a comparative study among waterlogged, desiccated and reflooded systems was carried out in August 2005 and February 2006 by analyzing nitrification, denitrification and N 2O depth profiles in the intertidal flats of the Yangtze estuary. Laboratory experiments showed that alternating emersion and inundation resulted in the significant changes in nitrification and denitrification rates in the intertidal sediment systems. Due to the desiccation-related effects, lowest nitrification and denitrification rates were observed in the desiccated sediment cores. Highest nitrification and denitrification rates were however detected in the waterlogged and reflooded systems, respectively. It is hypothesized that the highest nitrification rates in the waterlogged sediments were mainly attributed to higher nitrifier numbers and NH 4 + being more available, whereas the availability of NO 3 − might dominate denitrification in the reflooded sediments. In addition, the highest N 2O concentrations were detected in the reflooded sediment cores, and the lowest found in the dried sediment cores. It was also shown that N 2O in the intertidal sediments was mainly from nitrification under the desiccated condition. In contrast, N 2O in the intertidal sediments was produced mainly via denitrification under the waterlogged and reflooded conditions. It is therefore concluded that the semi-lunar tidal cycle has a significant influence on nitrification, denitrification and N 2O production in the intertidal sediment systems.

Microbial Community Diversity Associated with Carbon and Nitrogen Cycling in Permeable Shelf Sediments

Though a large fraction of primary production and organic matter cycling in the oceans occurs on continental shelves dominated by sandy deposits, the microbial communities associated with permeable shelf sediments remain poorly characterized. Therefore, in this study, we provide the first detailed characterization of microbial diversity in marine sands of the South Atlantic Bight through parallel analyses of small-subunit (SSU) rRNA gene (Bacteria), nosZ (denitrifying bacteria), and amoA (ammonia-oxidizing bacteria) sequences. Communities were analyzed by parallel DNA extractions and clone library construction from both sediment core material and manipulated sediment within column experiments designed for geochemical rate determinations. Rapid organic-matter degradation and coupled nitrification-denitrification were observed in column experiments at flow rates resembling in situ conditions over a range of oxygen concentrations. Numerous SSU rRNA phylotypes were affiliated with the phyla Proteobacteria (classes Alpha-, Delta-, and Gammaproteobacteria), Planctomycetes, Cyanobacteria, Chloroflexi, and Bacteroidetes. Detectable sequence diversity of nosZ and SSU rRNA genes increased in stratified redox-stabilized columns compared to in situ sediments, with the Alphaproteobacteria comprising the most frequently detected group. Alternatively, nitrifier communities showed a relatively low and stable diversity that did not covary with the other gene targets. Our results elucidate predominant phylotypes that are likely to catalyze carbon and nitrogen cycling in marine sands. Although overall diversity increased in response to redox stabilization and stratification in column experiments, the major phylotypes remained the same in all of our libraries, indicating that the columns sufficiently mimic in situ conditions.

Decomposition of Mytilus edulis: The effect on sediment nitrogen and carbon cycling

The benthic degradation of mussel tissue ( Mytilus edulis) was studied in a continuous flow-through system over a 32 day incubation period. Sediment chambers without mussels served as controls. The inflowing artificial seawater and the outflow water were analyzed for dissolved organic nitrogen (DON), short chain fatty acids (SCFA), dissolved inorganic nitrogen (DIN), ∑CO 2 and O 2 during the course of incubation. Sediment profiles of particulate organic carbon (POC), particulate organic nitrogen (PON), total hydrolyzable amino acids (THAA), pore water concentrations of DON and DIN and turnover rate of dissolved free amino acids (DFAA) were measured at four different times during the 32 day experiment. Immediately after the addition of mussel tissue, the chambers became completely anoxic and there was an increase in carbon oxidation and the efflux of DON, SCFA and NH 4 + from the sediment + mussel layer to the overlaying water. During the first 9 days there was a net buildup of DON, and NH 4 + in the sediment followed by a net consumption of the respective N-species during the remainder of the experiment. During the course of incubation 41% of the organic content of the added mussel tissue was released from the sediment as DON, whereas most of the other mussel-N effluxed the sediment as NH 4 +. Only 8% of the added mussel-N remained by the end of the experiment. There were indications of stimulated bacterial growth in both the mussel amended and the unamended sediments. This was measured as a net increase in THAA, which could only be explained by net bacterial growth and/or protein synthesis. During mussel decomposition both the estimated bacterial carbon incorporation efficiency and the C:N ratio of the substrates used by the bacteria were low. This resulted in a low bacterial nitrogen demand. As a consequence, almost all of the nitrogen mineralized within the sediment was released to the water column as NH 4 +.

Ammonium oxidation coupled to dissimilatory reduction of iron under anaerobic conditions in wetland soils

In exploring the dynamics of iron and nitrogen cycling in sediments from riparian forests we have observed a redox reaction that has not been previously described. During incubations of soil slurries under strictly anaerobic conditions, we repeatedly measured an unexpected production of both nitrite ( NO 2 − ) and ferrous iron [Fe(II)]. Using this indirect evidence we hypothesize that, under anaerobic conditions, there is a biological process that uses ferric iron [Fe(III)] as an electron acceptor while oxidizing ammonium ( NH 4 + ) to NO 2 − for energy production. This NH 4 + oxidation under iron reducing anaerobic conditions is thermodynamically feasible and is potentially a critical component of the N cycle in saturated sediments.

Nitrogen cycling in deep-sea sediments of the Porcupine Abyssal Plain, NE Atlantic

Rates of transformation, recycling and burial of nitrogen and their temporal and spatial variability were investigated in deep-sea sediments of the Porcupine Abyssal Plain (PAP), NE Atlantic during eight cruises from 1996 to 2000. Benthic fluxes of ammonium (NH 4) and nitrate (NO 3) were measured in situ using a benthic lander. Fluxes of dissolved organic nitrogen (DON) and denitrification rates were calculated from pore water profiles of DON and NO 3, respectively. Burial of nitrogen was calculated from down core profiles of nitrogen in the solid phase together with 14C-based sediment accumulation rates and dry bulk density. Average NH 4 and NO 3-effluxes were 7.4 ± 19 μmol m −2 d −1 ( n = 7) and 52 ± 30 μmol m −2 d −1 ( n = 14), respectively, during the period 1996–2000. During the same period, the DON-flux was 11 ± 5.6 μmol m −2 d −1 ( n = 5) and the denitrification rate was 5.1 ± 3.0 μmol m −2 d −1 ( n = 22). Temporal and spatial variations were only found in the benthic NO 3 fluxes. The average burial rate was 4.6 ± 0.9 μmol m −2 d −1. On average over the sampling period, the recycling efficiency of the PON input to the sediment was ∼94% and the burial efficiency hence ∼6%. The DON flux constituted ∼14% of the nitrogen recycled, and it was of similar magnitude as the sum of burial and denitrification. By assuming the PAP is representative of all deep-sea areas, rates of denitrification, burial and DON efflux were extrapolated to the total area of the deep-sea floor (>2000 m) and integrated values of denitrification and burial of 8 ± 5 and 7 ± 1 Tg N year −1, respectively, were obtained. This value of total deep-sea sediment denitrification corresponds to 3–12% of the global ocean benthic denitrification. Burial in deep-sea sediments makes up at least 25% of the global ocean nitrogen burial. The integrated DON flux from the deep-sea floor is comparable in magnitude to a reported global riverine input of DON suggesting that deep-sea sediments constitute an important source of DON to the world ocean.