Direct Denitrification Research Articles

Dissimilatory nitrate reduction to ammonium (DNRA) is marginal relative to denitrification in emerging-eroding wetlands in a subtropical oligohaline and eutrophic coastal delta

Nitrate (NO3−) and ammonium (NH4+) are reactive nitrogen (Nr) forms that can exacerbate eutrophication in coastal regions. NO3− can be lost to the atmosphere as N2 gas driven by direct denitrification, coupled nitrification-denitrification and annamox or retained within the ecosystems through conversion of NO3− to NH4+ via dissimilatory nitrate reduction to ammonium (DNRA). Denitrification and DNRA are competitive pathways and hence it is critical to evaluate their functional biogeochemical role. However, there is limited information about the environmental factors driving DNRA in oligohaline habitats, especially within deltaic regions where steep salinity gradients define wetland spatiotemporal distribution. Here we use the Isotope Pairing Technique to evaluate the effect of temperature (10, 20, 30 °C) and in situ soil/sediment organic matter (OM%) on total denitrification (Dtotal = direct + coupled nitrification) and DNRA rates in oligohaline forested/marsh wetlands soils and benthic sediment habitats at two sites representing prograding (Wax Lake Delta, WLD) and eroding (Barataria- Lake Cataouatche, BLC) deltaic stages in the Mississippi River Delta Plain (MRDP). Both sites receive MR water with high NO3− (>40 μM) concentrations during the year via river diversions. Denitrification rates were significantly higher (range: 18.0 ± 0.4–113.0 ± 10.6 μmol m−2 h−1) than DNRA rates (range: 0.7 ± 0.2–9.2 ± 0.3 μmol m−2 h−1). Therefore, DNRA represented on average < 10% of the total NO3− reduction (DNRA + Dtotal). Unlike denitrification, DNRA showed no consistent response to temperature. These results indicate that DNRA in wetland soils and benthic sediment is not a major nitrogen transformation in oligohaline regions across the MRDP regardless of wide range of OM% content in these eroding and prograding delta lobes.

Emerging Wetlands From River Diversions Can Sustain High Denitrification Rates in a Coastal Delta

AbstractIt is assumed that to treat excess NO3− high soil organic matter content (%OM) is required to maintain high denitrification rates in natural or restored wetlands. However, this excess also represents a risk by increasing soil decomposition rates triggering peat collapse and wetland fragmentation. Here, we evaluated the role of %OM and temperature interactions controlling denitrification rates in eroding (Barataria Bay‐BLC) and emerging (Wax Lake Delta‐WLD) deltaic regions in coastal Louisiana using the isotope pairing (IPT) and N2:Ar techniques. We also assessed differences between total (direct denitrification + coupled nitrification‐denitrification) and net (total denitrification minus nitrogen fixation) denitrification rates in benthic and wetland habitats with contrasting %OM and bulk density (BD). Sediment (benthic) and soil (wetland) cores were collected during summer, spring, and winter (2015–2016) and incubated at close to in‐situ temperatures (30°C, 20°C, and 10°C, respectively). Denitrification rates were linearly correlated with temperature; maximum mean rates ranged from 40.1–124.1 μmol m−2 h−1 in the summer with lower rates (&lt;26.2 ± 5.3 μmol m−2 h−1) in the winter seasons. Direct denitrification was higher than coupled denitrification in all seasons. Denitrification rates were higher in WLD despite lower %OM, lower total N concentration, and higher BD in wetland soils. Therefore, in environments with low carbon availability, high denitrification rates can be sustained as long as NO3− concentrations are high (&gt;30 μM) and water temperature is &gt;10°C. In coastal Louisiana, substrates under these regimes are represented by emergent supra‐tidal flats or land created by sediment diversions under oligohaline conditions (&lt;1 ppt).

Nitrogen Dynamics of Inundated Sediments in an Emerging Coastal Deltaic Floodplain in Mississippi River Delta Using Isotope Pairing Technique to Test Response to Nitrate Enrichment and Sediment Organic Matter

Nitrate (NO3–) enrichment in rivers over the last four decades has potentially shifted how coastal ecosystems process nitrogen (N). Shifts in N dynamics may be particularly significant in coastal deltaic floodplains when sediments are inundated during river flood stage as this may change the fate of NO3– transported to the coastal ocean. We evaluated the relative importance of denitrification, dissimilative nitrate reduction to ammonium (DNRA), and anaerobic ammonium oxidation (anammox) in intact sediment cores of Wax Lake Delta (WLD) using continuous flow-through system. We manipulated this experimental system with two concentrations of riverine NO3– (lower concentration at 5 μM and ambient concentration at 100 μM) to test how nitrate-enriched waters may modify N cycling in coastal deltaic floodplains. Inundated sediments in a coastal deltaic floodplain removed bio-reactive N as a function of sediment organic matter (SOM) concentrations, indicating how N dynamics vary as a result of deltaic succession. Direct denitrification was the dominant N pathway in inundated sediments in WLD, which was several times greater than coupled nitrification-denitrification, DNRA, and anammox. Gross denitrification (direct denitrification + coupled nitrification-denitrification) rates generally increased with SOM concentrations and ranged from 4 to 12 μmol N m–2 h–1 under lower NO3– enrichment (5 μM) and 68 to 276 μmol N m–2 h–1 under higher NO3– enrichment (100 μM). Most of these changes in N fluxes with SOM and NO3– enrichment resulted from the increase in direct denitrification rates rather than coupled nitrification-denitrification rates. DNRA rates varied from 0 to 65 μmol N m–2 h–1, and the relative significance of DNRA increased as a percentage of total NO3– reduction (%DNRA) with increasing ratio of organic carbon to NO3– concentrations (OC/NO3–). Anammox rates increased among sites as SOM increased, but rates were lower from 0.0 to 38 μmol N m–2 h–1. The dominance of direct denitrification and increased anammox rates with SOM concentrations indicate that dynamic deltaic floodplains can permanently remove bio-reactive N in the proximal zone of river deposition of tidal freshwater wetlands.

A pilot-scale test on the treatment of biological pretreated leachate by the synergy of ozonation-biological treatment-catalytic ozonation

In this pilot-scale test, the ozonation-biological treatment-catalytic ozonation system was performed to treat complex organics and highly-concentrated total nitrogen (TN) in biological pretreated incineration leachate. The test results showed that the ratio of five-day biochemical oxygen demand (BOD5) / chemical oxygen demand (COD) increased from 0.059 to 0.237, which indicated that the concentration of biodegradable COD (CODbio) increased by ozonation pre-treatment process. In addition, the TN removal mainly occurred in anaerobic zone due to direct denitrification by the activated bacteria, which were domesticated through different influent ratio. Moreover, it was necessary to add catalytic ozonation process to reach higher direct effluent discharge criteria. After 60 days repeated debugging, the removal rate of COD and TN reached 88.5% and 98.2%, respectively. Finally, the total cost of this system was ￥ 6.65 /m&lt;sup&gt;3&lt;/sup&gt; ($ 0.95 /m&lt;sup&gt;3&lt;/sup&gt;), which was acceptable for the treatment of biological pretreated leachate. This pilot-scale test could provide some guiding information for the treatment of leachate containing highly-concentrated TN with low CODbio/N by the composite system.

Development of a Combined Aerobic–Anoxic and Methane Oxidation Bioreactor System Using Mixed Methanotrophs and Biogas for Wastewater Denitrification

We developed a lab-scale aerobic–methane oxidation bioreactor (MOB)–anoxic system, combining a MOB and the aerobic–anoxic denitrification process, and evaluated its potential for advanced nitrogen treatment in wastewater treatment plants (WWTPs). The MOB used biogas generated from a WWTP and secondary-treated wastewater to support mixed methanotroph cultures, which mediated the simultaneous direct denitrification by methanotrophs and methanol production necessary for denitrifying bacteria in the anoxic chamber for denitrification. Compared to the aerobic–anoxic process, the aerobic–MOB–anoxic system with an influent concentration of 4.8 L·day−1 showed a marked increase in the reduction efficiency for total nitrogen (41.9% vs. 85.9%) and PO4−3-P (41.1% vs. 69.5%). However, the integrated actions of high nitrogen and phosphorus consumption are required for methanotroph growth, as well as the production and supply of methanol as a carbon source for denitrification and methane monooxygenase-mediated oxidation of NH3 into N2O by methanotrophs. After three months of continuous operation using actual wastewater, the total nitrogen removal rate was 76.3%, equivalent to the rate observed in a tertiary-advanced WWTP, while the total phosphorus removal rate reached 83.7%.

Evaluation of predominant factor for shortcut biological nitrogen removal in sequencing batch reactor at ambient temperature.

The shortcut biological nitrogen removal (SBNR) process requires less aeration and external carbon due to the oxidization of ammonia into nitrite and its direct denitrification to nitrogen gas during the biological nitrogen removal process. However, this process produces a poor effluent containing NH4+, since the system has to maintain a high free ammonia (FA, NH3) concentration. To overcome this drawback, in this study, the solid retention time (SRT) and the dissolved oxygen (DO) concentration were controlled to achieve both a high ammonia removal rate and nitrite accumulation in the sequencing batch reactor (SBR) process, which can remove nitrogen from wastewater to the desired concentration and provide high free ammonia inhibition and continuous shock loading. When sufficient DO was supplied, nitrite did not accumulate with a 20-day SRT, but the wash-out of nitrite oxidizers in a shorter SRT resulted in a high nitrite accumulation. When DO acted as a limitation, nitrite accumulated at all SRTs. This indicates that nitrite accumulation is more highly influenced by SRT and DO concentration than by FA inhibition. Also, as nitrite accumulated over a 10-day SRT regardless of DO concentration, the accumulation was more highly influenced by SRT than by DO concentration.

Relative Contributions of DNRA and Denitrification to Nitrate Reduction in Thalassia testudinum Seagrass Beds in Coastal Florida (USA)

Seagrass beds are vulnerable to eutrophication (nutrient loading) and declining worldwide. To quantify the fate of nitrogen (N) inputs, intact sediment cores were incubated in a continuous-flow system with 15N enrichments to compare N consumption and efflux pathways within and near seagrass (Thalassia testudinum) beds in St. Joseph Bay, Florida. Sediment oxygen demand and total ammonium (NH4+) efflux were greater (p < 0.001) in vegetated versus unvegetated sediments, suggesting that seagrasses enhance organic matter remineralization. Denitrification rates were 2–20× greater than estimates of potential dissimilatory nitrate reduction to ammonium (DNRA). The effect of vegetation on denitrification rates was inconsistent. Direct denitrification of overlying water nitrate and N fixation rates were low, suggesting tight coupling between remineralization, nitrification, and denitrification. DNRA rates were lower than denitrification and consistently greater in vegetated sediments. DNRA rate measurements are conservative if sediment cation exchange decreases the fraction of 15NH4+ reaching overlying water, especially in vegetated sediments, where rates of nitrate-induced ammonium fluxes were observed. Coupled nitrification-denitrification was the major N loss pathway in this system, as evidenced by the lack of 15N-labeled N2 production in isotope-enriched cores. Using measured sediment oxygen demand and NH4+ fluxes as an indicator of organic matter quality and quantity, these results are consistent with previous work showing that labile organic matter helps regulate the balance between N removal and internal recycling pathways in seagrass systems, which has implications for coastal management strategies to address eutrophication.

Isotopic Evidence that Nitrogen Enrichment Intensifies Nitrogen Losses to the Atmosphere from Subtropical Mangroves

Nitrogen (N) enrichment can have large effects on mangroves’ capacity to provide critical ecosystem services by affecting fundamental functions such as N cycling and primary productivity. However, our understanding of excess N input effects on N cycling in mangroves remains quite limited. To advance our understanding of how N enrichment via water or air pollution affects mangroves, we evaluated whether increasing N inputs would decrease biological N fixation (BNF), but intensify N dynamics and N losses to the atmosphere in these systems. We measured N concentrations in sediment and vegetation, rates of BNF in sediment and litter, and net sediment ammonification and nitrification rates. We also evaluated long-term integrated N dynamics and N losses to the atmosphere using the natural abundance of N stable isotopes (δ15N) in the sediment–plant system and in estuarine water. We performed these analyses at non-N-enriched and N-enriched (that is, polluted) fringe and basin mangroves in southeastern Brazil. The δ15N in the sediment–plant system was higher at N-enriched than non-N-enriched fringe sites, indicating increased N losses to the atmosphere from N-enriched sites. However, N concentrations in sediment and vegetation were similar or lower at N-enriched relative to non-N-enriched sites. BNF and net ammonification and nitrification rates were also similar between N-enriched and non-N-enriched sites. Excess N inputs intensified N losses to the atmosphere from mangroves, but N pools, BNF, and net ammonification and nitrification rates were not affected by N enrichment, likely because excess N was quickly lost from the system by direct denitrification and volatilization.

Contributions of freshwater mussels (Unionidae) to nutrient cycling in an urban river: filtration, recycling, storage, and removal

Consumers contribute to nutrient cycling in aquatic ecosystems by nutrient retention in tissues, metabolic transformations and excretion, and promoting microbial processes that remove nutrients (i.e., nitrogen (N) loss via denitrification). Freshwater mussels (Unionidae) form dense assemblages in rivers, and affect nutrient transformations through feeding, biodeposition, and bioturbation. However, the effects of Unionid mussels on N and phosphorus (P) retention are not commonly measured. We quantified rates of filtration, retention, and biodeposition of carbon (C), N, and P for two Unionid species: Lasmigona complanata and Pyganodon grandis. We used continuous-flow cores with 15N tracers to measure denitrification in sediments alone and with a single individual of each species. We conducted measurements in an urban river near Chicago, IL, USA that is a target for Unionid restoration. Both Unionid species showed high rates of P-specific feeding and retention, but low N-specific rates. This was in agreement with high N:P ratio in the water column. Each species significantly increased denitrification relative to sediment alone. 15N tracers suggested direct denitrification of nitrate increased denitrification, although enhanced coupled nitrification–denitrification likely also contributed to denitrification. Finally, denitrification rates with and without mussels were used to estimate the value of N loss under different scenarios for mussel restoration at the river scale. Overall, restored Unionid populations may enhance P retention in soft tissues and shells and N loss via denitrification. Ecosystem managers may find greater support for restoration of Unionid populations with careful calculations of their ecosystem role in nutrient retention and removal.

Ambient groundwater flow diminishes nitrate processing in the hyporheic zone of streams

AbstractModeling and experimental studies demonstrate that ambient groundwater reduces hyporheic exchange, but the implications of this observation for stream N‐cycling is not yet clear. Here we utilize a simple process‐based model (the Pumping and Streamline Segregation or PASS model) to evaluate N‐cycling over two scales of hyporheic exchange (fluvial ripples and riffle‐pool sequences), ten ambient groundwater and stream flow scenarios (five gaining and losing conditions and two stream discharges), and three biogeochemical settings (identified based on a principal component analysis of previously published measurements in streams throughout the United States). Model‐data comparisons indicate that our model provides realistic estimates for direct denitrification of stream nitrate, but overpredicts nitrification and coupled nitrification‐denitrification. Riffle‐pool sequences are responsible for most of the N‐processing, despite the fact that fluvial ripples generate 3–11 times more hyporheic exchange flux. Across all scenarios, hyporheic exchange flux and the Damköhler Number emerge as primary controls on stream N‐cycling; the former regulates trafficking of nutrients and oxygen across the sediment‐water interface, while the latter quantifies the relative rates of organic carbon mineralization and advective transport in streambed sediments. Vertical groundwater flux modulates both of these master variables in ways that tend to diminish stream N‐cycling. Thus, anthropogenic perturbations of ambient groundwater flows (e.g., by urbanization, agricultural activities, groundwater mining, and/or climate change) may compromise some of the key ecosystem services provided by streams.

Spatial variation in rates of benthic denitrification and environmental controls in four New Zealand estuaries

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 556:59-77 (2016) - DOI: https://doi.org/10.3354/meps11865 Spatial variation in rates of benthic denitrification and environmental controls in four New Zealand estuaries Catherine Gongol1, Candida Savage1,2,* 1Department of Marine Science, University of Otago, PO Box 56, Dunedin, New Zealand 2Department of Biological Sciences, University of Cape Town, Rondebosch, South Africa *Corresponding author: candida.savage@otago.ac.nz ABSTRACT: Denitrification plays a key role in global biogeochemical cycles as the main nitrogen (N) loss process that limits availability of N to primary producers. This study presents the first measurements of sediment denitrification rates for New Zealand estuaries. Spatially explicit rates of direct denitrification (Dw) and coupled nitrification-denitrification (Dn) were quantified using the isotope pairing method in 4 oligotrophic to moderately nutrient-enriched estuaries on the South Island, New Zealand (Avon-Heathcote, Waikouaiti, Tokomairiro, Tautuku). Coupled nitrification-denitrification dominated (>86%), with most sites having low dissolved inorganic nitrogen (DIN) concentrations (<30 µmol l-1). Rates of total denitrification (Dtot) and Dn were negatively correlated with percent sand and positively related to oxygen penetration depth (OPD). Surface sediment chlorophyll a concentrations and percent carbon (%C) were also identified as significant predictors of Dtot and Dn in the single-factor models. By contrast, Dw was the main nitrogen removal process at moderately nutrient-enriched sites and was positively related to DIN concentrations in the water column and negatively correlated with percent sand. OPD and %C were also identified as significant predictors of Dw. While Dw was positively related to DIN concentrations, overall rates of denitrification were not, demonstrating that predictive relationships between N loading and removal are not always possible in oligotrophic estuaries. Our findings demonstrate the importance of grain size as a driver of denitrification rates in estuaries and imply that an increase in fine-grained sediments from land-use intensification may alter rates of N removal in estuaries through complex direct and indirect processes. KEY WORDS: Denitrification · Estuaries · New Zealand · Nitrogen · Benthic · Sediment grain size · Isotope pairing method · Oxygen flux Full text in pdf format PreviousNextCite this article as: Gongol C, Savage C, (2016) Spatial variation in rates of benthic denitrification and environmental controls in four New Zealand estuaries. Mar Ecol Prog Ser 556:59-77. https://doi.org/10.3354/meps11865 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 556. Online publication date: September 08, 2016 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2016 Inter-Research.

Denitrification in a subtropical, semi-arid North American savanna: field measurements and intact soil core incubations

Information on denitrification (particularly N2) losses from dry ecosystems is limited despite their large area. Here, we present the first direct denitrification measurements for a northern hemisphere savanna, a Prosopis-dominated grassland/grove matrix in south Texas. We used the gas-flow intact soil core method to quantify N2, N2O and CO2 losses and compared these with field measurements of N2O, NOy, NH3 and CO2. Under field-realistic soil moisture and O2 conditions (average 17.5–20 % O2, minimum 15 %) incubated soils produced no measurable N2 flux (detection limit 52.2 µg N m−2 h−1). Only in a subset of grove soils were fluxes of 70–75 µg N m−2 h−1 recorded after 102 h of incubation at 5–10 % O2 following wetting of very dry soils. Making the assumption that potential N2 production falls just below the detection limit (likely an overestimate given the conditions needed to generate measurable fluxes), N2 flux rates would fall on the low end of that recorded for a tropical Australian savanna (45–110 µg N m−2 h−1) under comparable abiotic conditions. Assuming maximum possible production rates, N2 could comprise <32–76 % of total soil N gas flux following soil wetting in summer. Lack of flux response to soil wetting in winter suggests that cold-season N2 fluxes are negligible. N2O fluxes for core incubations were significantly higher than for field chambers; thus it is likely that incubations may overestimate N2O flux by reducing soil column consumption. Overall, results indicate that soil N2 fluxes are less dominant in this savanna than in other ecosystems investigated.

Bedforms as Biocatalytic Filters: A Pumping and Streamline Segregation Model for Nitrate Removal in Permeable Sediments.

Bedforms are a focal point of carbon and nitrogen cycling in streams and coastal marine ecosystems. In this paper, we develop and test a mechanistic model, the "pumping and streamline segregation" or PASS model, for nitrate removal in bedforms. The PASS model dramatically reduces computational overhead associated with modeling nitrogen transformations in bedforms and reproduces (within a factor of 2 or better) previously published measurements and models of biogeochemical reaction rates, benthic fluxes, and in-sediment nutrient and oxygen concentrations. Application of the PASS model to a diverse set of marine and freshwater environments indicates that (1) physical controls on nitrate removal in a bedform include the pore water flushing rate, residence time distribution, and relative rates of respiration and transport (as represented by the Damkohler number); (2) the biogeochemical pathway for nitrate removal is an environment-specific combination of direct denitrification of stream nitrate and coupled nitrification-denitrification of stream and/or sediment ammonium; and (3) permeable sediments are almost always a net source of dissolved inorganic nitrogen. The PASS model also provides a mechanistic explanation for previously published empirical correlations showing denitrification velocity (N2 flux divided by nitrate concentration) declines as a power law of nitrate concentration in a stream (Mulholland et al. Nature, 2008, 452, 202-205).

Eastern oyster (Crassostrea virginica) filtration, biodeposition, and sediment nitrogen cycling at two oyster reefs with contrasting water quality in Great Bay Estuary (New Hampshire, USA)

Benthic deposition of carbon (C) and nitrogen (N)-rich oyster biodeposits may increase denitrification, or anaerobic respiration of nitrate (NO3 −) to di-nitrogen gas (N2). However, environmental drivers of C and N dynamics in oyster biodeposits and reef-adjacent sediments require clarification. In July 2012, we collected intact sediment cores adjacent to and 15–20 m away from two oyster reefs (Crassostrea virginica) in Great Bay, New Hampshire, USA: one reference site and one site with cultural eutrophication. We also measured seston, chlorophyll a, and in situ oyster feeding and biodeposition. Cores were incubated in continuous-flow chambers where inflow water received 15N-ammonium (NH4 +), 15NO3 −, or no isotopes (control). We quantified fluxes of dissolved nutrients and gasses (oxygen, 28N2, 29N2, 30N2, and argon) after 24 h. Finally, we measured size-fractionated sediment organic matter. At the eutrophic site, abundant phytoplankton in the 5–28 µm size range was correlated with enhanced oyster feeding rates and biodeposit quality (lower C:N). This site had greater denitrification rates in reef-adjacent cores relative to distal cores. Low production of 29,30N2 in 15NH4 + amended cores suggested water column or biodeposit NH4 + were unlikely to be converted to N2. At both sites, reef-adjacent cores had more shell and higher 29,30N2 production with 15NO3 − addition relative to distal cores, suggesting direct denitrification enhancement near reefs. Oysters likely increased sediment N2 production via high quality biodeposits (eutrophic site), and NO3 − diffusion via structural complexity of reef-adjacent sediment (both sites). Overall, results suggest oyster-mediated ecosystems services may be expected to vary with environmental conditions.

A network model shows the importance of coupled processes in the microbial N cycle in the Cape Fear River Estuary

Estuaries serve important ecological and economic functions including habitat provision and the removal of nutrients. Eutrophication can overwhelm the nutrient removal capacity of estuaries and poses a widely recognized threat to the health and function of these ecosystems. Denitrification and anaerobic ammonium oxidation (anammox) are microbial processes responsible for the removal of fixed nitrogen and diminish the effects of eutrophication. Both of these microbial removal processes can be influenced by direct inputs of dissolved inorganic nitrogen substrates or supported by microbial interactions with other nitrogen transforming pathways such as nitrification and dissimilatory nitrate reduction to ammonium (DNRA). The coupling of nitrogen removal pathways to other transformation pathways facilitates the removal of some forms of inorganic nitrogen; however, differentiating between direct and coupled nitrogen removal is difficult. Network modeling provides a tool to examine interactions among microbial nitrogen cycling processes and to determine the within-system history of nitrogen involved in denitrification and anammox. To examine the coupling of nitrogen cycling processes, we built a nitrogen budget mass balance network model in two adjacent 1 cm3 sections of bottom water and sediment in the oligohaline portion of the Cape Fear River Estuary, NC, USA. Pathway, flow, and environ ecological network analyses were conducted to characterize the organization of nitrogen flow in the estuary and to estimate the coupling of nitrification to denitrification and of nitrification and DNRA to anammox. Centrality analysis indicated NH4+ is the most important form of nitrogen involved in removal processes. The model analysis further suggested that direct denitrification and coupled nitrification-denitrification had similar contributions to nitrogen removal while direct anammox was dominant to coupled forms of anammox. Finally, results also indicated that partial nitrification-anammox may play an important role in anammox nitrogen removal in the Cape Fear River Estuary.

The fate of assimilated nitrogen in streams: an in situ benthic chamber study

Summary1. Nitrogen (N) processing in streams has been investigated using whole‐stream 15N addition experiments that, in general, have found that a large proportion of added nitrate removed from the water column appears to be assimilated by the stream benthos. The long‐term fate of this retained N is unknown, and of particular interest is the possibility that it becomes denitrified through coupled mineralisation–nitrification–denitrification processes (indirect denitrification).2. We used in situ chambers to produce highly 15N‐enriched benthic biofilms and removed the chambers to allow biofilms to interact with ambient stream conditions. Nitrogen assimilation and direct denitrification were estimated from the first chamber deployment. Chambers were periodically reinstalled over 4 weeks to measure tracer 15N in ammonium (), nitrate () and dinitrogen (N2), from which we estimated subsequent rates of biotic N transformations, including N mineralisation (ammonification), nitrification and indirect denitrification. We also estimated rates of depuration of 15N tracer from benthic biomass compartments.3. Nitrate uptake was roughly equivalent in the sand and cobble habitats that dominated the stream. Direct denitrification (denitrification of from the water column) was an order of magnitude higher in cobble habitats than in sand habitats, accounting for c. 26 and 2% of total nitrate uptake in cobble and sand, respectively.4. Mean residence times of actively cycling organic N in stream benthos (algae and microbes) were 16 days in cobble habitats and 9 days in sand habitats. The difference between habitat types was driven by the influence of N residence time in epilithic biofilms (18 days) on cobbles.5. Release of enriched 15 was the primary flux of remineralised N, while release of enriched 15 was an order of magnitude less. We detected slight 15N enrichment in dissolved nitrogen gas (N2) in post‐enrichment sampling, indicating that indirect denitrification was taking place. However, indirect denitrification accounted for &lt;0.1% of the assimilated N.6. These experiments agree with results of whole‐stream 15N additions, in that most added N was assimilated rather than directly denitrified. Assimilation was primarily a short‐term N retention mechanism in this stream, and indirect denitrification of assimilated N accounted for only a minor proportion of the observed 15N loss over time.7. Remaining possible fates include export of N as particulate organic matter, which may lead to additional storage of assimilated N in downstream habitats, and consumption by grazers.

Shortcut biological nitrogen removal in hybrid biofilm/suspended growth reactors

A shortcut biological nitrogen removal (SBNR) process converts ammonium directly through nitrite to nitrogen gas, thus requiring less aeration and carbon. We evaluated a hybrid SBNR (HSBNR) reactor containing an anoxic tank followed by an aerobic tank and a settling tank. The aerobic tank was filled with polyvinyl alcohol sponge media (20%, v/v) to attach and retain ammonium oxidizers. Two configurations of the HSBNR reactor were tested for treating a wastewater with high strength ammonium and organic electron donor. The HSNBR reactors accumulated nitrite stably for 1.5 years and maintained a high free ammonia (FA) concentration (20–25 mg/L) and a low dissolved oxygen (DO) concentration (<1 mg/L) in the aerobic tank. Apparently, the biofilm carriers increased the solids retention time (SRT) for ammonium oxidizers, while high FA and low DO selected against nitrite oxidizers and promoted direct denitrification of nitrite in the aerobic tank. The significant amount of chemical oxygen demand (COD) was removed by shortcut denitrification of nitrite in the anoxic tank.

Processing watershed‐derived nitrogen in a well‐flushed New England estuary

Isotopically labeled nitrate (15NO3−) was added continuously to the Rowley estuary, Massachusetts, for 22 d to assess the transport, uptake, and cycling of terrestrially derived nitrogen during a period of high river discharge and low phytoplankton activity. Isotopic enrichment of the 3.5‐km tidal prism (150,000 m3) was achieved for the 3 weeks and allowed us to construct a nitrogen mass balance model for the upper estuary. Mean δ15NO3− in the estuary ranged from 300‰ to 600‰, and approximately 75%–80% of the 15N was exported conservatively as 15NO3− to the coastal ocean. Essentially all of the 20%–25% of the 15N processed in the estuary occurred in the benthos and was evenly split between direct denitrification and autotrophic assimilation. The lack of water‐column 15N uptake was attributed to low phytoplankton stocks and short water residence times (1.2–1.4 d). Uptake of watercolumn NO3− by benthic autotrophs (enriched in excess of 100‰) was a function of NO3− concentration and satisfied up to 15% and 25% of the total nitrogen demand for benthic microalgae and macroalgae, respectively. Approximately 10% of tracer assimilated by benthic autotrophs was mineralized and released back to the water column as 15NH4+. By the end of the study, 15N storage in sediments and marsh macrophytes accounted for 50%–70% of the 15N assimilated in the estuary. These compartments may sequester watershed‐derived nitrogen in the estuary for time scales of months to years.

Sediment din fluxes and preferential recycling of benthic microalgal nitrogen in a shallow macrotidal estuary

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 257:25-36 (2003) - doi:10.3354/meps257025 Sediment din fluxes and preferential recycling of benthic microalgal nitrogen in a shallow macrotidal estuary Craig Tobias1,2,*, Anne Giblin1, James McClelland1, Jane Tucker1, Bruce Peterson1 1The Ecosystems Center, Marine Biological Laboratory, Woods Hole, Massachusetts 02543, USA 2Present address: US Geological Survey, 431 National Center, 12201 Sunrise Valley Drive, Reston, Virginia 20192, USA *Email: crtobias@usgs.gov ABSTRACT: Sediment-water fluxes of NH4 +, NO3 -, dissolved inorganic carbon, and O2 were measured in cores collected from the upper Rowley River estuary, Massachusetts, an organic nitrogen (N) mineralization, nitrification, and coupled and direct denitrification (DNF). The cores contained 15N label in benthic microalgae (BMA) and in NO3- in the overlying water as a result of an ongoing whole-estuary 15NO3- enrichment study (NISOTREX II). The tracer allowed for estimation of gross NO3- regeneration in sediments and the contribution of BMA derived N to total mineralization. The mean mineralization rate between sites was 16.0 ± 2.0 mmol N m-2 d-1. Approximately 13 to 56% of the mineralized N was nitrified at rates ranging from 1.8 to 10.1 mmol N m-2 d-1. Total denitrification was dominated by direct DNF (3.6 mmol N m-2 d-1) furthest upstream, where NO3- concentrations were highest. Coupled DNF was most important (8.0 mmol N m-2 d-1) in the sediments with high nitrification and low water column NO3-. A gross NO3- flux from sediments to water of 0.9 to 2.1 mmol N m-2 d-1 was estimated from the isotope dilution of δ15NO 3- in the overlying water of the cores. The isotope dilution seen in the cores was also detected as a deviation from conservative δ15NO3- mixing along estuarine transects. Incorporation of this NO3- regeneration into the DNF calculations effectively increased the estimate of direct DNF by up to 50% and decreased the coupled DNF estimate by up to 220%. Increasing δ15NH4+ in the water of the cores indicated that the 15N labelled BMA were preferentially mineralized over bulk sediment organic N. Additional 15N enrichments in the sediment bacterial biomarker diaminopimelic acid showed a link among 15N labeled BMA, active bacteria, and 15NH4+released to the overlying water. Based on δ15NH4+ enrichments in the cores, BMA accounted for approximately 50 to 100% of the N mineralized. An isotopic enrichment of δ15NH4+ above background in the estuary was observed at a magnitude consistent with the core-based rates of BMA mineralization. These results provide further evidence that BMA are not unidirectional sinks for water column-dissolved organic nitrogen, but instead act to turn over N between sediments and estuarine water on the scale of days. KEY WORDS: Nitrogen · Benthic microalgae · Microphytobenthos · Stable isotopes · Biomarkers · Nutrient flux · Denitrification Full text in pdf format PreviousNextExport citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 257. Online publication date: August 07, 2003 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2003 Inter-Research.

Denitrification and neutralization treatment by direct feeding of an acidic wastewater containing copper ion and high-strength nitrate to a bio-electrochemical reactor process

The feasibility of the direct denitrification treatment of copper metal pickling wastewater by using a bio-electrochemical reactor process was investigated experimentally. Carbon electrodes were installed in the reactor as the anode and cathode and denitrifying microorganisms were fixed on the surface of the cathode. The reactor was continuously operated by applying an electric current and feeding acetate. In this reactor, copper ion removal and denitrification proceeded simultaneously and the pH value of the treated water was increased almost to neutral. The electric current that passed through the cathode contributed to the removal of the copper ion and the generation of hydrogen gas. The generated hydrogen gas as well as the added acetate was effectively utilized for denitrification. A theoretical evaluation of pH in the effluent suggested that the pH increase was mainly caused by the generation of hydroxyl ion during denitrification. In addition, the inorganic carbon species generated during denitrification with acetate and by the electrochemical oxidation of anodic carbon acted as a buffer to minimize a further increase of pH at higher nitrate removal efficiencies. These results demonstrated that copper ion removal, denitrification and neutralization could be achieved simultaneously by using a single bioelectrochemical reactor.