Rates, controls and potential adverse effects of nitrate removal in a denitrification bed

Denitrification beds are a simple approach for removing nitrate (NO₃⁻) from a range of point sources prior to discharge into receiving waters. These beds are large containers filled with woodchips that act as an energy source for microorganisms to convert NO₃⁻ to nitrogen (N) gases (N₂O, N₂) through denitrification. This study investigated the biological mechanism of NO₃⁻ removal, its controlling factors and its adverse effects in a large denitrification bed (176 m × 5 m × 1.5 m) receiving effluent with a high NO₃⁻ concentration (>100 g N m⁻³) from a hydroponic glasshouse (Karaka, Auckland, New Zealand). Samples of woodchips and water were collected from 12 sites along the bed every two months for one year, along with measurements of gas fluxes from the bed surface. Denitrifying enzyme activity (DEA), factors limiting denitrification (availability of carbon, dissolved organic carbon (DOC), dissolved oxygen (DO), temperature, pH, and concentrations of NO₃⁻, nitrite (NO₂⁻) and sulfide (S²⁻)), greenhouse gas (GHG) production - as nitrous oxide (N₂O), methane (CH₄), carbon dioxide (CO₂) - and carbon (C) loss were determined. NO₃⁻-N concentration declined along the bed with total NO₃⁻-N removal rates of 10.1 kg N d⁻¹ for the whole bed or 7.6 g N m⁻³ d⁻¹. NO₃⁻-N removal rates increased with temperature (Q₁₀ = 2.0). In laboratory incubations, denitrification was always limited by C availability rather than by NO₃⁻. DO levels were above 0.5 mg L⁻¹ at the inlet but did not limit NO₃⁻-N removal. pH increased steadily from about 6 to 7 along the length of the bed. Dissolved inorganic carbon (C-CO₂) increased in average about 27.8 mg L⁻¹, whereas DOC decreased slightly by about 0.2 mg L⁻¹ along the length of the bed. The bed surface emitted on average 78.58 μg m⁻² min⁻¹ N₂O-N (reflecting 1% of the removed NO₃⁻-N), 0.238 μg m⁻² min⁻¹ CH₄ and 12.6 mg m⁻² min⁻¹ CO₂. Dissolved N₂O-N increased along the length of the bed and the bed released on average 362 g dissolved N₂O-N per day coupled with N₂O emission at the surface about 4.3% of the removed NO₃⁻-N as N₂O. Mechanisms to reduce the production of this GHG need to be investigated if denitrification beds are commonly used. Dissolved CH₄ concentrations showed no trends along the length of the bed, ranging from 5.28 μg L⁻¹ to 34.24 μg L⁻¹. Sulfate (SO₄²⁻) concentrations declined along the length of the bed on three of six samplings; however, declines in SO₄²⁻ did not appear to be due to SO₄²⁻ reduction because S²⁻ concentrations were generally undetectable. Ammonium (NH₄⁺) (range: <0.0007 mg L⁻¹ to 2.12 mg L⁻¹) and NO₂⁻ concentrations (range: 0.0018 mg L⁻¹ to 0.95 mg L⁻¹) were always very low suggesting that anammox was an unlikely mechanism for NO₃⁻ removal in the bed. C longevity was calculated from surface emission rates of CO₂ and release of dissolved carbon (DC) and suggested that there would be ample C available to support denitrification for up to 39 years.This study showed that denitrification beds can be an efficient tool for reducing high NO₃⁻ concentrations in effluents but did produce some GHGs. Over the course of a year NO₃⁻ removal rates were always limited by C and temperature and not by NO₃⁻ or DO concentration.     

Simulated Nitrogen Deposition Reduces CH4 Uptake and Increases N2O Emission from a Subtropical Plantation Forest Soil in Southern China

To date, few studies are conducted to quantify the effects of reduced ammonium (NH₄ ⁺) and oxidized nitrate (NO₃ ⁻) on soil CH₄ uptake and N₂O emission in the subtropical forests. In this study, NH₄Cl and NaNO₃ fertilizers were applied at three rates: 0, 40 and 120 kg N ha⁻¹ yr⁻¹. Soil CH₄ and N₂O fluxes were determined twice a week using the static chamber technique and gas chromatography. Soil temperature and moisture were simultaneously measured. Soil dissolved N concentration in 0–20 cm depth was measured weekly to examine the regulation to soil CH₄ and N₂O fluxes. Our results showed that one year of N addition did not affect soil temperature, soil moisture, soil total dissolved N (TDN) and NH₄ ⁺-N concentrations, but high levels of applied NH₄Cl and NaNO₃ fertilizers significantly increased soil NO₃ ⁻-N concentration by 124% and 157%, respectively. Nitrogen addition tended to inhibit soil CH₄ uptake, but significantly promoted soil N₂O emission by 403% to 762%. Furthermore, NH₄ ⁺-N fertilizer application had a stronger inhibition to soil CH₄ uptake and a stronger promotion to soil N₂O emission than NO₃ ⁻-N application. Also, both soil CH₄ and N₂O fluxes were driven by soil temperature and moisture, but soil inorganic N availability was a key integrator of soil CH₄ uptake and N₂O emission. These results suggest that the subtropical plantation soil sensitively responses to atmospheric N deposition, and inorganic N rather than organic N is the regulator to soil CH₄ uptake and N₂O emission.     

Denitrification and nitrous oxide emissions from riparian forests soils exposed to prolonged nitrogen runoff

Compared to upland forests, riparian forest soils have greater potential to remove nitrate (NO₃) from agricultural runoff through denitrification. It is unclear, however, whether prolonged exposure of riparian soils to nitrogen (N) loading will affect the rate of denitrification and its end products. This research assesses the rate of denitrification and nitrous oxide (N₂O) emissions from riparian forest soils exposed to prolonged nutrient runoff from plant nurseries and compares these to similar forest soils not exposed to nutrient runoff. Nursery runoff also contains high levels of phosphate (PO₄). Since there are conflicting reports on the impact of PO₄ on the activity of denitrifying microbes, the impact of PO₄ on such activity was also investigated. Bulk and intact soil cores were collected from N-exposed and non-exposed forests to determine denitrification and N₂O emission rates, whereas denitrification potential was determined using soil slurries. Compared to the non-amended treatment, denitrification rate increased 2.7- and 3.4-fold when soil cores collected from both N-exposed and non-exposed sites were amended with 30 and 60 μg NO₃-N g⁻¹ soil, respectively. Net N₂O emissions were 1.5 and 1.7 times higher from the N-exposed sites compared to the non-exposed sites at 30 and 60 μg NO₃-N g⁻¹ soil amendment rates, respectively. Similarly, denitrification potential increased 17 times in response to addition of 15 μg NO₃-N g⁻¹ in soil slurries. The addition of PO₄ (5 μg PO₄-P g⁻¹) to soil slurries and intact cores did not affect denitrification rates. These observations suggest that prolonged N loading did not affect the denitrification potential of the riparian forest soils; however, it did result in higher N₂O emissions compared to emission rates from non-exposed forest soils.     

Spatial and Temporal Variability in Sediment Denitrification Within an Agriculturally Influenced Reservoir

Reservoirs are intrinsically linked to the rivers that feed them, creating a river–reservoir continuum in which water and sediment inputs are a function of the surrounding watershed land use. We examined the spatial and temporal variability of sediment denitrification rates by sampling longitudinally along an agriculturally influenced river–reservoir continuum monthly for 13 months. Sediment denitrification rates ranged from 0 to 63 μg N₂O g ash free dry mass of sediments (AFDM)⁻¹ h⁻¹ or 0–2.7 μg N₂O g dry mass of sediments (DM)⁻¹ h⁻¹ at reservoir sites, vs. 0–12 μg N₂O gAFDM⁻¹ h⁻¹ or 0–0.27 μg N₂O gDM⁻¹ h⁻¹ at riverine sites. Temporally, highest denitrification activity traveled through the reservoir from upper reservoir sites to the dam, following the load of high nitrate (NO₃⁻-N) water associated with spring runoff. Annual mean sediment denitrification rates at different reservoir sites were consistently higher than at riverine sites, yet significant relationships among theses sites differed when denitrification rates were expressed per gDM vs. per gAFDM. There was a significant positive relationship between sediment denitrification rates and NO₃⁻-N concentration up to a threshold of 0.88 mg NO₃⁻ -N l⁻¹, above which it appeared NO₃⁻-N was no longer limiting. Denitrification assays were amended seasonally with NO₃⁻-N and an organic carbon source (glucose) to determine nutrient limitation of sediment denitrification. While organic carbon never limited sediment denitrification, all sites were significantly limited by NO₃⁻-N during fall and winter when ambient NO ₃⁻-N was low.     

Exploring the relationship between groundwater geochemical factors and denitrification potentials on a dairy farm in southeast Ireland

Nitrate (NO₃⁻) loss from agriculture to shallow groundwater and transferral to sensitive aquatic ecosystems is of global concern. Denitrifying bioreactor technology, where a solid carbon (C) reactive media intercepts contaminated groundwater, has been successfully used to convert NO₃⁻ to di-nitrogen (N₂) gas. One of the challenges of groundwater remediation research is how to track denitrification potential spatially and temporally within reactive media and subsoil. First, using δ¹⁵N/δ¹⁸O isotopes, eight wells were divided into indicative transformational processes of ‘nitrification’ or ‘denitrification’ wells. Then, using N₂/argon (Ar) ratios these wells were divided into ‘low denitrification potential’ or high denitrification potential’ categories. Secondly, using falling head tests, the saturated hydraulic conductivity (K_sat) in each well was estimated, creating two groups of ‘slow’ (0.06 m day⁻¹) and ‘fast’ (0.13 m day⁻¹) wells, respectively. Thirdly, two ‘low denitrification potential’ wells (one fast and one slow) with high NO₃⁻ concentration were amended with woodchip to enhance denitrification. Water samples were retrieved from all wells using a low flow syringe to avoid de-gassing and analysed for N₂/Ar ratio using membrane inlet mass spectrometry. Results showed that there was good agreement between isotope and chemical (N₂/Ar ratio and dissolved organic C (DOC)) and physio-chemical (dissolved oxygen, temperature, conductivity and pH) parameters. To explain the spatial and temporal distribution of NO₃⁻ and other parameters on site, the development of predictive models using the available datasets for this field site was examined for NO₃⁻, Cl⁻, N₂/Ar and DOC. Initial statistical analysis was directed towards the testing of the effect of woodchip amendment. The analysis was formulated as a repeated measures analysis of the factorial structure for treatment and time. Nitrate concentrations were related to K_sat and water level (p < 0.0001 and p = 0.02, respectively), but did not respond to woodchip addition (p = 0.09). This non-destructive technique allows elucidation of denitrification potential over time and could be used in denitrifying bioreactor technology to assess denitrification hotspots in reactive media, while developing a NO₃⁻ spatial and temporal predictive model for bioreactor site specific conditions.     

An in-depth look into a tropical lowland forest soil: nitrogen-addition effects on the contents of N2O, CO2 and CH4 and N2O isotopic signatures down to 2-m depth

Atmospheric nitrogen (N) deposition is rapidly increasing in tropical regions. We investigated how a decade of experimental N addition (125 kg N ha^?1 year^?1) to a seasonal lowland forest affected depth distribution and contents of soil nitrous oxide (N₂O), carbon dioxide (CO₂) and methane (CH₄), as well as natural abundance isotopic signatures of N₂O, nitrate (NO₃ ^?) and ammonium (NH₄ ⁺). In the control plots during dry season, we deduced limited N₂O production by denitrification in the topsoil (0.05–0.40 m) as indicated by: ambient N₂O concentrations and ambient ¹⁵N-N₂O signatures, low water-filled pore space (35–60%), and similar ¹⁵N signatures of N₂O and NO₃ ^?. In the subsoil (0.40–2.00 m), we detected evidence of N₂O reduction to N₂ during upward diffusion, indicating denitrification activity. During wet season, we found that N₂O at 0.05–2.00 m was mainly produced by denitrification with substantial further reduction to N₂, as indicated by: lighter ¹⁵N-N₂O than ¹⁵N-NO₃ ^? throughout the profile, and increasing N₂O concentrations with simultaneously decreasing ¹⁵N-N₂O enrichment with depth. These interpretations were supported by an isotopomer map and by a positive correlation between ¹⁸O-N₂O and ¹⁵N-N₂O site preferences. Long-term N addition did not affect dry-season soil N₂O-N contents, doubled wet-season soil N₂O-N contents, did not affect ¹⁵N signatures of NO₃ ^?, and reduced wet-season ¹⁵N signatures of N₂O compared to the control plots. These suggest that the increased NO₃ ^? concentrations have stimulated N₂O production and decreased N₂O-to-N₂ reduction. Soil CO₂-C contents did not differ between treatments, implying that N addition essentially did not influence soil C cycling. The pronounced seasonality in soil respiration was largely attributable to enhanced topsoil respiration as indicated by a wet-season increase in the topsoil CO₂-C contents. The N-addition plots showed reduced dry-season soil CH₄-C contents and threshold CH₄ concentrations were reached at a shallower depth compared to the control plots, revealing an N-induced stimulation of methanotrophic activity. However, the net soil CH₄ uptake rates remained similar between treatments possibly because diffusive CH₄ supply from the atmosphere largely limited CH₄ oxidation.     

Influence of bioturbation on denitrification and dissimilatory nitrate reduction to ammonium (DNRA) in freshwater sediments

Intensive agriculture leads to increased nitrogen fluxes (mostly as nitrate, NO₃ ^?) to aquatic ecosystems, which in turn creates ecological problems, including eutrophication and associated harmful algal blooms. These problems have focused scientific attention on understanding the controls on nitrate reduction processes such as denitrification and dissimilatory nitrate reduction to ammonium (DNRA). Our objective was to determine the effects of nutrient-tolerant bioturbating invertebrates (tubificid oligochaetes) on nitrogen cycling processes, specifically coupled nitrification–denitrification, net denitrification, DNRA, and biogeochemical fluxes (O₂, NO₃ ^?, NH₄ ⁺, CO₂, N₂O, and CH₄) in freshwater sediments. A mesocosm experiment determined how tubificid density and increasing NO₃ ^? concentrations (using N¹⁵ isotope tracing) interact to affect N cycling processes. At the lowest NO₃ ^? concentration and in the absence of bioturbation, the relative importance of denitrification to DNRA was similar (i.e., 49.6 and 50.4 ± 8.1 %, respectively). Increasing NO₃ ^? concentrations in the control cores (without fauna) stimulated denitrification, but did not enhance DNRA, which significantly altered the relative importance of denitrification compared to DNRA (94.6 vs. 5.4 ± 0.9 %, respectively). The presence of tubificid oligochaetes enhanced O₂, NO₃ ^?, NH₄ ⁺ fluxes, greenhouse gas production, and N cycling processes. The relative importance of denitrification to DNRA shifted towards favoring denitrification with both the increase in NO₃ ^? concentrations and the increase of bioturbation activity. Our study highlights that understanding the interactions between nutrient-tolerant bioturbating species and nitrate contamination is important for determining the nitrogen removal capacity of eutrophic freshwater ecosystems.     

Denitrification and nitrous oxide effluxes in boreal, eutrophic river sediments under increasing nitrate load: a laboratory microcosm study

Intact sediment cores from rivers of the Bothnian Bay (Baltic Sea) were studied for denitrification based on benthic fluxes of molecular nitrogen (N₂) and nitrous oxide (N₂O) in a temperature controlled continuous water flow laboratory microcosm under 10, 30, 100, and 300 μM of ¹⁵N enriched nitrate (NO₃ ^?, ~98 at. %). Effluxes of both N₂ and N₂O from sediment to the overlying water increased with increasing NO₃ ^? load. Although the ratio of N₂O to N₂ increased with increasing NO₃ ^? load, it remained below 0.04, N₂ always being the main product. At the NO₃ ^? concentrations most frequently found in the studied river water (10–100 μM), up to 8% of the NO₃ ^? was removed in denitrification, whereas with the highest concentration (300 μM), the removal by denitrification was less than 2%. However, overall up to 42% of the NO₃ ^? was removed by mechanisms other than denitrification. As the microbial activity was simultaneously enhanced by the NO₃ ^? load, shown as increased oxygen consumption and dissolved inorganic carbom efflux, it is likely that a majority of the NO₃ ^? was assimilated by microbes during their growth. The ¹⁵N content in ammonium (NH₄ ⁺) in the efflux was low, suggesting that reduction of NO₃ ^? to NH₄ ⁺ was not the reason for the NO₃ ^? removal. This study provides the first published information on denitrification and N₂O fluxes and their regulation by NO₃ ^? load in eutrophic high latitude rivers.     

Effects of nitrate concentration on the denitrification potential of a calcic cambisol and its fractions of N2, N2O and NO

Background and aims

The direct measurement of denitrification dynamics and its product fractions is important for parameterizing process-oriented model(s) for nitrogen cycling in various soils. The aims of this study are to a) directly measure the denitrification potential and the fractions of nitrogenous gases as products of the process in laboratory, b) investigate the effects of the nitrate (NO ₃ ^? ) concentration on emissions of denitrification gases, and c) test the hypothesis that denitrification can be a major pathway of nitrous oxide (N₂O) and nitric oxide (NO) production in calcic cambisols under conditions of simultaneously sufficient supplies of carbon and nitrogen substrates and anaerobiosis as to be found to occur commonly in agricultural lands.

Methods

Using the helium atmosphere (with or without oxygen) gas-flow-soil-core technique in laboratory, we directly measured the denitrification potential of a silt clay calcic cambisol and the production of nitrogen gas (N₂), N₂O and NO during denitrification under the conditions of seven levels of NO ₃ ^? concentrations (ranging from 10 to 250 mg N kg^?1 dry soil) and an almost constant initial dissolved organic carbon concentration (300 mg C kg^?1 dry soil).

Results

Almost all the soil NO ₃ ^? was consumed during anaerobic incubation, with 80–88 % of the consumed NO ₃ ^? recovered by measuring nitrogenous gases. The results showed that the increases in initial NO ₃ ^? concentrations significantly enhanced the denitrification potential and the emissions of N₂ and N₂O as products of this process. Despite the wide range of initial NO ₃ ^? concentrations, the ratios of N₂, N₂O and NO products to denitrification potential showed much narrower ranges of 51–78 % for N₂, 14–36 % for N₂O and 5–22 % for NO.

Conclusions

These results well support the above hypothesis and provide some parameters for simulating effects of variable soil NO ₃ ^? concentrations on denitrification process as needed for biogeochemical models.     

Capacity for Denitrification and Reduction of Nitrate to Ammonia in a Coastal Marine Sediment

The capacity for dissimilatory reduction of NO₃⁻ to N₂ (N₂O) and NH₄⁺ was measured in ¹⁵NO₃⁻-amended marine sediment. Incubation with acetylene (7 × 10⁻³ atmospheres [normal]) caused accumulation of N₂O in the sediment. The rate of N₂O production equaled the rate of N₂ production in samples without acetylene. Complete inhibition of the reduction of N₂O to N₂ suggests that the “acetylene blockage technique” is applicable to assays for denitrification in marine sediments. The capacity for reduction of NO₃⁻ by denitrification decreased rapidly with depth in the sediment, whereas the capacity for reduction of NO₃⁻ to NH₄⁺ was significant also in deeper layers. The data suggested that the latter process may be equally as significant as denitrification in the turnover of NO₃⁻ in marine sediments.     

Coordinated nitroxyl anion is produced and released as nitrous oxide by the decomposition of iridium-coordinated nitrosothiols

The aqueous decomposition of the iridium coordinated nitrosothiols (RSNOs) trans-K[IrCl₄(CH₃CN)NOSPh] (1), and K₂[IrCl₅(NOECyS)] (2, ECyS = cysteine ethyl ester), was studied by MS analysis of the gaseous products, ESI-MS, NMR, and UV-Vis spectroscopy. Bent NO (NO⁻, nitroxyl anion), sulfenic acids and nitrite were observed as coordinated products in solution, while nitrous oxide (N₂O) and nitrogen were detected in the gas phase. The formation of coordinated NO⁻ and N₂O, a nitroxyl dimerization product, allows us to propose the formation of free nitroxyl (HNO) as an intermediate. Complex 1 decomposes 300 times slower than free PhSNO does. In both cases (1 and 2) kinetic results show a first order decomposition behavior and a very negative ΔS^≠, which strongly indicates an associative rate-determining step. A proposed decomposition mechanism, supported by the experimental data and DFT calculations, involves, as the first step, nucleophilic attack of H₂O on to the sulfur atom of the coordinated RSNO, producing an NO⁻ complex and free sulfenic acid, followed by two competing reactions: a ligand exchange reaction of this NO⁻ with the sulfenic acid or, to a minor extent, coordination of N₂O to produce an NO⁻/N₂O complex which finally renders free N₂ and coordinated NO₂⁻. Some of the produced NO⁻ is likely to be released from the metal center producing nitroxyl by protonation and finally N₂O by dimerization and loss of H₂O. In conclusion, the decomposition of these coordinated RSNOs occurs through a different mechanism than for the decomposition of free RSNOs. It involves the formation of sulfenic acids and coordinated NO⁻, which is released from the complexes and protonated at the reaction pH producing nitroxyl (HNO), and ultimately N₂O.     

Nitrous oxide in brackish Lake Nakaumi, Japan II: the role of nitrification and denitrification in N2O accumulation

In order to understand the role of nitrification and denitrification in the accumulation of nitrous oxide (N₂O) in the hypolimnetic water of brackish Lake Nakaumi, the effects of dissolved oxygen (DO) concentration on these activities were investigated by incubation experiments. N₂O was produced during the oxidation of NH₄ ⁺ to NO₂ ⁻ in nitrification and during the reduction of NO₃ ⁻ to N₂ in denitrification. N₂O-producing activity by nitrification (N₂O_N) increased markedly with decreasing concentrations of DO. Low DO (10%–30% saturation) induced high N₂O_N. In contrast to nitrification, N₂O-producing activity by denitrification (N₂O_D) decreased with decreasing concentrations of DO. Little N₂O was accumulated during denitrification under low-level conditions of DO (10%–30%), because of further reduction of N₂O to N₂. It can therefore be assumed that N₂O produced as the by-product of nitrification is concurrently reduced to N₂ by denitrification under low-DO conditions. This would result in no substantial accumulation of N₂O during active nitrification in the hypolimnetic water of Lake Nakaumi. Received: July 6, 2001 / Accepted: December 10, 2001     

Selective Inhibition of Ammonium Oxidation and Nitrification-Linked N2O Formation by Methyl Fluoride and Dimethyl Ether

Methyl fluoride (CH₃F) and dimethyl ether (DME) inhibited nitrification in washed-cell suspensions of Nitrosomonas europaea and in a variety of oxygenated soils and sediments. Headspace additions of CH₃F (10% [vol/vol]) and DME (25% [vol/vol]) fully inhibited NO₂^- and N₂O production from NH₄⁺ in incubations of N. europaea, while lower concentrations of these gases resulted in partial inhibition. Oxidation of hydroxylamine (NH₂OH) by N. europaea and oxidation of NO₂^- by a Nitrobacter sp. were unaffected by CH₃F or DME. In nitrifying soils, CH₃F and DME inhibited N₂O production. In field experiments with surface flux chambers and intact cores, CH₃F reduced the release of N₂O from soils to the atmosphere by 20- to 30-fold. Inhibition by CH₃F also resulted in decreased NO₃^- + NO₂^- levels and increased NH₄⁺ levels in soils. CH₃F did not affect patterns of dissimilatory nitrate reduction to ammonia in cell suspensions of a nitrate-respiring bacterium, nor did it affect N₂O metabolism in denitrifying soils. CH₃F and DME will be useful in discriminating N₂O production via nitrification and denitrification when both processes occur and in decoupling these processes by blocking NO₂^- and NO₃^- production.     

Distinguishing between Nitrification and Denitrification as Sources of Gaseous Nitrogen Production in Soil

The source of N₂O produced in soil is often uncertain because denitrification and nitrification can occur simultaneously in the same soil aggregate. A technique which exploits the differential sensitivity of these processes to C₂H₂ inhibition is proposed for distinguishing among gaseous N losses from soils. Denitrification N₂O was estimated from 24-h laboratory incubations in which nitrification was inhibited by 10-Pa C₂H₂. Nitrification N₂O was estimated from the difference between N₂O production under no C₂H₂ and that determined for denitrification. Denitrification N₂ was estimated from the difference between N₂O production under 10-kPa C₂H₂ and that under 10 Pa. Laboratory estimates of N₂O production were significantly correlated with in situ N₂O diffusion measurements made during a 10-month period in two forested watersheds. Nitrous oxide production from nitrification was most important on well-drained sites of a disturbed watershed where ambient NO₃⁻ was high. In contrast, denitrification N₂O was most important on poorly drained sites near the stream of the same watershed. Distinction between N₂O production from nitrification and denitrification was corroborated by correlations between denitrification N₂O and water-filled pore space and between nitrification N₂O and ambient NO₃⁻. This technique permits qualitative study of environmental parameters that regulate gaseous N losses via denitrification and nitrification.     

Biogeochemical regime shifts in coastal landscapes: the contrasting effects of saltwater incursion and agricultural pollution on greenhouse gas emissions from a freshwater wetland

Many coastal plain wetlands receive nutrient pollution from agricultural fields and are particularly vulnerable to saltwater incursion. Although wetlands are a major source of the greenhouse gases methane (CH₄) and nitrous oxide (N₂O), the consequences of salinization for greenhouse gas emissions from wetlands with high agricultural pollution loads is rarely considered. Here, we asked how saltwater exposure alters greenhouse gas emissions from a restored freshwater wetland that receives nutrient loading from upstream farms. During March to November 2012, we measured greenhouse gases along a ~2 km inundated portion of the wetland. Sampling locations spanned a wide chemical gradient from sites receiving seasonal fertilizer nitrogen and sulfate (SO₄ ^2?) loads to sites receiving seasonal increases in marine salts. Concentrations and fluxes of CH₄ were low (<100 µg L^?1 and <10 mg m^?2 h^?1) for all sites and sampling dates when SO₄ ^2? was high (>10 mg L^?1), regardless of whether the SO₄ ^2? source was agriculture or saltwater. Elevated CH₄ (as high as 1,500 µg L^?1 and 45 mg m^?2 h^?1) was only observed on dates when air temperatures were >27 °C and SO₄ ^2? was <10 mg L^?1. Despite elevated ammonium (NH₄ ⁺) for saltwater exposed sites, concentrations of N₂O remained low (<5 µg L^?1 and <10 µg m^?2 h^?1), except when fertilizer derived nitrate (NO₃ ^?) concentrations were high and N₂O increased as high as 156 µg L^?1. Our results suggest that although both saltwater and agriculture derived SO₄ ^2? may suppress CH₄, increases in N₂O associated with fertilizer derived NO₃ ^? may offset that reduction in wetlands exposed to both agricultural runoff and saltwater incursion.     

Rapid Conversion of Added Nitrate to Nitrous Oxide and Dinitrogen in Northern Forest Soil

The aims of this study were to simulate wet deposition of atmospheric nitrate (NO₃^?) onto forest soils and trace its fate via conversion spatially and temporally into gaseous products nitrous oxide (N₂O) and dinitrogen (N₂). The most likely mechanism is microbial denitrification, but an intermediate product nitrite (NO₂^?) can fuel N₂O production via a chemical pathway involving reactions with iron and/or organic matter referred to as chemodenitrification. During summer months, we applied tracer amounts of ¹⁵N-labeled NO₃^? onto forest soils (pH ～ 4) at three sites in the White Mountain Region of New Hampshire, USA. We recovered ¹⁵N as N₂O in 210 of 504 measurements (42%) versus ¹⁵N as N₂ in 51 of 504 measurements (10%), suggesting partial microbial denitrification and/or chemodenitrification. When recovery occurred, the mean percent recovery of added ¹⁵N was just 1.1% as N₂O, although N₂ recovery was 33%. A site with old-growth trees had a larger percentage recovery as N₂ (48%), whereas a site that had burned 100 years ago had a small percentage recovery as N₂O (0.24%). The ¹⁵N composition of N₂O in ambient air, collected before addition of the label, was markedly enriched in ¹⁵N. Since flux measurements were made 2 h after the addition, the results suggest that denitrification enzymes and conditions for chemodenitrification are present throughout the summer months but account for small amounts of NO₃^? conversion into N₂O and N₂.     

Stimulating denitrification in a stream mesocosm with elemental sulfur as an electron donor

The heavy use of fertilizers in agricultural lands can result in significant nitrate (NO₃⁻) loadings to the aquatic environment. We hypothesized that biological denitrification in agricultural ditches and streams could be enhanced by adding elemental sulfur (S^o) to the sediment layer, where it could act as a biofilm support and electron donor. Using a bench-scale stream mesocosm with a bed of S^o granules, we explored NO₃⁻ removal fluxes as a function of the effluent NO₃⁻ concentrations. With effluent NO₃⁻ ranging from 0.5 mg N L⁻¹ to 4.1 mg N L⁻¹, NO₃⁻ removal fluxes ranged from 228 mg N m⁻² d⁻¹ to 708 mg N m⁻² d⁻¹. This is as much as 100 times higher than for agricultural drainage streams. Sulfate (SO₄²⁻) production was high due to aerobic sulfur oxidation. Molecular studies demonstrated that the S^o amendment selected for Thiobacillus species, and that no special inoculum was required for establishing a S^o-based autotrophic denitrifying community. Modeling studies suggested that denitrification was diffusion limited, and advective flow through the bed would greatly enhance NO₃⁻ removal fluxes. Our results indicate that amendment with S^o is an effective means to stimulate denitrification in a stream environment. To minimize SO₄²⁻ production, it may be better to place S^o deeper in the sediment layer.     

Microzonation of Denitrification Activity in Stream Sediments as Studied with a Combined Oxygen and Nitrous Oxide Microsensor

Microzonation of denitrification was studied in stream sediments by a combined O₂ and N₂O microsensor technique. O₂ and N₂O concentration profiles were recorded simultaneously in intact sediment cores in which C₂H₂ was added to inhibit N₂O reduction in denitrification. The N₂O profiles were used to obtain high-resolution profiles of denitrification activity and NO₃⁻ distribution in the sediments. O₂ penetrated about 1 mm into the dark-incubated sediments, and denitrification was largely restricted to a thin anoxic layer immediately below that. With 115 μM NO₃⁻ in the water phase, denitrification was limited to a narrow zone from 0.7 to 1.4 mm in depth, and total activity was 34 nmol of N cm⁻² h⁻¹. With 1,250 μM NO₃⁻ in the water, the denitrification zone was extended to a layer from 0.9 to 4.8 mm in depth, and total activity increased to 124 nmol of N cm⁻² h⁻¹. Within most of the activity zone, denitrification was not dependent on the NO₃⁻ concentration and the apparent K_m for NO₃⁻ was less than 10 μM. Denitrification was the only NO₃⁻-consuming process in the dark-incubated stream sediment. Even in the presence of C₂H₂, a significant N₂O reduction (up to 30% of the total N₂O production) occurred in the reduced, NO₃⁻-free layers below the denitrification zone. This effect must be corrected for during use of the conventional C₂H₂ inhibition technique.     

Aggregation of imino-phosphonate monoester complexes

The aggregates {[Zn(L¹)]H₂O}_∞ and {[Y(L²)]₄Na₃(H₂O)₂(MeOH)_1.2}(NO₃)₃·2H₂O·5.6MeOH have been assembled from complexes of imino-phosphonate monoester ligands [L¹]²⁻ {CH₂[CH₂NC(CH₃)PO₂(OMe)]₂}²⁻ and [L²]³⁻ {N[CH₂CH₂NC(CH₃)PO₂(OMe)]₃}³⁻, the topology of these materials differing from that of their imino-carboxylate analogues.     

Hexanuclear copper(II) complex of a novel poly(pyridine) ligand exhibiting unique long distance ferromagnetic interactions through a nitrato-O,O bridge

A novel hexanuclear copper complex [Cu₆(NO₃)₁₂(opytrizediam)₂(H₂O)][(CH₃)₂CO]_0.5(CH₃CH₂CH₂OH)_0.5 (1) with a NO₃ bridge has been synthesized by reaction of Cu(NO₃)₂ · 3H₂O with the new potentially octadentate ligand opytrizediam in n-propanol/acetone solution (opytrizediam=N,N^′-{2,4-di[(di-pyridin-2-yl)amine]-1,3,5-triazine} ethylenediamine). A single-crystal X-ray diffraction analysis showed the presence of six structurally different copper centres. The coordination spheres of four copper(II) ions are best described as square-pyramidal CuN₂O₃ chromophores while the two other copper(II) ions are in a trigonal-bipyramidal CuN₄O environment. Variable-temperature studies on 1 revealed a unique ferromagnetic coupling of two copper(II) ions bridged by a didentate nitrate anion and separated by a distance of 6.391(6) Å, with J=8.6(1) cm⁻¹.