Nitrate Pool Research Articles

Evidence that the efficacy of the nitrification inhibitor dicyandiamide (DCD) is affected by soil properties in UK soils

A laboratory incubation study was conducted with nine UK soils to determine the effect of soil physical and chemical properties, and temperature, on the efficacy the nitrification inhibitor dicyandiamide (DCD). Nitrogen was applied to soil as ammonium chloride at a rate of 100 μg N g−1 dry soil, and incubated at 60% water-filled-pore-space at either 5, 15 or 25 °C. The ammonium (NH4+) pool was enriched with 15N to 60 atom% excess and DCD was applied at a rate of 10 μg g−1 dry soil. The concentrations and enrichments of the NH4+ and nitrate (NO3−) pools, along with nitrous oxide (N2O) flux measurements, were determined regularly for 60 days after N application. Gross soil N transformation rates were quantified with a 15N tracing model. The persistence of DCD was strongly related (P < 0.001) to temperature with the measured half-life across all soils of 89, 37 and 18 days at 5, 15, and 25 °C, respectively. There was wide variation in the half-life of DCD among soils; which was predominantly associated with the soil oxalate extractable Fe concentration. Greater (P < 0.001) inhibition in autotrophic nitrification by DCD occurred at 5 and 15 °C compared to 25 °C. Across all soils and temperatures DCD increased the rate of mineralisation of recalcitrant organic-N and the rate of adsorption of free NH4+, however, effects varied between soils. DCD did not have a significant effect on the rate of oxidation of recalcitrant organic-N to NO3− or on any of the immobilisation processes or mineralisation of labile N to NH4+. The efficacy of DCD in inhibiting net NO3− production best correlated with soil Cu (r = −0.82), % clay (r = −0.71), total N (r = −0.66) and LOI (r = −0.61). Stepwise multiple regression showed that Cu, oxalate extractable Fe and oxalate extractable Al explained 85.0% of the variation in the percentage inhibition of net NO3− production by DCD. The inhibitor also reduced cumulative N2O emissions, with reductions negatively correlated with a range of soil properties associated with organic matter. We provide evidence that the interaction between temperature, soil clay content and soil organic matter governs the efficacy of DCD. The grassland soils had higher native total N concentrations than the arable soils, hence the inhibition of net NO3− production by DCD was lower and this resulted in an overall inhibition in N2O emissions of 58% and 81% for grassland and arable soils respectively.

Evidence for denitrification as main source of N2O emission from residue-amended soil

Catch crops, especially leguminous catch crops, may increase crop nitrogen (N) supply and decrease environmental impacts in cropping systems, but they may also stimulate nitrous oxide (N2O) emissions following spring incorporation. In this 28-day laboratory incubation study, we examined the carbon (C) and N dynamics and N2O evolution after simulated incorporation of residues from three catch crop species into a loamy sand soil, with variable soil moisture (40, 50 or 60% water-filled pore space (WFPS)). The catch crops include two leguminous (red clover and winter vetch) and one non-leguminous species (ryegrass). Plant material was placed in a discrete layer surrounded by soil in which the nitrate (NO3−) pool was enriched with 15N to distinguish N2O derived from denitrification and nitrification. Net N mineralisation from leguminous catch crops was significant (30–48 mg N kg−1 soil, accounting for 41–56% of the added residue-N), whereas ryegrass incorporation resulted in net N immobilisation. The evolution of N2O was probably enhanced by N release from the residues, especially during the second week, which can explain the lower N2O evolution after application of ryegrass. Emission of N2O occurred at all moisture levels, but was higher at 50 and 60% WFPS than at 40% in soil with leguminous residues. The 15N enrichment of N2O indicated that denitrification was the dominant source independent of moisture level and residue type. We conclude that catch crop residues will stimulate N2O emissions via denitrification over a wide range of soil moisture conditions, but that emission levels may depend significantly on residue quality and soil moisture.

Spatial distribution and temporal variation of nitrate nitrogen and oxygen isotopes in the upper equatorial Pacific Ocean

Equatorial Pacific nitrate 15N/14N and 18O/16O (δ15N and δ18O) measurements from repeat station occupations are used to: (1) identify the source of surface waters, (2) estimate the nitrate assimilation isotope effect, (3) characterize the temporal relationship between the supply and consumption of nitrate in surface waters, and (4) quantify the degree of nitrate consumption in nitrate-bearing surface waters. Surface nitrate assimilation was consistent with consumption of a closed nitrate pool at 63–80% of stations. Those stations yielded nitrate assimilation isotope effects of 6.0‰ ± 0.4‰ for N and 5.9‰ ± 0.6‰ for O, with an N:O isotope effect ratio of 1.0 ± 0.1, consistent with nitrate consumption being the dominant biogeochemical flux in the euphotic zone. The euphotic zone data point to the local Equatorial Under Current as the dominant source of surface nitrate along the equator, although surface waters at 110°W and ≥5° south of the equator show signs of surface nitrate input from further east. The collected findings suggest that nitrate delivery along the equatorial Pacific is dominated by Ekman upwelling rather than surface advection or vertical or lateral mixing. Surface nitrate δ15N and δ18O are correlated with seasonality and El Niño-Southern Oscillation, with the implied degree of surface ocean nitrate consumption decreasing with increased upwelling strength. Incomplete consumption of surface nitrate near the equator lowers the δ15N of subsurface equatorial “thermostad” nitrate through the remineralization of low δ15N sinking organic matter while the continued consumption of the residual surface nitrate transported away from the equator raises the nitrate δ15N in off-equatorial subsurface waters.

Differences in Ecosystem Carbon Distribution and Nutrient Cycling Linked to Forest Tree Species Composition in a Mid-Successional Boreal Forest

In the boreal forest of Alaska, increased fire severity associated with climate change is expanding deciduous forest cover in areas previously dominated by black spruce (Picea mariana). Needle-leaf conifer and broad-leaf deciduous species are commonly associated with differences in tree growth, carbon (C) and nutrient cycling, and C accumulation in soils. Although this suggests that changes in tree species composition in Alaska could impact C and nutrient pools and fluxes, few studies have measured these linkages. We quantified C, nitrogen, phosphorus, and base cation pools and fluxes in three stands of black spruce and Alaska paper birch (Betula neoalaskana) that established following a single fire event in 1958. Paper birch consistently displayed characteristics of more rapid C and nutrient cycling, including greater aboveground net primary productivity, higher live foliage and litter nutrient concentrations, and larger ammonium and nitrate pools in the soil organic layer (SOL). Ecosystem C stocks (aboveground + SOL + 0–10 cm mineral soil) were similar for the two species; however, in black spruce, 78% of measured C was found in soil pools, primarily in the SOL, whereas aboveground biomass dominated ecosystem C pools in birch forest. Radiocarbon analysis indicated that approximately one-quarter of the black spruce SOL C accumulated prior to the 1958 fire, whereas no pre-fire C was observed in birch soils. Our findings suggest that tree species exert a strong influence over C and nutrient cycling in boreal forest and forest compositional shifts may have long-term implications for ecosystem C and nutrient dynamics.

Spatial variation in nutrient dynamics among five different peatland types in the Alberta oil sands region

AbstractWetlands are found extensively throughout the Western Boreal Plain, a region under pressure because of disturbance by the oil and gas industries. To understand how wetland systems may respond to disturbance and set targets for reclamation efforts, it is necessary to understand natural variability in nutrient dynamics in the landscape. The purpose of this study was to characterize spatial variability in peatland nutrient (nitrogen, N, and phosphorus, P) dynamics in the Athabasca Oil Sands (AOS) region. N and P availability and net mineralization rates in the upper 10 cm layer of peat were examined during the peak growing season in five peatlands that fell along an apparent moisture gradient. N and P dynamics within and among the sites were related to water table position, peat moisture content and temperature. Phosphorus supply rates and total inorganic N pools and supply rates were generally elevated under wetter conditions, whereas nitrate (NO3−) pools and supply rates and P pools did not vary along a moisture gradient. In general, net immobilization was observed at the wetter sites where nutrient pools were elevated and net mineralization was observed at drier sites where nutrient pools were lower. Nutrient transformation rates were most strongly driven by warmer temperatures. Nutrient availability and immobilization rates were anomalously high at one peatland (a disturbed fen with a semi‐permanent road and decommissioned well pads). We suggest that reclamation and management practices should focus on regulating peatland hydrologic conditions, optimizing these for the most desirable nutrient levels for vegetation growth. Copyright © 2015 John Wiley &amp; Sons, Ltd.

THE FEATURES THE AUTOTROPHIC AND HETEROTROPHIC NITRIFICATION IN THE ROOT ZONE OF WINTER RYE PLANTS UNDER THE USE OF MINERAL FERTILIZERS AND DIAZOBAKTERYN

The paper depicts the study of mineral nitrogen and pre-sowing seeds bacteryzation on the development of nitrogen fixing bacteria and the activity of autotrophic and heterotrophic nitrification. It was revealed that nitrification activity in the root zone of winter rye plants rises together with the increase of mineral nitrogen doses. Heterotrophic nitrification plays a significant role in the formation of nitrate pool in the root zone, especially during the early stages of plants organogenesis. Application of the microbial preparation Diazobakteryn had enhanced the activity autotrophic and heterotrophic nitrification processes in the rhizosphere soil of plants in the spring, during the early stages of their development while during the next organogenesis phases the reduction of nitrification processes was observed. In plants-free soil the contribution of heterotrophic nitrification to biological transformation of nitrogen was negligible.

High turnover rates indicated by changes in the fixed N forms and their stable isotopes in Antarctic landfast sea ice

AbstractWe report concentration and nitrogen and oxygen isotopic measurements of nitrate, total dissolved nitrogen, and particulate nitrogen from Antarctic landfast sea ice, covering almost the complete seasonal cycle of sea ice growth and decay (from April to November). When sea ice forms in autumn, ice algae growth depletes nitrate and accumulates organic N within the ice. Subsequent low biological activity in winter imposes minor variations in the partitioning of fixed N. In early spring, the coupling between nitrate assimilation and brine convection at the sea ice bottom traps a large amount of fixed N within sea ice, up to 20 times higher than in the underlying seawater. At this time, remineralization and nitrification also accelerate, yielding nitrate concentrations up to 5 times higher than in seawater. Nitrate δ15N and δ18O are both elevated, indicating a near‐balance between nitrification and nitrate assimilation. These findings require high microbially mediated turnover rates for the large fixed N pools, including nitrate. When sea ice warms in the spring, ice algae grow through the full thickness of the ice. The warming stratifies the brine network, which limits the exchange with seawater, causing the once‐elevated nitrate pool to be nearly completely depleted. The nitrate isotope data point to light limitation at the base of landfast ice as a central characteristic of the environment, affecting its N cycling (e.g., allowing for nitrification) and impacting algal physiology (e.g., as reflected in the N and O isotope effects of nitrate assimilation).

Surface nitrate utilization in the Bering sea since 180 kA BP: Insight from sedimentary nitrogen isotopes

We present high-resolution records of sedimentary nitrogen (δ15Nbulk) and carbon isotope ratios (δ13Cbulk) from piston core SO201-2-85KL located in the western Bering Sea. The records reflect changes in surface nitrate utilization and terrestrial organic matter contribution in submillennial resolution that span the last 180kyr. The δ15Nbulk record is characterized by a minimum during the penultimate interglacial indicating low nitrate utilization (~62–80%) despite the relatively high export production inferred from opal concentrations along with a significant reduction in the terrestrial organic matter fraction (mterr). This suggests that the consumption of the nitrate pool at our site was incomplete and even more reduced than today (~84%). δ15Nbulk increases from Marine Isotope Stage (MIS) 5.4 and culminates during the Last Glacial Maximum, which indicates that nitrate utilization in the Bering Sea was raised during cold intervals (MIS 5.4, 5.2, 4) and almost complete during MIS 3 and 2 (~93–100%). This is in agreement with previous hypotheses suggesting that stronger glacial stratification reduced the nutrient supply from the subeuphotic zone, thereby increasing the iron-to-nutrient ratio and therefore the nitrate utilization in the mixed surface layer. Large variations in δ15Nbulk were also recorded from 180 to 130ka BP (MIS 6), indicating a potential link to insolation and sea-level forcing and its related feedbacks. Millennial-scale oscillations were observed in δ15Nbulk and δ13Cbulk that might be related to Greenland interstadials.

Restricted use of nitrate and a strong preference for ammonium reflects the nitrogen ecophysiology of a light-limited red alga.

Ammonium and nitrate are important sources of inorganic nitrogen for coastal primary producers. Nitrate has higher energy requirement for uptake and assimilation, compared with ammonium, suggesting that it might be a more efficient nitrogen source for slow-growing, light-limited macroalgae. To address this hypothesis, we examined the nitrogen ecophysiology of Anotrichium crinitum, a rhodophyte macroalgae common in low-light habitats in New Zealand. We measured seasonal changes in seawater nitrate and ammonium concentrations and the concentration of nitrate and ammonium stored internally by A.crinitum. We determined the maximal uptake rates of nitrate and ammonium seasonally and grew A.crinitum in the laboratory with these nitrogen sources under two ecologically relevant saturating light levels. Our results show that field-harvested A.crinitum has a high affinity for ammonium and although it will grow when supplied exclusively with nitrate, internal nitrate pools are low and it is unable to take up nitrate without several days of acclimation to saturating light. Our data predict that A.crinitum would be able to sustain growth with ammonium as the sole source of nitrogen, a strategy that would help it survive under low-light conditions that prevail in the field.

Soil bacterial community structure remains stable over a 5-year chronosequence of insect-induced tree mortality.

Extensive tree mortality from insect epidemics has raised concern over possible effects on soil biogeochemical processes. Yet despite the importance of microbes in nutrient cycling, how soil bacterial communities respond to insect-induced tree mortality is largely unknown. We examined soil bacterial community structure (via 16S rRNA gene pyrosequencing) and community assembly processes (via null deviation analysis) along a 5-year chronosequence (substituting space for time) of bark beetle-induced tree mortality in the southern Rocky Mountains, USA. We also measured microbial biomass and soil chemistry, and used in situ experiments to assess inorganic nitrogen mineralization rates. We found that bacterial community structure and assembly—which was strongly influenced by stochastic processes—were largely unaffected by tree mortality despite increased soil ammonium () pools and reductions in soil nitrate () pools and net nitrogen mineralization rates after tree mortality. Linear models suggested that microbial biomass and bacterial phylogenetic diversity are significantly correlated with nitrogen mineralization rates of this forested ecosystem. However, given the overall resistance of the bacterial community to disturbance from tree mortality, soil nitrogen processes likely remained relatively stable following tree mortality when considered at larger spatial and longer temporal scales—a supposition supported by the majority of available studies regarding biogeochemical effects of bark beetle infestations in this region. Our results suggest that soil bacterial community resistance to disturbance helps to explain the relatively weak effects of insect-induced tree mortality on soil N and C pools reported across the Rocky Mountains, USA.

Nitrate uptake by foraminifera and use in conjunction with endobionts under anoxic conditions

Among eukaryotes, some benthic foraminiferal species have been reported to be capable of nitrate respiration, although little is known about their denitrification processes. In this study, we incubated the shallow‐water benthic foraminifer Ammonia beccarii with isotopically labeled sodium nitrate (−73‰, 0‰, or +73‰) under oxic or anoxic conditions to investigate how nitrate is used in foraminiferal cells and whether those signatures remain in amino acids. The δ15N values of amino acids from bulk cells incubated under anoxic conditions were correlated with those of nitrate in seawater and were enriched in 15N by up to ∼ 50‰ compared to the isotopic compositions in seawater. There was no such relationship or enrichment in the cases of organic matter in the calcite tests under oxic or anoxic conditions or bulk cells under oxic conditions, suggesting that benthic foraminifera take up ambient nitrate under anoxic conditions and use it for denitrification; the remaining 15N‐enriched intracellular nitrate pool is used for amino acid synthesis, probably by endosymbiotic microbes, as suggested by observation of the cellular ultrastructure. The degree of 15N enrichment may depend on denitrification rates of the intracellular nitrate pool. Because the amino acids in the calcite test are synthesized by foraminifera, they were not enriched in 15N, even under anoxic conditions. Thus, differences between amino acid δ15N of bulk foraminiferal cells and organic matter in tests may serve as a proxy for denitrification in foraminiferal cells and microbial amino acid synthesis under oxygen‐depleted conditions.

Індукція нітрозативного стресу в мітохондріях серця щурів за експериментальної ішемії–реперфузії головного мозку та його корекція екдистероном

On the model of focal ischemia-reperfusion of the brain investigated the induction of nitrosative stress in mitochondria of rats hearts and possible mechanisms of protective action of ecdysterone. It is shown that focal ischemia-reperfusion of the brain induced in myocardial mitochondria the activation of constitutive and inducible de novo synthesis of NO by oxidation of L-arginine and not oxidative synthesis of NO through the recovery of oxidized stable metabolites of NO. Strong evidence of induction of nitrosative stress in heart mitochondria by focal ischemia-reperfusion of the brain, was a significant increase in mitochondrial pool of nitrate- and nitrite-anions and pools of nitrosothiols, that is proof of the formation and decay of peroxynitrite--a key marker of nitrosative stress. Also was observed increase in heart mitochondria by focal ischemia-reperfusion of the brain, content key regulator of de novo synthesis of NO-hydrogen sulfide and activity of inducible arginase II and, as a result, the pool of carbamide, which is also a regulator of the synthesis of NO. Previous introduction for animals herbal extract Serratsula coronata, enriched ecdysterone, reduces induction nitrosative stress in mitochondria of rats hearts under conditions of focal ischemia-reperfusion of the brain.

The fate of nitrate in intertidal permeable sediments.

Coastal zones act as a sink for riverine and atmospheric nitrogen inputs and thereby buffer the open ocean from the effects of anthropogenic activity. Recently, microbial activity in sandy permeable sediments has been identified as a dominant source of N-loss in coastal zones, namely through denitrification. Some of the highest coastal denitrification rates measured so far occur within the intertidal permeable sediments of the eutrophied Wadden Sea. Still, denitrification alone can often account for only half of the substantial nitrate (NO3 −) consumption. Therefore, to investigate alternative NO3 − sinks such as dissimilatory nitrate reduction to ammonium (DNRA), intracellular nitrate storage by eukaryotes and isotope equilibration effects we carried out 15NO3 − amendment experiments. By considering all of these sinks in combination, we could quantify the fate of the 15NO3 − added to the sediment. Denitrification was the dominant nitrate sink (50–75%), while DNRA, which recycles N to the environment accounted for 10–20% of NO3 − consumption. Intriguingly, we also observed that between 20 and 40% of 15NO3 − added to the incubations entered an intracellular pool of NO3 − and was subsequently respired when nitrate became limiting. Eukaryotes were responsible for a large proportion of intracellular nitrate storage, and it could be shown through inhibition experiments that at least a third of the stored nitrate was subsequently also respired by eukaryotes. The environmental significance of the intracellular nitrate pool was confirmed by in situ measurements which revealed that intracellular storage can accumulate nitrate at concentrations six fold higher than the surrounding porewater. This intracellular pool is so far not considered when modeling N-loss from intertidal permeable sediments; however it can act as a reservoir for nitrate during low tide. Consequently, nitrate respiration supported by intracellular nitrate storage can add an additional 20% to previous nitrate reduction estimates in intertidal sediments, further increasing their contribution to N-loss.

Sources of N2O–N following simulated animal treading of ungrazed pastures

It has been previously hypothesised that the treading of pastures by grazing animals can increase nitrous oxide (N2O) emissions as the result of reduced plant N uptake. In addition, grazing animals urinate and defecate on to soils which can also increase the N2O emissions. To avoid these additional N inputs, a treading machine was used in two field experiments where the pre-grazing dry matter (DM) was present or removed to test the above hypothesis. The N2O emissions were measured for 47 and 30 days afterwards, respectively. The soil nitrate (NO3−–N) pool was 15N labelled prior to treading in Experiment 2. Treading induced greater N2O emissions and reduced herbage DM yields (by 31%–41%) and soil NO3−–N concentrations when soil gravimetric water contents ranged from 0.45 to 0.63 g water/g soil. A comparison of treading in the presence and absence of pre-grazing DM showed that reduced plant N uptake did not induce greater N2O emissions. Rather, the 15N labelling of the NO3−–N pool indicated this pool contributed to the N2O emissions under treading. In addition, 15N labelling showed other soil–N pools became available as a consequence of either, (1) treading inducing soil perturbation and releasing N from the soil organic matter–N and/or plant root–N pools, or (2) as a result of simulated grazing causing the release of plant root–N. Soil water soluble carbon (C) also increased under treading in the absence of pasture, supporting the theory that organic–N was released from either the soil organic matter (OM) or plant root pools. Further research should investigate the effect of treading on the turnover and contribution of organic–N pools (roots, OM) in pasture soils in order to more fully understand the contributions these make to background N2O emissions and effects on N and C cycling in pasture soils.

Nitrate reduction functional genes and nitrate reduction potentials persist in deeper estuarine sediments. Why?

Denitrification and dissimilatory nitrate reduction to ammonium (DNRA) are processes occurring simultaneously under oxygen-limited or anaerobic conditions, where both compete for nitrate and organic carbon. Despite their ecological importance, there has been little investigation of how denitrification and DNRA potentials and related functional genes vary vertically with sediment depth. Nitrate reduction potentials measured in sediment depth profiles along the Colne estuary were in the upper range of nitrate reduction rates reported from other sediments and showed the existence of strong decreasing trends both with increasing depth and along the estuary. Denitrification potential decreased along the estuary, decreasing more rapidly with depth towards the estuary mouth. In contrast, DNRA potential increased along the estuary. Significant decreases in copy numbers of 16S rRNA and nitrate reducing genes were observed along the estuary and from surface to deeper sediments. Both metabolic potentials and functional genes persisted at sediment depths where porewater nitrate was absent. Transport of nitrate by bioturbation, based on macrofauna distributions, could only account for the upper 10 cm depth of sediment. A several fold higher combined freeze-lysable KCl-extractable nitrate pool compared to porewater nitrate was detected. We hypothesised that his could be attributed to intracellular nitrate pools from nitrate accumulating microorganisms like Thioploca or Beggiatoa. However, pyrosequencing analysis did not detect any such organisms, leaving other bacteria, microbenthic algae, or foraminiferans which have also been shown to accumulate nitrate, as possible candidates. The importance and bioavailability of a KCl-extractable nitrate sediment pool remains to be tested. The significant variation in the vertical pattern and abundance of the various nitrate reducing genes phylotypes reasonably suggests differences in their activity throughout the sediment column. This raises interesting questions as to what the alternative metabolic roles for the various nitrate reductases could be, analogous to the alternative metabolic roles found for nitrite reductases.

Fast photolysis of carbonyl nitrates from isoprene

Abstract. Photolysis is shown to be a major sink for isoprene-derived carbonyl nitrates, which constitute an important component of the total organic nitrate pool over vegetated areas. Empirical evidence from published laboratory studies on the absorption cross sections and photolysis rates of α-nitrooxy ketones suggests that the presence of the nitrate group (i) greatly enhances the absorption cross sections and (ii) facilitates dissociation to a point that the photolysis quantum yield is close to unity, with O–NO2 dissociation as a likely major channel. On this basis, we provide new recommendations for estimating the cross sections and photolysis rates of carbonyl nitrates. The newly estimated photo rates are validated using a chemical box model against measured temporal profiles of carbonyl nitrates in an isoprene oxidation experiment by Paulot et al. (2009). The comparisons for ethanal nitrate and for the sum of methacrolein- and methyl vinyl ketone nitrates strongly supports our assumptions of large cross-section enhancements and a near-unit quantum yield for these compounds. These findings have significant atmospheric implications: the photorates of key carbonyl nitrates from isoprene are estimated to be typically between ~ 3 and 20 times higher than their sink due to reaction with OH in relevant atmospheric conditions. Moreover, since the reaction is expected to release NO2, photolysis is especially effective in depleting the total organic nitrate pool.

Physiological and Morphological Responses of Phaseolus vulgaris Caused by Mercury Stress under Lab Conditions

One of the many new risks that effecting the early societies is the continuous exposure to pollutants, namely, heavy metals. Mercury (Hg) is perhaps the metal which has attracted the most attention in soil science and plant nutrition due to its potential toxicity to ecosystem. In the present study, the toxic effect of mercury was determined by morphological and physiological parameter on plant Phaseolus vulgaris. In germination studies, parameters such as germination percentage, root length, and shoot length were decreased with increasing dose of mercuric chloride (HgCl2) concentrations. Mercury also showed inhibition property towards physiological parameters such as chlorophyll, protein, nitrate, and endogenous pool. Higher concentrations of HgCl2 were found to be more toxic.

Response of the ubiquitous pelagic diatom Thalassiosira weissflogii to darkness and anoxia.

Thalassiosira weissflogii, an abundant, nitrate-storing, bloom-forming diatom in the world’s oceans, can use its intracellular nitrate pool for dissimilatory nitrate reduction to ammonium (DNRA) after sudden shifts to darkness and anoxia, most likely as a survival mechanism. T. weissflogii cells that stored 4 mM 15N-nitrate consumed 1.15 (±0.25) fmol NO3 - cell-1 h-1 and simultaneously produced 1.57 (±0.21) fmol 15NH4 + cell-1 h-1 during the first 2 hours of dark/anoxic conditions. Ammonium produced from intracellular nitrate was excreted by the cells, indicating a dissimilatory rather than assimilatory pathway. Nitrite and the greenhouse gas nitrous oxide were produced at rates 2-3 orders of magnitude lower than the ammonium production rate. While DNRA activity was restricted to the first few hours of darkness and anoxia, the subsequent degradation of photopigments took weeks to months, supporting the earlier finding that diatoms resume photosynthesis even after extended exposure to darkness and anoxia. Considering the high global abundance of T. weissflogii, its production of ammonium and nitrous oxide might be of ecological importance for oceanic oxygen minimum zones and the atmosphere, respectively.

Evidence of parallel denitrification and nitrite oxidation in the ODZ of the Arabian Sea from paired stable isotopes of nitrate and nitrite

The Arabian Sea is a major oceanic nitrogen sink, and its oxygen‐deficient zone (ODZ) extends from 150 m to 1200 m water depth. To identify the dominant transformation processes of reactive nitrogen and to quantify the amounts of nitrogen turned over in the different reactions of the nitrogen cycle, we use paired data on stable isotope ratios of nitrogen and oxygen in nitrate and nitrite measured at four near‐coastal and five open ocean stations in the Arabian Sea. We find significant nitrate reduction and denitrification between 100 m and 400 m in the open Arabian Sea, which are most intense in the eastern and northern part of the basin, and estimate that about 50% of initial nitrate is being reduced either to dinitrogen gas (denitrification) or to nitrite (nitrate reduction) in the core zone of denitrification. Nitrite accumulates in concentrations above 4 µM in the water column of the eastern and northern Arabian Sea. Large differences in isotopic ratios of nitrate and nitrite and a decoupling of their stable nitrogen and oxygen isotopes can be explained by the reoxidation of nitrite. The observed decoupling of the paired isotopes may be due to the exchange of oxygen of nitrite with oxygen from ambient water. In agreement with model estimates from the literature, about 25% of the nitrate initially reduced to nitrite is returned to the nitrate pool by nitrification in the upper and lower denitrification layer while 40% is denitrified.

Coupled nitrate nitrogen and oxygen isotopes and organic matter remineralization in the Southern and Pacific Oceans

[1] The difference between nitrate δ15N and δ18O, or Δ(15–18), is sensitive to organic matter remineralization and tracks the modification of nitrate as it passes from the deep Pacific Ocean, through the Southern Ocean surface, and into the intermediate-depth Pacific. Circumpolar Deep Water (CDW) is upwelled with a nitrate Δ(15–18) of +3.0‰ and appears unaltered by nitrate assimilation in the Antarctic surface. However, within Subantarctic Mode Water (SAMW)—the primary conduit of deep-sea nutrients to the lower latitudes—nitrate δ15N and δ18O are both higher than CDW, while nitrate Δ(15–18) is as low as +1.5‰. The lower SAMW nitrate Δ(15–18) results from the production of low δ15N organic matter during partial assimilation of the surface nitrate pool followed by its sinking and remineralization back to nitrate, lowering the δ15N of thermocline nitrate more than its δ18O. As SAMW flows toward the lower latitudes, nitrate Δ(15–18) is expected and observed to increase because complete surface ocean nitrate consumption produces sinking nitrogen with a δ15N similar to that of the nitrate supplied from below such that remineralization lowers the δ18O of nitrate, but not its δ15N. Nitrate Δ(15–18) is also used to estimate a surprisingly high low latitude sinking nitrogen δ15N of ≈9.0‰, suggesting a major effect of 14NO3− loss by tropical denitrification on middepth nitrate basin-wide. The remineralization of this high sinking nitrogen δ15N increases nitrate Δ(15–18) as deep as the southward-moving Pacific Deep Water, which supplies CDW. This relatively high Δ(15–18) is then lowered to the observed CDW value by the remineralization of Southern Ocean sinking nitrogen with a low δ15N.