Uptake Of NO3 Research Articles

Nitrogen isotope gradients off Peru and Ecuador related to upwelling, productivity, nutrient uptake and oxygen deficiency

We present new nitrogen isotope data from the water column and surface sediments for paleo–proxy validation collected along the Peruvian and Ecuadorian margins between 1°N and 18°S. Productivity proxies in the bulk sediment (organic carbon, total nitrogen, biogenic opal, C37 alkenone concentrations) and 15N/14N ratios were measured at more than 80 locations within and outside the present-day Peruvian oxygen minimum zone (OMZ). Microbial N-loss to N2 in subsurface waters under O2 deficient conditions leaves a characteristic 15N-enriched signal in underlying sediments. We find that phytoplankton nutrient uptake in surface waters within the high nutrient, low chlorophyll (HNLC) regions of the Peruvian upwelling system influences the sedimentary signal as well. How the δ15Nsed signal is linked to these processes is studied by comparing core-top values to the 15N/14N of nitrate and nitrite (δ15NNOx) in the upper 200 m of the water column. Between 1°N and 10°S, subsurface O2 is still high enough to suppress N-loss keeping δ15NNOx values relatively low in the subsurface waters. However δ15NNOx values increase toward the surface due to partial nitrate utilization in the photic zone in this HNLC portion of the system. δ15Nsed is consistently lower than the isotopic signature of upwelled NO3−, likely due to the corresponding production of 15N depleted organic matter. Between 10°S and 15°S, the current position of perennial upwelling cells, HNLC conditions are relaxed and biological production and near-surface phytoplankton uptake of upwelled NO3− are most intense. In addition, subsurface O2 concentration decreases to levels sufficient for N-loss by denitrification and/or anammox, resulting in elevated subsurface δ15NNOx values in the source waters for coastal upwelling. Increasingly higher production southward is reflected by various productivity proxies in the sediments, while the north–south gradient towards stronger surface NO3− utilization and subsurface N-loss is reflected in the surface sediment 15N/14N ratios. South of 10°S, δ15Nsed is lower than maximum water column δ15NNOx values most likely because only a portion of the upwelled water originates from the depths where highest δ15NNOx values prevail. Though the enrichment of δ15NNOx in the subsurface waters is unambiguously reflected in δ15Nsed values, the magnitude of δ15Nsed enrichment depends on both the depth of upwelled waters and high subsurface δ15NNOx values produce by N-loss. Overall, the degree of N-loss influencing subsurface δ15NNOx values, the depth origin of upwelled waters, and the degree of near-surface nitrate utilization under HNLC conditions should be considered for the interpretation of paleo δ15Nsed records from the Peruvian oxygen minimum zone.

Thaw depth determines reaction and transport of inorganic nitrogen in valley bottom permafrost soils

Nitrate (NO3 (-) ) export coupled with high inorganic nitrogen (N) concentrations in Alaskan streams suggests that N cycles of permafrost-influenced ecosystems are more open than expected for N-limited ecosystems. We tested the hypothesis that soil thaw depth governs inorganic N retention and removal in soils due to vertical patterns in the dominant N transformation pathways. Using an in situ, push-pull method, we estimated rates of inorganic N uptake and denitrification during snow melt, summer, and autumn, as depth of soil-stream flowpaths increased in the valley bottom of an arctic and a boreal catchment. Net NO3 (-) uptake declined sharply from snow melt to summer and decreased as a nonlinear function of thaw depth. Peak denitrification rate occurred during snow melt at the arctic site, in summer at the boreal site, and declined as a nonlinear function of thaw depth across both sites. Seasonal patterns in ammonium (NH4 (+) ) uptake were not significant, but low rates during the peak growing season suggest uptake that is balanced by mineralization. Despite rapid rates of hydrologic transport during snow melt runoff, rates of uptake and removal of inorganic N tended to exceed water residence time during snow melt, indicating potential for retention of N in valley bottom soils when flowpaths are shallow. Decreased reaction rates relative to water residence time in subsequent seasons suggest greater export of inorganic N as the soil-stream flowpath deepens due to thawing soils. Using seasonal thaw as a proxy for longer term deepening of the thaw layer caused by climate warming and permafrost degradation, these results suggest increasing potential for export of inorganic N from permafrost-influenced soils to streams.

Multiphase Chemical Kinetics of NO3 Radicals Reacting with Organic Aerosol Components from Biomass Burning

Multiphase reactions with nitrate radicals are among the most important chemical aging processes of organic aerosol particles in the atmosphere especially at nighttime. Reactive uptake of NO(3) by organic compounds has been observed in a number of studies, but the pathways of mass transport and chemical reaction remained unclear. Here we apply kinetic flux models to experimental NO(3) exposure studies. The model accounts for gas phase diffusion within a cylindrical flow tube, reversible adsorption of NO(3), surface-bulk exchange, bulk diffusion, and chemical reactions from the gas-condensed phase interface to the bulk. We resolve the relative contributions of surface and bulk reactions to the uptake of NO(3) by levoglucosan and abietic acid, which serve as surrogates and molecular markers of biomass burning aerosol (BBA). Applying the kinetic flux model, we provide the first estimate of the diffusion coefficient of NO(3) in amorphous solid organic matrices (10(-8)-10(-7) cm(2) s(-1)) and show that molecular markers are well-conserved in the bulk of solid BBA particles but undergo rapid degradation upon deliquescence/liquefaction at high relative humidity, indicating that the observed concentrations and subsequent apportionment of the biomass burning source could be significantly underestimated.

Multiphase Chemical Kinetics of the Nitration of Aerosolized Protein by Ozone and Nitrogen Dioxide

Proteins contained in pollen and other biological particles are nitrated by ozone and nitrogen dioxide in polluted air. The nitration can enhance the allergenic potential of proteins, which may contribute to the increasing prevalence of allergic diseases. The reactive uptake of NO(2) by aerosolized protein (bovine serum albumin) was investigated in an aerosol flow tube using the short-lived radioactive tracer (13)N. In the absence of O(3), the NO(2) uptake coefficient was below detection limit (γ(NO2) < 10(-6)), but with 20-160 ppb O(3) γ(NO2) increased from ~10(-6) to ~10(-4). Using the kinetic multilayer model of surface and bulk chemistry (KM-SUB), the observed time and concentration dependence can be well reproduced by a multiphase chemical mechanism involving ozone-generated reactive oxygen intermediates (ROIs), but not by NO(3) radicals formed in the gas phase. Product studies show the formation of protein dimers, suggesting that the ROIs are phenoxy radical derivatives of the amino acid tyrosine (tyrosyl radicals) which are also involved in physiological protein nitration processes. Our results imply that proteins on the surface of aerosol particles undergo rapid nitration in polluted air, while the rate of nitration in bulk material may be low depending on phase state and surface-to-volume ratio.

The effect of inorganic nitrogen speciation on primary production in the San Francisco Estuary

We describe the results of a series of 96-h enclosure experiments conducted using water from stations in the northern San Francisco Estuary (SFE) along a gradient in ammonium (NH4) and nitrate (NO3) concentrations. Using dual-labeled 13C/15N tracers, we followed the timing and sequence of primary (carbon, C) production and phytoplankton nitrogen (N) use during experimental phytoplankton blooms. Our results show that diatoms consistently drive the phytoplankton blooms in the enclosures. By tracing both C and N uptake we provide clear evidence that high rates of C uptake are linked to phytoplankton NO3, and not NH4, use. Results from kinetics experiments demonstrated higher specific uptake rates (VMAx) for NO3 compared to NH4 in the SFE. Finally, dissolved inorganic carbon and nutrient drawdown ratios in the enclosures from the chronically high NH4 regions of the SFE were substantially lower than predicted from the Redfield ratio, suggesting suppressed C uptake, in relation to other elemental uptake. Our conceptual model of the DIN interactions that lead to higher primary production and phytoplankton blooms in the SFE suggests that higher rates of primary production that accompany phytoplankton NO3 uptake are sufficient to outpace phytoplankton losses, leading to blooms, compared to the lower rates associated with NH4 uptake (only 20% of that based upon NO3). Historical changes in wastewater practices have increased the proportion of NH4 to the DIN pool in the SFE leading to reduced access to NO3 by phytoplankton. This may help to explain some of the reduced primary production and phytoplankton biomass observed there since the 1970s.

Model organic compounds differ in priming effects on alkalinity release in soils through carbon and nitrogen mineralisation

The influence of organic matter and its cycling on soil pH change is still unclear. This study investigated the effect of organic compounds on carbon and nitrogen dynamics and their relationship with pH changes in two soils differing in initial soil pH (Podosol of pH 4.5 and Tenosol of pH 6.2). Seven organic compounds representing common compounds in decomposing plant residues or root exudates were added to the soils and incubated for 60 d. The largest cumulative soil respiration occurred when glucose, malic acid and citric acid were added. In addition, the Tenosol had the greater respiration compared to the Podosol. The addition of organic acids (acetic, malic, citric, ferulic and benzoic acid) instantly decreased soil pH due to the dissociation of H+ from the acids. The pH of both soils was then restored over time, which was positively correlated with decomposition % of these compounds. The pH of the Tenosol amended with all the organic acids and of the Podosol with malic acid exceeded that of the control, and net alkalization occurred, with the degree of alkalization being greater with malic and citric acid. Adding organic acids to the Tenosol generally increased NH4 concentrations but decreased NO3 concentrations. The addition of glucose decreased pH in Podosol but slightly increased it in the Tenosol. The addition of glucosamine hydrochloride decreased pH due to significant nitrification. The results suggest that the addition of organic acids stimulates microbial NO3 uptake, and ammonification and decomposition of indigenous soil organic matter, resulting in a priming effect on alkalinity release, and that the degree of the priming effect is influenced by the type of organic acid and initial soil pH.

Assimilation and regeneration of inorganic nitrogen in a coastal upwelling system: ammonium and nitrate utilization

The main processes involved in nitrogen cycling (as NO3 � and NH4 + assimilation and their re- generation) were studied using N-tracer experiments in the coastal upwelling system off central Chile (36°S). The study area shows seasonal surface fertil- ization and the development of subsurface hypoxia in austral spring and summer. The rates of NO3 � uptake during active upwelling were 5 times higher than in non-upwelling seasons. Uptake of NH4 + was almost half that of NO3 � uptake rates during upwelling peri- ods, and similar to NO3 � uptake rates in the absence of upwelling. Nitrification experiments showed higher rates during active, compared to non-active, upwelling seasons. NH4 + oxidation was coupled with NO2 � oxi- dation in near bottom (suboxic) waters, while in the oxycline (hypoxic water), total fluxes of NO3 � regener- ation via nitrification resulted from higher activity of NO2 � oxidation compared to NH4 + oxidation. On the other hand, archaeal NH4 + oxidation had the potential for processing a large fraction of NH4 + and could therefore co-occur with bacterial NO2 � oxidation. NH4 + utilization in this coastal up welling is thus in the same range as NO3 � assimilation. NH4 + oxidation is af- fected by oxygen concentration in the water column, leading to occasional decoupling of NH4 + and NO2 � oxidation, particularly at the oxycline. This study gives preliminary evidence of the importance of archaeal- bacterial interactions in the nitrification process and highlights the role of ammonium in fueling annual primary production in coastal upwelling systems.

SGC in SMC acts as Nitrite Reductase leading to NO formation

Several heme proteins including hemoglobin and nitric oxide synthase (NOS) have been documented to serve as nitrite reductase under anoxia. Since sGC is also a heme protein and an important signaling biomolecule, we hypothesized that it may function as nitrite reductase in cultured smooth muscle cells. Millimolar levels of nitrite were required to detect cellular NO as measured by NO specific fluorescent dye (CuFL2E). However, in chemiluminescence analysis of cell lysates, addition of 400–600 μM NO2− was sufficient to detect measurable NO formation under anoxic conditions. This NO formation was inhibited with ODQ showing sGC in the cell lysate as the source of nitrite reduction. CO is a known activator of sGC and thus CO donor (50 μM CORM‐2) significantly activated nitrite reduction to NO. The cellular uptake of NO2− was studied using the AE‐1 inhibitor, DIDS and intracellular NO2− was measured by tri‐iodide assay. DIDS (300 μM) significantly inhibited NO2− uptake and showing that NO2− uses AE‐1 transport mechanism. These results demonstrate that sGC mediates NO2− reduction to form NO and that the AE‐1 channel is one of the pathways for NO2− entry into SMC. This work was supported by NIH grant SC1HL095101.

Iron from melting glaciers fuels phytoplankton blooms in the Amundsen Sea (Southern Ocean): Phytoplankton characteristics and productivity

The phytoplankton community composition and productivity in waters of the Amundsen Sea and surrounding sea ice zone were characterized with respect to iron (Fe) input from melting glaciers. High Fe input from glaciers such as the Pine Island Glacier, and the Dotson and Crosson ice shelves resulted in dense phytoplankton blooms in surface waters of Pine Island Bay, Pine Island Polynya, and Amundsen Polynya. Phytoplankton biomass distribution was the opposite of the distribution of dissolved Fe (DFe), confirming the uptake of glacial DFe in surface waters by phytoplankton. Phytoplankton biomass in the polynyas ranged from 0.6 to 14μgChl aL−1, with lower biomass at glacier sites where strong upwelling of Modified Circumpolar Deep Water from beneath glacier tongues was observed. Phytoplankton blooms in the polynyas were dominated by the haptophyte Phaeocystis antarctica, whereas the phytoplankton community in the sea ice zone was a mix of P. antarctica and diatoms, resembling the species distribution in the Ross Sea. Water column productivity based on photosynthesis versus irradiance characteristics averaged 3.00gCm−2d−1 in polynya sites, which was approximately twice as high as in the sea ice zone. The highest water column productivity was observed in the Pine Island Polynya, where both thermally and salinity stratified waters resulted in a shallow surface mixed layer with high phytoplankton biomass. In contrast, new production based on NO3 uptake was similar between different polynya sites, where a deeper UML in the weakly, thermally stratified Pine Island Bay resulted in deeper NO3 removal, thereby offsetting the lower productivity at the surface. These are the first in situ observations that confirm satellite observations of high phytoplankton biomass and productivity in the Amundsen Sea. Moreover, the high phytoplankton productivity as a result of glacial input of DFe is the first evidence that melting glaciers have the potential to increase phytoplankton productivity and thereby CO2 uptake, resulting in a small negative feedback to anthropogenic CO2 emissions.

Phytoplankton biomass and pigment responses to Fe amendments in the Pine Island and Amundsen polynyas

Nutrient addition experiments were performed during the austral summer in the Amundsen Sea (Southern Ocean) to investigate the availability of organically bound iron (Fe) to the phytoplankton communities, as well as assess their response to Fe amendment. Changes in autotrophic biomass, pigment concentration, maximum photochemical efficiency of photosystem II, and nutrient concentration were recorded in response to the addition of dissolved free Fe (DFe) and Fe bound to different model ligands. Analysis of pigment concentrations indicated that the autotrophic community was dominated by the prymnesiophyte Phaeocystis antarctica throughout most of the Amundsen Sea, although diatoms dominated in two experiments conducted in the marginal ice zone. Few significant differences in bulk community biomass (particulate organic carbon, nitrogen, and chlorophyll a) were observed, relative to the controls, in treatments with Fe added alone or bound to the ligand phytic acid. In contrast, when Fe was bound to the ligand desferrioxamine B (DFB), decreases in the bulk biomass indices were observed. The concentration of the diatom accessory pigment fucoxanthin showed little response to Fe additions, while the concentration of the P. antarctica-specific pigment, 19′-hexanoyloxyfucoxanthin (19′-hex), decreased when Fe was added alone or bound to the model ligands. Lastly, differences in the nitrate:phosphate (NO3−:PO43−) utilization ratio were observed between the Fe-amended treatments, with Fe bound to DFB resulting in the lowest NO3−:PO43− uptake ratios (∼10) and the remaining Fe treatments having higher NO3−:PO43− uptake ratios (∼17). The data are discussed with respect to glacial inputs of Fe in the Amundsen Sea and the bioavailability of Fe. We suggest that the previously observed high NO3−:PO43− utilization ratio of P. antarctica is a consequence of its production of dissolved organic matter that acts as ligands and increases the bioavailability of Fe, thereby stimulating the uptake of NO3−.

MUTANTS OF FICUS PUMILA PRODUCED BY ION BEAM IRRADIATION WITH AN IMPROVED ABILITY TO UPTAKE AND ASSIMILATE ATMOSPHERIC NITROGEN DIOXIDE

Production of novel mutants with a high ability to mitigate pollutants is important for phytoremediation. We investigated the use of ion beam irradiation to produce mutants of Ficus pumila L. with an improved ability to mitigate atmospheric nitrogen dioxide (NO2). More than 25,000 shoot explants were irradiated with an ion beam (12C5+, 12C6+, or 4He2+), from which 263 independent plant lines were obtained. The plants were analyzed for NO2 uptake by fumigation with 1 ppm 15N-labeled NO2 for 8 h in light, followed by mass spectrometric analysis. The mean NO2 uptake values of each of the 263 lines differed over a 110-fold range. Propagation was attempted using cuttings from 44 lines showing the greatest NO2 uptake; in total, 15 lines were propagated. Two of the 15 lines showed a mean NO2 uptake 1.7- to 1.8-fold greater than that of the wild-type. This increase in NO2 uptake was heritable in both lines; their progenies showed a significantly greater ability to take up and assimilate NO2 than did the wild-type. RAPD analysis demonstrated DNA variation between the progeny plants and the wild type, suggesting that the progeny were true mutants. These mutants of F. pumila may prove useful in mitigating atmospheric NO2.

Sulfur dioxide and nitrogen dioxide adsorption on zinc oxide and zirconium hydroxide nanoparticles and the effect on photoluminescence

Nanoparticulate zinc oxide and micron-size zirconium hydroxide powders have been exposed to sulfur dioxide and nitrogen dioxide by flowing the gases, diluted with nitrogen, over powder samples. X-ray photoelectron spectroscopy (XPS), Raman spectroscopy, and thermogravimetric analysis (TGA) indicate strongly bound, chemisorbed SO3 and NO3 surface species. Two pre-treatments of the nanoparticulate ZnO samples prior to gas exposure have been investigated: (1) drying overnight in a vacuum oven and (2) hydrating the samples by placing them overnight in water-saturated air. A dramatic difference in reactivity of ZnO is observed, with approximately two-fold and ten-fold greater uptake of NO2 and SO2, respectively, measured by XPS for the hydrated samples relative to the dried ones. Transmission electron microscopy (TEM) demonstrates that the greater uptake arises from a morphology change in the case of the hydrated samples. For zirconium hydroxide, no morphology change is observed for hydrated samples, and SO4 (ads), in addition to SO3 (ads), is indicated by XPS. ZnO and Zr(OH)4 both exhibit photoluminescence (PL) spectra, with peak intensities that change dramatically due to hydration and subsequent exposure to SO2 and NO2 gases. Dosing of the powders with these gases effectively reverts the PL spectra to those corresponding to less hydration.

N2O flux from plant-soil systems in polar deserts switch between sources and sinks under different light conditions

Production and consumption of greenhouse gases such as CO2, CH4 and N2O are key factors driving climate change. While CO2 sinks are commonly reported and the mechanisms relatively well understood, N2O sinks have often been overlooked and the driving factors for these sinks are poorly understood. We examined CO2, CH4 and N2O flux in three High Arctic polar deserts under both light (measured in transparent chambers) and dark (measured in opaque chambers) conditions. We further examined if differences in soil moisture, evapotranspiration, Photosynthetically Active Radiation (PAR), and/or plant communities were driving gas fluxes measured in transparent and opaque chambers at each of our sites. Nitrous oxide sinks were found at all of our sites suggesting that N2O uptake can occur under extreme polar desert conditions, with relatively low soil moisture, soil temperature and limited soil N. Fluxes of CO2 and N2O switched from sources under dark conditions to sinks under light conditions, while CH4 fluxes at our sites were not affected by light conditions. Neither evapotranspiration nor PAR were significantly correlated with CO2 or N2O flux, however, soil moisture was significantly correlated with both gas fluxes. The relationship between soil moisture and N2O flux was different under light and dark conditions, suggesting that there are other factors, in addition to moisture, driving N2O sinks. We found significant differences in N2O and CO2 flux between plant communities under both light and dark conditions and observed individual communities that shifted between sources and sinks depending on light conditions. Failure of many studies to include plant-mediated N2O flux, as well as, N2O soil sinks may account for the currently unbalanced global N2O budget.

Experimental and kinetic modeling study of NO oxidation: Comparison of Fe and Cu-zeolite catalysts

NO oxidation is considered as the rate determining step of the standard selective catalytic reduction (SCR) reaction (NO+NH3+O2) on metal-exchanged zeolite catalysts. A comprehensive experimental and kinetic modeling study of NO oxidation reaction was carried out and a comparison made between Fe-ZSM-5, Cu-ZSM-5 and recently developed small pore Cu-chabazite catalysts. Steady state integral measurements of NO oxidation show Fe-zeolite to be a more active NO oxidation catalyst than Cu-zeolite. Both NO2 temperature programmed desorption and decomposition experiments show that the NO2 is more strongly bound on the Cu-chabazite compared to Fe-ZSM-5. Increasing amounts of NO2 in the NOx feed decrease the NO oxidation activity on both the catalysts. Differential kinetic studies show a positive order rate dependence of NO oxidation reaction with respect to both NO (∼1) and O2 (∼0.5) while showing a negative order dependence with respect to NO2 (−0.5 to −1), which confirms that the NO2 inhibits the NO oxidation reaction. NO2 inhibition on NO oxidation was found to be more severe on Cu-chabazite (∼ −1 order) compared to Fe-ZSM-5 (∼ −0.5 order). The overall activation energy of NO oxidation was found to be 39kJ/mol on Fe-ZSM-5 compared to 56kJ/mol on Cu-chabazite. The presence of feed water strongly inhibits the NO oxidation reaction. Transient NO2 uptake experiments reveal nitrate formation on both catalysts. Catalysts pretreatments by different oxidizing/reducing agents (O2, NO2, H2 and NH3) did not affect steady state NO oxidation results. Experiments with monolith catalysts having the same washcoat volume but different washcoat thicknesses rule out the presence of washcoat diffusion limitations for NO oxidation. A global kinetic model is developed which accurately predicts the experimentally observed reaction orders, NO conversions and NO2 and water inhibition effects for a wide temperature range and different feed conditions. The kinetic model accounts for the experimentally observed instability of nitrates in the presence of feed NO and shows that the steady state NO oxidation data can be predicted with a global kinetic model assuming the surface reaction between adsorbed O atoms and gaseous NO as the rate determining step.

Air-Pollutant-Philic Plants for Air Remediation

In this communication, we review our work over two decades on air-pollutant-philic plants that can grow with air pollutants as the sole nutrient source. We believe that such plants are instrumental in mitigating air pollution. Our target air pollutant has been atmospheric nitrogen dioxide (NO2), and our work on this subject has consisted of three parts: Variation in plants’ abilities to mitigate air pollutants among naturally occurring plants, genetic improvement of plants’ abilities to mitigate air pollutants, and the plant vitalization effect of NO2. So far, an estimation of the half-life of nitrogen derived from NO2 uptake in plants belonging to the 217 taxa studied to date has shown no plants to be naturally occurring air-pollutant-philic. However, we found that an enormous difference exists in plants’ ability to uptake and assimilate atmospheric NO2. Future studies on the causes of this process may provide an important clue to aid the genetic production of plants that are effectively air-pollutant-philic. Both genetic engineering of the genes involved in the primary nitrate metabolism and genetic modification by ion-beam irradiation failed to make plants air-pollutant-philic, but mutants obtained in these studies will prove useful in revealing those genes critical in doing so. During our study on air-pollutant-philic plants, we unexpectedly discovered that prolonged exposure of plants to a sufficient level of NO2 activates the uptake and metabolism of nutrients that fuel plant growth and development. We named this phenomenon “the plant vitalization effect of NO2” (PVEON). Investigations into the mechanisms and genes involved in PVEON will provide an important clue to making plants air-pollutant-philic in the future.

Nitrous Oxide Emission from Riparian Buffers in Relation to Vegetation and Flood Frequency

The nitrate (NO(3)(-)) removal capacity of riparian zones is well documented, but information is lacking with regard to N(2)O emission from riparian ecosystems and factors controlling temporal dynamics of this potent greenhouse gas. We monitored N(2)O fluxes (static chambers) and measured denitrification (C(2)H(2) block using soil cores) at six riparian sites along a fourth-order stretch of the White River (Indiana, USA) to assess the effect of flood regime, vegetation type, and forest maturity on these processes. The study sites included shrub/grass, aggrading (<15 yr-old), and mature (>80 yr) forests that were flooded either frequently (more than four to six times per year), occasionally (two to three times per year), or rarely (every 20 yr). While the effect of forest maturity and vegetation type (0.52 and 0.65 mg N(2)O-m(-2) d(-1) in adjacent grassed and forested sites) was not significant, analysis of variance (ANOVA) revealed a significant effect ( < 0.01) of flood regime on N(2)O emission. Among the mature forests, mean N(2)O flux was in this order: rarely flooded (0.33) < occasionally flooded (0.99) < frequently flooded (1.72). Large pulses of N(2)O emission (up to 80 mg N(2)O-m(-2) d(-1)) occurred after flood events, but the magnitude of the flux enhancement varied with flood event, being higher after short-duration than after long-duration floods. This pattern was consistent with the inverse relationship between soil moisture and mole fraction of N(2)O, and instances of N(2)O uptake near the river margin after flood events. These results highlight the complexity of N(2)O dynamics in riparian zones and suggest that detailed flood analysis (frequency and duration) is required to determine the contribution of riparian ecosystems to regional N(2)O budget.

Fluxes and production pathways of nitrous oxide in different types of tropical forest soils in Thailand

Nitrous oxide (N2O) emissions from the soil surface of five different forest types in Thailand were measured using the closed chamber method. Soil samples were also taken to study the N2O production pathways. The monthly average emissions (±SD, n = 12) of N2O from dry evergreen forest (DEF), hill evergreen forest (HEF), moist evergreen forest (MEF), mixed deciduous forest (MDF) and acacia reforestation (ARF) were 13.0 ± 8.2, 5.7 ± 7.1, 1.2 ± 12.1, 7.3 ± 8.5 and 16.7 ± 9.2 µg N m−2 h−1, respectively. Large seasonal variations in fluxes were observed. Emission was relatively higher during the wet season than during the dry season, indicating that soil moisture and denitrification were probably the main controlling factors. Net N2O uptake was also observed occasionally. Laboratory studies were conducted to further investigate the influence of moisture and the N2O production pathways. Production rates at 30% water holding capacity (WHC) were 3.9 ± 0.2, 0.5 ± 0.06 and 0.87 ± 0.01 ng N2O-nitrogen (N) g-dw−1day−1 in DEF, HEF and MEF respectively. At 60% WHC, N2O production rates in DEF, HEF and MEF soils increased by factors of 68, 9 and 502, respectively. Denitrification was found to be the main N2O production pathway in these soils except in MEF.

Nitrogen dioxide (NO2) uptake by vegetation controlled by atmospheric concentrations and plant stomatal aperture

Nitrogen dioxide (NO2) exchange between the atmosphere and five European tree species was investigated in the laboratory using a dynamic branch enclosure system (consisting of two cuvettes) and a highly specific NO2 analyzer. NO2 measurements were performed with a sensitive gas phase chemiluminescence NO detector combined with a NO2 specific (photolytic) converter, both from Eco-Physics (Switzerland). This highly specific detection system excluded bias from other nitrogen compounds. Investigations were performed at two light intensities (Photosynthetic Active Radiation, PAR, 450 and 900 μmol m−2 s−1) and NO2 concentrations between 0 and 5 ppb. Ambient parameters (air temperature and relative humidity) were held constant. The data showed dominant NO2 uptake by the respective tree species under all conditions. The results did not confirm the existence of a compensation point within a 95% confidence level, though we cannot completely exclude emission of NO2 under very low atmospheric concentrations. Induced stomatal stricture, or total closure, by changing light conditions, as well as by application of the plant hormone ABA (Abscisic Acid) caused a corresponding decrease of NO2 uptake. No loss of NO2 to plant surfaces was observed under stomatal closure and species dependent differences in uptake rates could be clearly related to stomatal behavior.

Kinetics and equilibrium studies on the treatment of nitric acid with Mg–Al oxide obtained by thermal decomposition of [formula omitted]-intercalated Mg–Al layered double hydroxide

Mg–Al oxide obtained by thermal decomposition of NO3--intercalated Mg–Al layered double hydroxide (NO3·Mg–Al LDH) was found to treat HNO3, acting as both a neutralizer and fixative for NO3-. The degree of NO3- removal increased with time, Mg–Al oxide quantity, and temperature. The NO3- removal could be represented by a first-order reaction. The apparent activation energy was 52.9kJmol−1, confirming that NO3- removal by Mg–Al oxide proceeded under chemical reaction control. Furthermore, the adsorption of NO3- on Mg–Al oxide could be expressed by a Langmuir-type adsorption isotherm. The maximum adsorption amount and equilibrium adsorption constant were 3.8mmolg−1 and 1.33, respectively. The Gibbs free energy change was −18kJmol−1, confirming that the uptake of NO3- from HNO3 by Mg–Al oxide proceeded spontaneously.

NO2-drived NO3– metabolism in leaves

Atmospheric nitrogen dioxide (NO2) can be dry/wet deposited into plant leaves mainly through stomata, with a small fraction of cuticle deposition. Plants in nature present a wide variation in NO2 assimilation and the resistance. The ability of leaf NO2 uptake and efficient remobilization of NO2-derived NO3/NO2 is important for normal plant growth and to withstand nitrogen deprivation. Moreover, leafand root-derived NO3 metabolisms have both links and differences. This review emphasizes on the fates of NO2-drived NO3, including i) the assimilation into amino acids, ii) accumulation in vacuoles, and iii) reemission by NOx (NO and/or NO2), and discuss metabolic differences of NO2-drived and root-derived NO3. Special attention is drawn to NOx evolution in apoplast and symplast of leaves and its control to stomatal dynamics. Moreover, further progress is proposed to get a better understanding of the dynamic uptake of NO2 and NO3 transporters in leaves.