Interaction of benthic microalgae and macrofauna in the control of benthic metabolism, nutrient fluxes and denitrification in a shallow sub-tropical coastal embayment (western Moreton Bay, Australia)

Benthic biogeochemistry and macrofauna were investigated six times over 1 year in a shallow sub-tropical embayment. Benthic fluxes of oxygen (annual mean ?918 μmol O₂ m^?2 h^?1), ammonium (NH₄ ⁺), nitrate (NO₃ ^?), dissolved organic nitrogen, dinitrogen gas (N₂), and dissolved inorganic phosphorus were positively related to OM supply (N mineralisation) and inversely related to benthic light (N assimilation). Ammonium (NH₄ ⁺), NO₃ ^? and N₂ fluxes (annual means +14.6, +15.9 and 44.6 μmol N m^?2 h^?1) accounted for 14, 16 and 53 % of the annual benthic N remineralisation respectively. Denitrification was dominated by coupled nitrification–denitrification throughout the study. Potential assimilation of nitrogen by benthic microalgae (BMA) accounted for between 1 and 30 % of remineralised N, and was greatest during winter when bottom light was higher. Macrofauna biomass tended to be highest at intermediate benthic respiration rates (?1,000 μmol O₂ m^?2 h^?1), and appeared to become limited as respiration increased above this point. While bioturbation did not significantly affect net fluxes, macrofauna biomass was correlated with increased light rates of NH₄ ⁺ flux which may have masked reductions in NH₄ ⁺ flux associated with BMA assimilation during the light. Peaks in net N₂ fluxes at intermediate respiration rates are suggested to be associated with the stimulation of potential denitrification sites due to bioturbation by burrowing macrofauna. NO₃ ^? fluxes suggest that nitrification was not significantly limited within respiration range measured during this study, however comparisons with other parts of Moreton Bay suggest that limitation of coupled nitrification–denitrification may occur in sub-tropical systems at respiration rates exceeding ?1,500 μmol O₂ m^?2 h^?1.     

Denitrification, Dissimilatory Reduction of Nitrate to Ammonium, and Nitrification in a Bioturbated Estuarine Sediment as Measured with 15N and Microsensor Techniques

Nitrogen and oxygen transformations were studied in a bioturbated (reworked by animals) estuarine sediment (Norsminde Fjord, Denmark) by using a combination of ¹⁵N isotope (NO₃^-), specific inhibitor (C₂H₂), and microsensor (N₂O and O₂) techniques in a continuous-flow core system. The estuarine water was NO₃^- rich (125 to 600 μM), and NO₃^- was consistently taken up by the sediment on the four occasions studied. Total NO₃^- uptake (3.6 to 34.0 mmol of N m^-2 day^-1) corresponded closely to N₂ production (denitrification) during the experimental steady state, which indicated that dissimilatory, as well as assimilatory, NO₃^- reduction to NH₄⁺ was insignificant. When C₂H₂ was applied in the flow system, denitrification measured as N₂O production was often less (58 to 100%) than the NO₃^- uptake because of incomplete inhibition of N₂O reduction. The NO₃^- formed by nitrification and not immediately denitrified but released to the overlying water, uncoupled nitrification, was calculated both from ¹⁵NO₃^- dilution and from changes in NO₃^- uptake before and after C₂H₂ addition. These two approaches gave similar results, with rates ranging between 0 and 8.1 mmol of N m^-2 day^-1 on the four occasions. Attempts to measure total nitrification activity by the difference between NH₄⁺ fluxes before and after C₂H₂ addition failed because of non-steady-state NH₄⁺ fluxes. The vertical distribution of denitrification and oxygen consumption was studied by use of N₂O and O₂ microelectrodes. The N₂O profiles measured during the experimental steady state were often irregularly shaped, and the buildup of N₂O after C₂H₂ was added was much too fast to be described by a simple diffusion model. Only bioturbation by a dense population of infauna could explain these observations. This was corroborated by the relationship between diffusive and total fluxes, which showed that only 19 to 36 and 29 to 62% of the total O₂ uptake and denitrification, respectively, were due to diffusion-reaction processes at the regular sediment surface, excluding animal burrows.     

Current dynamics and mechanisms for the instability of a non-self-sustained glow discharge in nitrogen

The current dynamics in a non-self-sustained glow discharge in atmospheric-pressure nitrogen (with a small admixture of oxygen) at cryogenic and room temperatures is studied experimentally and theoretically. For the first time, the theoretical model incorporates the processes of the decomposition of O ₂ ⁺ ·N₂ and NO⁺·N₂ complex ions in collisions with vibrationally excited nitrogen molecules and the associative ionization reactions with the participation of excited nitrogen and oxygen atoms. The computation results agree quite satisfactorily with the experimental data on the current dynamics and the duration of the stable phase of a non-self-sustained discharge for various applied voltages. Even a small (0.01%) oxygen admixture is found to greatly affect the dynamics of the ion composition and the characteristic duration of the stable phase of a non-self-sustained discharge in atmospheric-pressure nitrogen.     

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.     

Preparative Procedures and Culture Medium Affect the Success of Cryostorage of Holostemma annulare Shoot Tips

Shoot tip cryopreservation of Holostemma annulare, an endangered medicinal plant was carried out using Murashige-Skoog (MS) medium containing mM NH⁺ ₄+NO^– ₃; 20.6+39.4 (MS-1), 2.6+18.8 (MS-2) or 0.0+18.8 (MS-3). The three media combinations were tested during four preparative procedures viz.: development of cultures; preconditioning of shoot tip cuttings; preculture of encapsulated shoot tips; and post-freeze recovery to understand the most critical phase of NH₄NO₃ sensitivity. MS-1 used during the initial three preparative steps supported 10.9–16.6% post-freeze recovery of cryopreserved shoot tips. Development of culture in MS-1 and subsequent passages (2nd, 3rd and 4th preparative steps) in MS-2 or MS-3 improved the recovery rate to 26.4–35.8%. MS-3 used throughout the steps favoured 38.5% recovery. Shoot tips from shoot cultures raised in MS-2 upon preconditioning in MS-2 or MS-3 and subsequent preculture of encapsulated shoot tips and post-freeze recovery culture in MS-3 showed maximum regeneration (55%). MS-2 used throughout the procedure supported 48% regeneration of cryopreserved shoot tips.     

Reaction of N-hydroxyguanidine with the ferrous-oxy state of a heme peroxidase cavity mutant: A model for the reactions of nitric oxide synthase

Yeast cytochrome c peroxidase was used to construct a model for the reactions catalyzed by the second cycle of nitric oxide synthase. The R48A/W191F mutant introduced a binding site for N-hydroxyguanidine near the distal heme face and removed the redox active Trp-191 radical site. Both the R48A and R48A/W191F mutants catalyzed the H₂O₂ dependent conversion of N-hydroxyguanidine to N-nitrosoguanidine. It is proposed that these reactions proceed by direct one-electron oxidation of NHG by the Fe⁺⁴O center of either Compound I (Fe⁺⁴O, porph⁺) or Compound ES (Fe⁺⁴O, Trp⁺). R48A/W191F formed a Fe⁺²O₂ complex upon photolysis of Fe⁺²CO in the presence of O₂, and N-hydroxyguanidine was observed to react with this species to produce products, distinct from N-nitrosoguanidine, that gave a positive Griess reaction for nitrate + nitrite, a positive Berthelot reaction for urea, and no evidence for formation of NO. It is proposed that HNO and urea are produced in analogy with reactions of nitric oxide synthase in the pterin-free state.     

Mechanism of nitrite oxidation and oxidoreductase systems inNitrobacter agilis

Chemiosmotic coupling mechanisms operate in the electron transfer reactions from: nitrite to O₂, NO₂ ^– to NAD⁺, ascorbate to O₂, NADH to O₂, and NADH to NO₃ ^–. The enzyme systems catalyzing these reactions are named NO₂ ^–:O₂ oxidoreductase, ATP-dependent NO₂ ^–:NAD⁺ oxidoreductase, ascorbate:O₂ oxidoreductase, NADH:O₂ oxidoreductase, and NADH:NO₃ ^– oxidoreductase, respectively. All of the oxidoreduction reactions are exergonic with the exception of the ATP-dependent NO₂ ^–:NAD⁺ oxidoreductase system, which involves reversed electron flow against the thermodynamic gradients. The mechanism for nitrite oxidation was found to be quite different from that of ascorbate oxidation; both systems were insensitive, however, to rotenone, amytal, antimycin A, and 2-n-heptyl 4-hydroxyquinolineN-oxide. These compounds, on the other hand, severely inhibited the electron transfer reactions catalyzed by NADH:O₂ oxidoreductase, NADH:NO₃ ^– oxidoreductase, and the ATP-dependent NO₂ ^–:NAD⁺ oxidoreductase, indicating a common pathway of electron transport in these oxidoreductase systems. Cyanide inhibited all systems except the NADH:NO₃ ^– oxidoredctase. The uncoupler carbonyl cyanide-m-chlorophenyl hydrazone strongly inhibited NO₂ ^–:O₂ oxidoreductase and ATP-dependent NO₂ ^–:NAD⁺ oxidoreductase, which indicates the involvement of energy-linked reactions in both systems; the uncoupler caused a marked stimulation of the NADH:O₂ oxidoreductase and NADH:NO₃ ^– oxidoreductase without affecting the ascorbate:O₂ oxidoreductase activities.     

Charge Balance in NO(3)-Fed Soybean: Estimation of K and Carboxylate Recirculation

Soybeans (Glycine max L. Merr., cv Kingsoy) were grown on media containing NO₃⁻ or urea. The enrichments of shoots in K⁺, NO₃⁻, and total reduced N (N_r), relative to that in Ca²⁺, were compared to the ratios K⁺/Ca²⁺,NO₃⁻/Ca²⁺, and N_r/Ca²⁺ in the xylem saps, to estimate the cycling of K⁺, and N_r. The net production of carboxylates (R⁻) was estimated from the difference between the sums of the main cations and inorganic anions. The estimate for shoots was compared to the theoretical production of R⁻ associated with NO₃⁻ assimilation in these organs, and the difference was attributed to export of R⁻ to roots. The net exchange rates of H⁺ and OH⁻ between the medium and roots were monitored. The shoots were the site of more than 90% of total NO₃⁻ reduction, and N_r was cycling through the plants at a high rate. Alkalinization of the medium by NO₃⁻-fed plants was interrupted by stem girdling, and not restored by glucose addition to the medium. It was concluded that the majority of the base excreted in NO₃⁻ medium originated from R⁻ produced in the shoots, and transported to the roots together with K⁺. As expected, cycling of K⁺ and reduced N was favoured by NO₃⁻ nutrition as compared to urea nutrition.     

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.     

Chemiluminescence of lucigenin by electrogenerated superoxide ions in aqueous solutions

The chemiluminescence reaction of lucigenin (Luc²⁺?2NO₃^?, N,N′‐dimethyl‐9,9′‐biacridinium dinitrate) at gold electrodes in dioxygen‐saturated alkaline aqueous solutions (pH 10) was investigated in detail by the use of electrochemical emission spectroscopy. We noted that both O₂ and Luc²⁺ are reduced on a gold electrode in aqueous solution of pH 10 in almost the same potential region. From this fact, we expected chemiluminescence based on a radical–radical coupling reaction of superoxide ion (O₂·^?) and one‐electron reduced form of Luc²⁺ (Luc·⁺, a radical cation). Chemiluminescence was actually observed in the potential range where O₂ and Luc²⁺ were simultaneously reduced at the electrodes. The effects were examined upon addition of enzymes, i.e. superoxide dismutase (SOD) and catalase, into the solution and the substitution of heavy water (D₂O) for light water (H₂O) as a solvent on the chemiluminescence. In the presence of native and active SOD, chemiluminescence was completely absent. On the other hand, chemiluminescence was observed, unchanged in the presence of either denatured and inert SOD or catalase. In addition, the amount of chemiluminescence in D₂O solution was about three times greater than that in H₂O solution. These results, together with cyclic voltammetric results, suggest that O₂·^? participates directly in the chemiluminescence but H₂O₂ does not, and the chemiluminescence results from the coupling reaction between O₂·^? and Luc^·+ under the present experimental conditions. These chemically unstable species, O₂·^? and Luc·⁺, are produced during the simultaneous electroreduction of O₂ and Luc²⁺. The coupling reaction between those radical species would lead to the formation of a dioxetane‐type intermediate and, finally, to chemiluminescence. The chemiluminescence reaction mechanism is discussed. Copyright © 2002 John Wiley & Sons, Ltd.     

Denitrification and Assimilatory Nitrate Reduction in Aquaspirillum magnetotacticum

Aquaspirillum magnetotacticum MS-1 grew microaerobically but not anaerobically with NO₃⁻ or NH₄⁺ as the sole nitrogen source. Nevertheless, cell yields varied directly with NO₃⁻ concentration under microaerobic conditions. Products of NO₃⁻ reduction included NH₄⁺, N₂O, NO, and N₂. NO₂⁻ and NH₂OH, each toxic to cells at 0.2 mM, were not detected as products of cells growing on NO₃⁻. NO₃⁻ reduction to NH₄⁺ was completely repressed by the addition of 2 mM NH₄⁺ to the growth medium, whereas NO₃⁻ reduction to N₂O or to N₂ was not. C₂H₂ completely inhibited N₂O reduction to N₂ by growing cells. These results indicate that A. magnetotacticum is a microaerophilic denitrifier that is versatile in its nitrogen metabolism, concomitantly reducing NO₃⁻ by assimilatory and dissimilatory means. This bacterium appears to be the first described denitrifier with an absolute requirement for O₂. The process of NO₃⁻ reduction appears well adapted for avoiding accumulation of several nitrogenous intermediates that are toxic to cells.     

The hemoglobin of the crossopterygian fish, Latimeria chalumnae (Smith). Subunit structure and oxygen equilibria

There are five oxidation-reduction states of horseradish peroxidase which are interconvertible. These states are ferrous, ferric, Compound II (ferryl), Compound I (primary compound of peroxidase and H₂O₂), and Compound III (oxy-ferrous). The presence of heme-linked ionization groups was confirmed in the ferrous enzyme by spectrophotometric and pH stat titration experiments. The values of pK were 5.87 for isoenzyme A and 7.17 for isoenzymes (B + C). The proton was released when the ferrous enzyme was oxidized to the ferric enzyme while the uptake of the proton occurred when the ferrous enzyme reacted with oxygen to form Compound III. The results could be explained by assuming that the heme-linked ionization group is in the vicinity of the sixth ligand and forms a stable hydrogen bond with the ligand.The measurements of uptake and release of protons in various reactions also yielded the following stoichiometries: Ferric peroxidase + H₂O₂ → Compound I, Compound I + e^? + H⁺ → Compound II, Compound II + e^? + H⁺ → ferric peroxidase, Compound II + H₂O₂ → Compound III, Compound III + 3e^? + 3H⁺ → ferric peroxidase.Based on the above stoichiometries and assuming the interaction between the sixth ligand and heme-linked ionization group of the protein, it was possible to picture simple models showing structural relations between five oxidation-reduction states of peroxidase. Tentative formulae are as follows: [Pr·Po·Fe-(II) $\text{\$}\text{?}$PrH⁺·Po·Fe(II)] is for the ferrous enzyme, Pr·Po·Fe(III)OH₂ for the ferric one, Pr·Po·Fe(IV)OH^? for Compound II, Pr(OH^?)·Po⁺·Fe(IV)OH^? for Compound I, and PrH⁺·Po·Fe(III)O₂^? for Compound III, in which Pr stands for protein and Po for porphyrin. And by Fe(IV)OH^?, for instance, is meant that OH^? is coordinated at the sixth position of the heme iron and the formal oxidation state of the iron is four.     

Dissimilatory Reduction of NO2− to NH4+ and N2O by a Soil Citrobacter sp.

Dissimilatory reduction of NO₂⁻ to N₂O and NH₄⁺ by a soil Citrobacter sp. was studied in an attempt to elucidate the physiological and ecological significance of N₂O production by this mechanism. In batch cultures with defined media, NO₂⁻ reduction to NH₄⁺ was favored by high glucose and low NO₃⁻ concentrations. Nitrous oxide production was greatest at high glucose and intermediate NO₃⁻ concentrations. With succinate as the energy source, little or no NO₂⁻ was reduced to NH₄⁺ but N₂O was produced. Resting cell suspensions reduced NO₂⁻ simultaneously to N₂O and free extracellular NH₄⁺. Chloramphenicol prevented the induction of N₂O-producing activity. The K_m for NO₂⁻ reduction to N₂O was estimated to be 0.9 mM NO₂⁻, yet the apparent K_m for overall NO₂⁻ reduction was considerably lower, no greater than 0.04 mM NO₂⁻. Activities for N₂O and NH₄⁺ production increased markedly after depletion of NO₃⁻ from the media. Amendment with NO₃⁻ inhibited N₂O and NH₄⁺ production by molybdate-grown cells but not by tungstate-grown cells. Sulfite inhibited production of NH₄⁺ but not of N₂O. In a related experiment, three Escherichia coli mutants lacking NADH-dependent nitrite reductase produced N₂O at rates equal to the wild type. These observations suggest that N₂O is produced enzymatically but not by the same enzyme system responsible for dissimilatory reduction of NO₂⁻ to NH₄⁺.     

Contemporary and pre-industrial global reactive nitrogen budgets

Increases and expansion of anthropogenic emissions of both oxidized nitrogen compounds, NO_x, and a reduced nitrogen compound, NH₃, have driven an increase in nitrogen deposition. We estimate global NO_x and NH₃ emissions and use a model of the global troposphere, MOGUNTIA, to examine the pre-industrial and contemporary quantities and spatial patterns of wet and dry NO_y and NH_x deposition. Pre-industrial wet plus dry NO_x and NH_x deposition was greatest for tropical ecosystems, related to soil emissions, biomass burning and lightning emissions. Contemporary NO_y+NH_x wet and dry deposition onto Northern Hemisphere (NH) temperate ecosystems averages more than four times that of preindustrial N deposition and far exceeds contemporary tropical N deposition. All temperate and tropical biomes receive more N via deposition today than pre-industrially. Comparison of contemporary wet deposition model estimates to measurements of wet deposition reveal that modeled and measured wet deposition for both NO ₃ ^– and NH ₄ ⁺ were quite similar over the U.S. Over Western Europe, the model tended to underestimate wet deposition of NO ₃ ^– and NH ₄ ⁺ but bulk deposition measurements were comparable to modeled total deposition. For the U.S. and Western Europe, we also estimated N emission and deposition budgets. In the U.S., estimated emissions exceed interpolated total deposition by 3-6 Tg N, suggesting that substantial N is transported offshore and/or the remote and rural location of the sites may fail to capture the deposition of urban emissions. In Europe, by contrast, interpolated total N deposition balances estimated emissions within the uncertainty of each.Abbreviations EMEP European Monitoring and Evaluation Program - GEIA Global Emissions Inventory Activity - NADP/NTN National Atmospheric Deposition Program/National Trends Network in the US - NH Northern Hemisphere - NH_x=NH₃+NH ₊ ⁴ NO_x=NO+NO₂ NO_y total odd nitrogen=NO_x+HNO₃+HONO+HO₂NO₂+NO₃+radical (NO₃ ^.)+Peroxyacetyl nitrates+N₂O₅+organic nitrates - SH Southern Hemisphere - Gg 10⁹ g - Tg 10¹² g     

Influence of carbon availability on the production of NO, N2O, N2 and CO2 by soil cores during anaerobic incubation

Net productions of permanent soil atmosphere gases (N₂, CO₂, O₂) and temporary gases (N₂O, NO) were monitored in soil cores using a non-interfering, fully automated measuring technique allowing highly time resolved measurements over prolonged periods. The influence of changes in available organic carbon on CO₂, N₂O, NO and N₂ production was studied by changing the soil carbon content through aerobic preincubations of different length, up to 21 days.The aerobic preincubation caused an increase in NO₃ ^- concentration and a decrease in available carbon content. Available carbon content dominated both CO₂ and total N gas (N₂+N₂O+NO) production during anaerobiosis. Both CO₂ and total N gas production rates decreased with increasing length of the previous aerobic preincubation, this in spite of the higher initial NO₃ ^- concentration.Total denitrification rates were closely related to the anaerobic CO₂ production rates. No relation was found between water soluble carbon content and total denitrification. The N₂O/N₂ ratio could be explained by an interaction of carbon availability, NO₃ ^- concentration and enzyme status. Net N₂O consumption was monitored. The balance between cumulative total N gas production and NO₃ ^- consumption varied according to the different treatments. Cumulative N₂O production exceeded cumulative N₂ production for 0 up to 5 days.     

Efficiency of respiration and energy requirements of N assimilation in roots of Pisum sativum

The function of alternative path respiration in roots was investigated in pea plants (Pisum sativum L. cv. Rondo). Plants were grown in symbiosis with Rhizobium leguminosarum (strain PF2), completely dependent on N₂ fixation, or non-nodulated, receiving nitrate or ammonium at the same rate as N₂ was fixed in symbiosis. Under these conditions, relative growth rates of plants grown with N₂, NO^-₃ or NH⁺₄ were the same. This facilitated interpretation of the effect of the N source on the efficiency of root respiration, as determined by the relative activity of the non-phosphorylating alternative path. The ‘wasteful’ oxidation of carbohydrate via this pathway was defined as the glucose equivalent of the difference between the amounts of ATP (mol O₂)^-1 produced in cytochrome and alternative path respiration. ‘Wasteful’ carbohydrate oxidation maximally amounted to 4% (N₂), 15% (NO^-₃) and 25% (NH⁺₄) of the daily carbohydrate oxidation in the roots. It is concluded that the ‘wasteful’ oxidation of carbohydrate via the alternative path is of minor importance for the adaptation of root respiratory metabolism to different energy requirements of N assimilation. The total carbohydrate import by roots fixing N₂ was ca 60 and 30% higher than the import by roots assimilating NO^-₃ or NH⁺₄, respectively. Two factors are shown to account for these differences: the high carbohydrate cost of N₂ fixation, and the small contribution (30%) of the roots to NO^-₃ reduction by the plant. The high carbohydrate requirements of roots fixing N₂ were met by higher rates of photosynthesis as compared with plants utilizing NO^-₃ or NH⁺₄.     

Nitrous Oxide Emission Associated with Autotrophic Ammonium Oxidation in Acid Coniferous Forest Soil

Aerobic N₂O production was studied in nitrifying humus from urea-fertilized pine forest soil. Acetylene and nitrapyrin inhibited both NH₄⁺ oxidation and N₂O production, indicating that N₂O production was closely associated with autotrophic NH₄⁺ oxidation. N₂O production was enhanced by low soil pH; it was negligible above pH 4.7. When soil pH decreased from 4.7 to 4.1, the relative amount of N₂O-N produced from NH₄⁺-N oxidized increased exponentially to 20%. There was also some evidence that N₂O formation was stimulated by salts (potassium sulfate and sodium phosphates). The maximum rate of N₂O-N production was 0.17 μg of N₂O-N per g of soil per h. When humus was treated with NO₂⁻, N₂O evolved immediately, indicating chemical formation, but no N₂O was formed on the addition of NO₃⁻. The amount of N₂O-N evolved was 0.6 to 4.6% of NO₂⁻-N added. A high concentration of NO₂⁻ and low soil pH enhanced chemical production of N₂O. There was no accumulation of NO₂⁻ during nitrification. The calculations indicated that chemical formation of N₂O was not the main source of N₂O during NH₄⁺ oxidation. After the addition of inhibitors of NH₄⁺ oxidation the soils contained NO₃⁻, but no N₂O was produced. The results suggest that enhanced autotrophic NH₄⁺ oxidation is a potential source of N₂O in fertilized acid forest soil.     

Nitrate reduction in marine sediment: Pathways and interactions with iron and sulfur cycling

Abstract

In coastal marine sediments, the interactions between NO₃ ^? reduction and transformations of Fe and S compounds often occur in a strong gradient of electron activity ("redoxcline"). Denitrification activity is observed throughout the NC₃ ^?‐containing surface zone, although the reduction step from N₂O to N₂ can be inhibited by H₂S in the “redoxcline.”; Survival of denitrifiers is generally poor in NO₃ ^?‐free, reduced sediment; such populations are likely to employ Fe³⁺ reduction in their energy metabolism. At depth, the sediments often contain a larger capacity for “nitrate ammonification”; (dissimilatory NO₃ ^? reduction to NH₄ ⁺) than for denitrification. The “nitrate ammonification”; is found commonly among fermenting bacteria, although SO₄ ^2? reducers may also be involved. In situ activities observed in whole sediment cores indicate that “nitrate ammonification”; may account for as much as one‐third of the carbon oxidation in organic‐rich sediments. The control of partitioning between denitrification and “nitrate ammonification”; at low NO₃ ^? concentrations is poorly investigated, but the larger metabolic capacity of fermenting and S O₄ ^2?‐reducing baceria in relatively reduced sediment could be important. In addition to bacterial reduction, chemical NO₃ ^? reduction is possible where significant amounts of Fe²⁺ (or H₂S) accumulate in the “redoxcline.”;     

Distribution of nitrous oxide and regulators of its production across a tropical rainforest catena in the Luquillo Experimental Forest, Puerto Rico

Understanding of N₂O fluxes to the atmosphere is complicated by interactions between chemical and physical controls on both production and movement of the gas. To better understand how N₂O production is controlled in the soil, we measured concentrations of N₂O and of the proximal controllers on its production in soil water and soil air in a field study in the Rio Icacos basin of the Luquillo Experimental Forest, Puerto Rico. A toposequence (ridge, slope-ridge break, slope, slope-riparian break, riparian, and streambank) was used that has been previously characterized for groundwater chemistry and surface N₂O fluxes. The proximal controls on N₂O production include NO₃ ^–, NH₄ ⁺, DOC, and O₂. Nitrous oxide and O₂ were measured in soil air and NO₃ ^–, NH₄ ⁺, and DO were measured in soil water. Nitrate and DOC disappeared from soil solution at the slope-riparian interface, where soil N₂O concentrations increased dramatically. Soil N₂O concentrations continued to increase through the flood plain and the streambank. Nitrous oxide concentrations were highest in soil air probes that had intermediate O₂ concentrations. Changes in N₂O concentrations in groundwater and soil air in different environments along the catena appear to be controlled by O₂ concentrations. In general, N processing in the unsaturated and saturated zones differs within each topographic position apparently due to differences in redox status.     

Nitrite-driven nitrous oxide production under aerobic soil conditions: kinetics and biochemical controls

Nitrite (NO₂⁻) can accumulate during nitrification in soil following fertilizer application. While the role of NO₂⁻ as a substrate regulating nitrous oxide (N₂O) production is recognized, kinetic data are not available that allow for estimating N₂O production or soil‐to‐atmosphere fluxes as a function of NO₂⁻ levels under aerobic conditions. The current study investigated these kinetics as influenced by soil physical and biochemical factors in soils from cultivated and uncultivated fields in Minnesota, USA. A linear response of N₂O production rate () to NO₂⁻ was observed at concentrations below 60 μg N g⁻¹ soil in both nonsterile and sterilized soils. Rate coefficients (K_p) relating to NO₂⁻ varied over two orders of magnitude and were correlated with pH, total nitrogen, and soluble and total carbon (C). Total C explained 84% of the variance in K_p across all samples. Abiotic processes accounted for 31–75% of total N₂O production. Biological reduction of NO₂⁻ was enhanced as oxygen (O₂) levels were decreased from above ambient to 5%, consistent with nitrifier denitrification. In contrast, nitrate (NO₃⁻)‐reduction, and the reduction of N₂O itself, were only stimulated at O₂ levels below 5%. Greater temperature sensitivity was observed for biological compared with chemical N₂O production. Steady‐state model simulations predict that NO₂⁻ levels often found after fertilizer applications have the potential to generate substantial N₂O fluxes even at ambient O₂. This potential derives in part from the production of N₂O under conditions not favorable for N₂O reduction, in contrast to N₂O generated from NO₃⁻ reduction. These results have implications with regard to improved management to minimize agricultural N₂O emissions and improved emissions assessments.