Fluxes and chemistry of nitrogen oxides in the Niwot Ridge,Colorado, snowpack

The effect of snow cover on surface-atmosphere exchanges of nitrogen oxides (nitrogen oxide (NO) + nitrogen dioxide (NO₂); note, here ‘NO₂’ is used as surrogate for a series of oxidized nitrogen gases that were detected by the used monitor in this analysis mode) was investigated at the high elevation, subalpine (3,340 m asl) Soddie site, at Niwot Ridge, Colorado. Vertical (NO + NO₂) concentration gradient measurements in interstitial air in the deep (up to ~2.5 m) snowpack were conducted with an automated sampling and analysis system that allowed for continuous observations throughout the snow-covered season. These measurements revealed sustained, highly elevated (NO + NO₂) mixing ratios inside the snow. Nitrogen oxide concentrations were highest at the bottom of the snowpack, reaching levels of up to 15 ppbv during mid-winter. Decreasing mixing ratios with increasing distance from the soil–snow interface were indicative of an upwards flux of NO from the soil through the snowpack, and out of the snow into the atmosphere, and imply that biogeochemical processes in the subnival soil are the predominant NO source. Nitrogen dioxide reached maximum levels of ~3 ppbv in the upper layers of the snowpack, i.e., ~20–40 cm below the surface. This behavior suggests that a significant fraction of NO is converted to NO₂ during its diffusive transport through the snowpack. Ozone showed the opposite behavior, with rapidly declining levels below the snow surface. The mirroring of vertical profiles of ozone and the NO₂/(NO + NO₂) ratio suggest that titration of ozone by NO in the snowpack contributes to the ozone reaction in the snow and to the ozone surface deposition flux. However, this surface efflux of (NO + NO₂) can only account for a minor fraction of ozone deposition flux over snow that has been reported at other mid-latitude sites. Neither (NO + NO₂) nor ozone levels in the interstitial air showed a clear dependence on incident solar irradiance, much in contrast to observations in polar snow. Comparisons with findings from polar snow studies reveal a much different (NO + NO₂) and ozone snow chemistry in this alpine environment. Snowpack concentration gradients and diffusion theory were applied to estimate an average, wintertime (NO + NO₂) flux of 0.005–0.008 nmol m⁻² s⁻¹, which is of similar magnitude as reported (NO + NO₂) fluxes from polar snow. While fluxes are similar, there is strong evidence that processes controlling (NO + NO₂) fluxes in these environments are very different, as subnivial soil at Niwot Ridge appears to be the main source of the (NO + NO₂) efflux, whereas in polar snow (NO + NO₂) has been found to be primarily produced from photochemical de-nitrification of snow nitrate.     

Nitrate reductase activity and nitrogen compounds in xylem exudate of Juglans nigra seedlings: relation to nitrogen source and supply

Nitrogen (N) limits plant productivity and its uptake and assimilation may be regulated by N source, N availability, and nitrate reductase activity (NRA). Knowledge of how these factors interact to affect N uptake and assimilation processes in woody angiosperms is limited. We fertilized 1-year-old, half-sib black walnut (Juglans nigra L.) seedlings with ammonium (NH₄ ⁺) [as (NH₄)₂SO₄], nitrate (NO₃ ⁻) (as NaNO₃), or a mixed N source (NH₄NO₃) at 0, 800, or 1,600 mg N plant⁻¹ season⁻¹. Two months following final fertilization, growth, in vivo NRA, plant N status, and xylem exudate N composition were assessed. Specific leaf NRA was higher in NO₃ ⁻-fed and NH₄NO₃-fed plants compared to observed responses in NH₄ ⁺-fed seedlings. Regardless of N source, N addition increased the proportion of amino acids (AA) in xylem exudate, inferring greater NRA in roots, which suggests higher energy cost to plants. Root total NRA was 37% higher in NO₃ ⁻-fed than in NH₄ ⁺-fed plants. Exogenous NO₃ ⁻ was assimilated in roots or stored, so no difference was observed in NO₃ ⁻ levels transported in xylem. Black walnut seedling growth and physiology were generally favored by the mixed N source over NO₃ ⁻ or NH₄ ⁺ alone, suggesting NH₄NO₃ is required to maximize productivity in black walnut. Our findings indicate that black walnut seedling responses to N source and level contrast markedly with results noted for woody gymnosperms or herbaceous angiosperms.     

Changes in nitrogen and base cation concentrations in soil water due to the tree cutting in a wetland alder forest in the Kushiro Wetland, northern Japan

As part of the Kushiro Wetland Restoration Project, we evaluated the effect of the tree-cutting (February 2003) on soil water chemistry for 3 years in an alder (Alnus japonica) shrubland on the peat soil in the Kushiro Wetland, the largest wetland in Japan. The alder stand was divided into two types; low stature and high stature stands, mean heights being 1.5 and 2.6 m, respectively. The treatment plot with tree cutting and the reference plot measuring 25 m × 25 m each were established in both stands, and a soil incubation experiment was also conducted by trenching the root zone of wetland vegetation in the treatment plots in both stands in the summer of 2004. The tree cutting did not substantially increase the concentrations of ammonium ion (NH₄ ⁺) and nitrate ion (NO₃ ⁻) in the soil water, although a gradual and slight increase of NO₃ ⁻ concentration was found after the tree-cutting only in the high stature stand (<10 μmol/l). This increase in the NO₃ ⁻ concentration in the high stature stand was probably due to the rhizosphere oxidation of wetland herbaceous vegetation, and was accompanied by the increase of Ca²⁺ concentration. The soil incubation experiment showed no significant change of the nitrogen concentration, suggesting that inorganic nitrogen was immobilized and NO₃ ⁻ was denitrified intensively in this wetland peat soil. The results of this study suggested that this wetland ecosystem had a high potential to stabilize the soil water chemistry.     

Adventitious root suspension cultures of <Emphasis Type="BoldItalic">Hypericum perforatum</Emphasis>: effect of nitrogen source on production of biomass and secondary metabolites

The present study investigated the effect of nitrogen source (NH₄⁺; NO₃⁻) at different concentrations on the accumulation of biomass and secondary metabolites in adventitious root cultures of Hypericum perforatum L. Cultures were initiated in shake flasks by using half-strength Murashige and Skoog (MS) medium with B5 vitamins, 1.0 mg l⁻¹ indole-3-butyric acid, 0.1 mg l⁻¹ kinetin, 3% (w/v) sucrose, and different ratios of ammonium and nitrate (0:30, 5:25, 10:20, 15:15, 20:10, 25:5, and 30:0 mM, using NH₄Cl and KNO₃). The cultures were maintained in darkness. The medium supplemented with 5:25 (mM) NH₄⁺/NO₃⁻ resulted in the optimum accumulation of biomass and total phenols and flavonoids. The antioxidant potential of a methanolic extract, measured as the 1, 1-diphenyl-2-picrylhydrazyl and 2, 2-azino-bis-(3-ethylbenzothiazoline-6-sulfonic acid) radical scavenging activities, of H. perforatum adventitious roots showed that antioxidant activity was high from root extracts that were grown on higher concentrations of NO₃⁻ nitrogen (15, 20, and 25 mM). Further, assessment of hydrogen peroxide (H₂O₂) and malondialdehyde content of the root extracts revealed that cultures supplemented with higher levels of NO₃⁻ nitrogen (15–30 mM) were under oxidative stress, which boosted the levels of secondary metabolites in the adventitious roots. These results suggest that optimal adventitious root biomass could be achieved with the supplementation of cultures with 5:25 ratios of MS nitrogen sources.     

Whole‐plant and organ‐level nitrogen isotope discrimination indicates modification of partitioning of assimilation,fluxes and allocation of nitrogen in knockout lines of Arabidopsis thaliana

The nitrogen isotope composition (δ¹⁵N) of plants has potential to provide time‐integrated information on nitrogen uptake, assimilation and allocation. Here, we take advantage of existing T‐DNA and γ‐ray mutant lines of Arabidopsis thaliana to modify whole‐plant and organ‐level nitrogen isotope composition. Nitrate reductase 2 (nia2), nitrate reductase 1 (nia1) and nitrate transporter (nrt2) mutant lines and the Col‐0 wild type were grown hydroponically under steady‐state NO₃^– conditions at either 100 or 1000 μM NO₃^– for 35 days. There were no significant effects on whole‐plant discrimination and growth in the assimilatory mutants (nia2 and nia1). Pronounced root vs leaf differences in δ¹⁵N, however, indicated that nia2 had an increased proportion of nitrogen assimilation of NO₃^– in leaves while nia1 had an increased proportion of assimilation in roots. These observations are consistent with reported ratios of nia1 and nia2 gene expression levels in leaves and roots. Greater whole‐plant discrimination in nrt2 indicated an increase in efflux of unassimilated NO₃^– back to the rooting medium. This phenotype was associated with an overall reduction in NO₃^– uptake, assimilation and decreased partitioning of NO₃^– assimilation to the leaves, presumably because of decreased symplastic intercellular movement of NO₃^– in the root. Although the results were more varied than expected, they are interpretable within the context of expected mechanisms of whole‐plant and organ‐level nitrogen isotope discrimination that indicate variation in nitrogen fluxes, assimilation and allocation between lines.     

Tracing uptake and assimilation of NO2 in spruce needles with 13N

For the first time, spruce shoots (Picea abies [L.] Karst.)were fumigated in vivo with ¹³N-labelled NO₂ with the aim ofelucidating the mechanism of NO₂^– trapping in the apoplastof the substomatal cavity. Uptake by the needles could be monitoredon-line, and a quantitative analysis of the activity recordsdelivered a deposition velocity in agreement with the commondry deposition estimates and ruled out rapid export processes.A fast extraction procedure was applied which revealed thatNO₂ did not produce any detectable traces of nitrite. In needlesin which the enzymes of nitrate reduction were not induced byprior fumigation with NO₂, incorporation of NO₂ was partiallyinhibited as compared to the fully induced shoots which tookup and assimilated NO₂ as expected from a constant influx. Theonly labelled inorganic species found in the extracts was nitrate(60%), whereas the rest of the label (40%) was assimilated organicnitrogen.A quantitative analysis of the data shows that thereaction of NO₂ in the apoplast yields at least three timesmore nitrate than nitrite, so that the existing models aboutthe apoplastic trapping reaction, disproportionation or antioxidantscavenging, which both postulate substantial production of nitrite,have to be reconsidered. Key words: ¹³N, nitrogen dioxide, spruce, air pollutants, deposition     

Nitrite utilization by Chaetoceros muelleri under elevated CO2 concentration

Chaetoceros muelleri (Lemn.) was cultured with nitrite (NO₂⁻) or nitrate (NO₃⁻) as the sole nitrogen source and aerated with air or with CO₂-enriched air. Cells of C. muelleri excreted into the medium nitrite produced by reduction of nitrate when grown with 100 μM NaNO₃ as nitrogen source. Accordingly, NO₂⁻ concentration reached 10.4 μM after 95 h at the low CO₂ condition (aerated with air); while the maximum NO₂⁻ concentration was only around 2.0 μM at the high CO₂ condition (aerated with 5% CO₂ in air), furthermore, after 30 h it decreased to no more than 1.0 μM. NO₂⁻ was almost assimilated in 80 h when C. muelleri was cultured at the high CO₂ condition with 100 μM NaNO₂ as sole nitrogen source. At the high CO₂ condition, after 3 h the activity of nitrite reductase was as much as 50% higher than that at the low CO₂ condition. It was indicated that enriched CO₂ concentration could inhibit nitrite excretion and enhance nitrite assimilation by cells. Therefore, aeration with enriched CO₂ might be an effective way to control nitrite content in aquaculture systems.     

Influence of nitrogen source and concentration on nitrogen isotopic discrimination in two barley genotypes (Hordeum vulgare L.)

The occurrence of nitrogen isotope discrimination with absorption and assimilation of nitrate (NO₃^–) and ammonium (NH₄⁺) was investigated using two genotypes of barley, Hordeum vulgare L. cv. Steptoe and Az12 : Az70, the latter of which lacks the characterized nitrate reductase isozymes. Plants were grown under two situations: a closed system with limited nitrogen or an open system with unlimited nitrogen, to elucidate the conditions and processes that influence discrimination. There was no discrimination observed for Az12 : Az70 when supplied with limited nitrogen. Discrimination was observed for Steptoe seedlings at high external NO₃^– concentrations, but not with low NO₃^– when assimilation is probably rapid and complete. The same pattern was observed for Steptoe when NH₄⁺ was supplied; indicating that for both nitrogen forms discrimination is dependent upon the presence of the assimilatory enzyme and the external concentration. The implications of this study are that both internal (assimilatory enzyme distribution) and external (source concentration) factors may have a larger impact on tissue δ ¹⁵N than the form of nitrogen utilized. This suggests that tissue δ ¹⁵N may not always be a reliable indicator of a plant's integrated nitrogen nutrition.     

葡萄糖、根浸出液对丛枝菌根真菌吸收不同外源氮产生精氨酸的影响

以大葱(Allium fistulosum)为宿主植物, 接种丛枝菌根(Arbuscular mycorrhizal, AM)真菌Glomus intraradices, 采用三室隔离盆栽培养系统, 在菌丝室施加浓度为4 mmol/L的不同形态外源氮、1%葡萄糖及根浸出液, 通过测定根外菌丝(Extraradical mycelium, ERM)和菌根中精氨酸的含量, 探究葡萄糖、根浸出液对AM真菌吸收不同形式外源氮产生精氨酸的影响。结果表明, 不同外源氮对ERM中精氨酸含量的影响为尿素>Gln>NH₄NO₃>Arg/Gly>NH₄Cl>KNO₃, 对菌根中精氨酸含量的影响为Arg>Gln>尿素>NH₄NO₃>Gly>NH₄Cl>KNO₃; 施加葡萄糖和根浸出液在不同程度上提高ERM干重和菌丝室孢子数量, 但使ERM和菌根中的精氨酸含量降低。说明AM真菌吸收同化不同外源氮产生精氨酸的能力不同, 葡萄糖和根浸出液降低AM真菌吸收同化外源氮产精氨酸的能力。     

Relationship between growth,nitrogen fixation and assimilation in a legume (Medicago sativa L.)

Summary Symbiotic N₂ fixation, NO ₃ ⁻ assimilation and protein accumulation in the shoots were measured simultaneously in alfalfa (Medicago sativa L.) grown in the field or in pots, in order to study how the balance between the two modes of nitrogen nutrition could be influenced by agronomic factors, such as harvest, mineral nitrogen supply and drought stress. During periods of rapid growth, fixation and assimilation may function simultaneously; they are antagonistic at the beginning and at the end of the growth cycle, when the nitrogen requirement of the plant is lower. When nitrogen nutrition does not limit growth, mineral nitrogen supply favours assimilation at the expense of fixation, but does not modify the amount of nitrogen accumulated, which is adjusted to the growth capacity of the plant. After cutting, nitrate assimilation compensated for the decrease in fixation and supplied the plant with the nitrogen required by the regrowth, the proliferation of which determined the fixation recovery. Drought stress decreased N₂ fixation much more than NO ₃ ⁻ assimilation. The latter made growth recovery possible when water supply conditions became normal again. These results suggested the existence of an optimum level of nitrate assimilation, which differed depending on the age of the plants and allowed both maximum growth and fixing activity.     

Anaerobic ammonia oxidation with nitrogen dioxide by Nitrosomonas eutropha

Nitrosomonas eutropha, an obligately lithoautotrophic bacterium, was able to nitrify and denitrify simultaneously under anoxic conditions when gaseous nitrogen dioxide (NO₂) was supplemented to the atmosphere. In the presence of gaseous NO₂, ammonia was oxidized, nitrite and nitric oxide (NO) were formed, and hydroxylamine occurred as an intermediate. Between 40 and 60% of the produced nitrite was denitrified to dinitrogen (N₂). Nitrous oxide (N₂O) was shown to be an intermediate of denitrification. Under an N₂ atmosphere supplemented with 25 ppm NO₂ and 300 ppm CO₂, the amount of cell protein increased by 0.87 mg protein per mmol ammonia oxidized, and the cell number of N. eutropha increased by 5.8 × 10⁹ cells per mmol ammonia oxidized. In addition, the ATP and NADH content increased by 4.3 μmol ATP (g protein)^–1 and 6.3 μmol NADH (g protein)^–1 and was about the same in both anaerobically and aerobically grown cells. Without NO₂, the ATP content decreased by 0.7 μmol (g protein)^–1, and the NADH content decreased by 1.2 μmol (g protein)^–1. NO was shown to inhibit anaerobic ammonia oxidation. Received: 9 October 1996 / Accepted: 5 December 1996     

Mitochondrial Respiration Can Support NO(3) and NO(2) Reduction during Photosynthesis : Interactions between Photosynthesis, Respiration, and N Assimilation in the N-Limited Green Alga Selenastrum minutum

Mass spectrometric analysis shows that assimilation of inorganic nitrogen (NH₄⁺, NO₂⁻, NO₃⁻) by N-limited cells of Selenastrum minutum (Naeg.) Collins results in a stimulation of tricarboxylic acid cycle (TCA cycle) CO₂ release in both the light and dark. In a previous study we have shown that TCA cycle reductant generated during NH₄⁺ assimilation is oxidized via the cytochrome electron transport chain, resulting in an increase in respiratory O₂ consumption during photosynthesis (HG Weger, DG Birch, IR Elrifi, DH Turpin [1988] Plant Physiol 86: 688-692). NO₃⁻ and NO₂⁻ assimilation resulted in a larger stimulation of TCA cycle CO₂ release than did NH₄⁺, but a much smaller stimulation of mitochondrial O₂ consumption. NH₄⁺ assimilation was the same in the light and dark and insensitive to DCMU, but was 82% inhibited by anaerobiosis in both the light and dark. NO₃⁻ and NO₂⁻ assimilation rates were maximal in the light, but assimilation could proceed at substantial rates in the light in the presence of DCMU and in the dark. Unlike NH₄⁺, NO₃⁻ and NO₂⁻ assimilation were relatively insensitive to anaerobiosis. These results indicated that operation of the mitochondrial electron transport chain was not required to maintain TCA cycle activity during NO₃⁻ and NO₂⁻ assimilation, suggesting an alternative sink for TCA cycle generated reductant. Evaluation of changes in gross O₂ consumption during NO₃⁻ and NO₂⁻ assimilation suggest that TCA cycle reductant was exported to the chloroplast during photosynthesis and used to support NO₃⁻ and NO₂⁻ reduction.     

Sampling spatial and temporal variation in soil nitrogen availability

There are few studies in natural ecosystems on how spatial maps of soil attributes change within a growing season. In part, this is due to methodological difficulties associated with sampling the same spatial locations repeatedly over time. We describe the use of ion exchange membrane spikes, a relatively nondestructive way to measure how soil resources at a given point in space fluctuate over time. We used this method to examine spatial patterns of soil ammonium (NH⁺ ₄) and nitrate (NO⁻ ₃) availability in a mid-successional coastal dune for four periods of time during the growing season. For a single point in time, we also measured soil NH⁺ ₄ and NO⁻ ₃ concentrations from soil cores collected from the mid-successional dune and from an early and a late successional dune. Soil nitrogen concentrations were low and highly variable in dunes of all ages. Mean NH⁺ ₄ and NO⁻ ₃ concentrations increased with the age of the dune, whereas coefficients of variation for NH⁺ ₄ and NO⁻ ₃ concentrations decreased with the age of the dune. Soil NO⁻ ₃ concentration showed strong spatial structure, but soil NH⁺ ₄ concentration was not spatially structured. Plant-available NH⁺ ₄ and NO⁻ ₃ showed relatively little spatial structure: only NO⁻ ₃ availability in the second sampling period had significant patch structure. Spatial maps of NH⁺ ₄ and NO⁻ ₃ availability changed greatly over time, and there were few significant correlations among soil nitrogen availability at different points in time. NO⁻ ₃ availability in the second sampling period was highly correlated (r = 0.90) with the initial soil NO⁻ ₃ concentrations, providing some evidence that patches of plant-available NO⁻ ₃ may reappear at the same spatial locations at irregular points in time. Received: 20 February 1998 / Accepted: 23 November 1998     

Nitrate reductase activity of two leafy vegetables as affected by nickel and different nitrogen forms

The author studied the effect of different nickel concentrations (0, 0.4, 40 and 80 μM Ni) on the nitrate reductase (NR) activity of New Zealand spinach (Tetragonia expansa Murr.) and lettuce (Lactuca sativa L. cv. Justyna) plants supplied with different nitrogen forms (NO₃ ⁻–N, NH₄ ⁺–N, NH₄NO₃). A low concentration of Ni (0.4 μM) did not cause statistically significant changes of the nitrate reductase activity in lettuce plants supplied with nitrate nitrogen (NO₃ ⁻–N) or mixed (NH₄NO₃) nitrogen form, but in New Zealand spinach leaves the enzyme activity decreased and increased, respectively. The introduction of 0.4 μM Ni in the medium containing ammonium ions as a sole source of nitrogen resulted in significantly increased NR activity in lettuce roots, and did not cause statistically significant changes of the enzyme activity in New Zealand spinach plants. At a high nickel level (Ni 40 or 80 μM), a significant decrease in the NR activity was observed in New Zealand spinach plants treated with nitrate or mixed nitrogen form, but it was much more marked in leaves than in roots. An exception was lack of significant changes of the enzyme activity in spinach leaves when plants were treated with 40 μM Ni and supplied with mixed nitrogen form, which resulted in the stronger reduction of the enzyme activity in roots than in leaves. The statistically significant drop in the NR activity was recorded in the aboveground parts of nickel-stressed lettuce plants supplied with NO₃ ⁻–N or NH₄NO₃. At the same time, there were no statistically significant changes recorded in lettuce roots, except for the drop of the enzyme activity in the roots of NO₃ ⁻-fed plants grown in the nutrient solution containing 80 μM Ni. An addition of high nickel doses to the nutrient solution contained ammonium nitrogen (NH₄ ⁺–N) did not affect the NR activity in New Zealand spinach plants and caused a high increase of this enzyme in lettuce organs, especially in roots. It should be stressed that, independently of nickel dose in New Zealand spinach plants supplied with ammonium form, NR activity in roots was dramatically higher than that in leaves. Moreover, in New Zealand spinach plants treated with NH₄ ⁺–N the enzyme activity in roots was even higher than in those supplied with NO₃ ⁻–N.     

Antioxidative system in maize roots as affected by osmotic stress and different nitrogen sources

The activities of antioxidative enzymes and contents of proline and total phenolics were assayed in roots of two maize (Zea mays L.) genotypes grown in a medium containing nitrate (NO₃ ⁻) or both nitrogen forms, nitrate and ammonium (NH₄ ⁺/NO₃ ⁻). An increase in the activities of class III peroxidases (POD), superoxide dismutase (SOD), ascorbate peroxidase (APX), ascorbate oxidase (AO) and proline content, and decrease in phenolic content were observed in NH₄ ⁺/NO₃ ⁻ in comparison with NO₃ ⁻ grown plants. When polyethylene glycol (PEG) was added to both nitrogen treatments, the content of total phenolics and proline was increased, especially in NH₄ ⁺/NO₃ ⁻ treatment. The PEG treatment decreased enzyme activities in NH₄ ⁺/NO₃ ⁻ grown plants, but in NO₃ ⁻ grown plants activities of POD and SOD were increased, opposite to decreased APX and AO. Isoelectric focusing demonstrated increased activities of acidic POD isoforms in PEG treated NO₃ ⁻ grown plants, and lower activities of both, acidic and basic isoforms in NH₄ ⁺/NO₃ ⁻grown plants.     

Denitrification,dissimilatory nitrate reduction to ammonium,and nitrogen fixation along a nitrate concentration gradient in a created freshwater wetland

Wetlands are often highly effective nitrogen (N) sinks. In the Lake Waco Wetland (LWW), near Waco, Texas, USA, nitrate (NO₃⁻) concentrations are reduced by more than 90% in the first 500 m downstream of the inflow, creating a distinct gradient in NO₃⁻ concentration along the flow path of water. The relative importance of sediment denitrification (DNF), dissimilatory NO₃⁻ reduction to ammonium (DNRA), and N₂ fixation were examined along the NO₃⁻ concentration gradient in the LWW. “Potential DNF” (hereafter potDNF) was observed in all months and ranged from 54 to 278 μmol N m⁻² h⁻¹. “Potential DNRA” (hereafter potDNRA) was observed only in summer months and ranged from 1.3 to 33 μmol N m⁻² h⁻¹. Net N₂ flux ranged from 184 (net denitrification) to −270 (net N₂ fixation) μmol N m⁻² h⁻¹. Nitrogen fixation was variable, ranging from 0 to 426 μmol N m⁻² h⁻¹, but high rates ranked among the highest reported for aquatic sediments. On average, summer potDNRA comprised only 5% (±2% SE) of total NO₃⁻ loss through dissimilatory pathways, but was as high as 36% at one site where potDNF was consistently low. Potential DNRA was higher in sediments with higher sediment oxygen demand (r ² = 0.84), and was related to NO₃⁻ concentration in overlying water in one summer (r ² = 0.81). Sediments were a NO₃⁻ sink and accounted for 50% of wetland NO₃⁻ removal (r ² = 0.90). Sediments were an NH₄⁺ source, but the wetland was often a net NH₄⁺ sink. Although DNRA rates in freshwater wetlands may rival those observed in estuarine systems, the importance of DNRA in freshwater sediments appears to be minor relative to DNF. Furthermore, sediment N₂ fixation can be extremely high when NO₃⁻ in overlying water is consistently low. The data suggest that newly fixed N can support sustained N transformation processes such as DNF and DNRA when surface water inorganic N supply rates are low.     

The influence of salinity on the utilization of root anaplerotic carbon and nitrogen metabolism in tomato seedlings

In hydroponically grown Lycopersicon esculentum (L.) Mill. cv.F144 the site of NO₃^– reduction and assimilation withinthe plant was shifted from the shoot to the root by salinity.Uptake of NO₃^– from the root solution was strongly inhibitedby salinization. Consequently, NO₃^– concentrations inthe leaf, stem and root tissues as well as the nitrate reductaseactivities of the leaves were lower in salinized than in controlplants. Lower NO₃^–, but higher reduced-N, concentrationswere observed in the xylem sap as a result of the enhanced participationof the root in NO₃^– reduction in salinized plants. Lowerstem K⁺ concentrations and leaf malate concentrations were foundin salinized compared to control plants which indicates reducedfunctioning of the K⁺–shuttle in the salinized plants. Incorporation of inorganic carbon by the root was determinedby supplying a pulse of NaH¹⁴CO₃ followed by extraction andseparation of the labelled products on ion exchange resins.The rate of H¹⁴CO₃^– incorporation was c. 2-fold higherin control than in salinized plants. In salinized plants theproducts of H¹⁴CO₃^– incorporation within the roots werediverted into amino acids, while the control plants divertedrelatively more ¹⁴C into organic acids. Products of inorganiccarbon incorporation in the roots of salinized plants providean anaplerotic source of carbon for assimilation of reducedNO₃^– into amino acids, while in control plants the productswere predominantly organic acids as part of mechanisms to maintainionic balance in the cells and in the xylem sap. Key words: Tomato, nitrate, PEPc, respiration, salinity     

The effect of water exchange on bacterioplankton depletion and inorganic nutrient dynamics in coral reef cavities

We studied the effect of water exchange on the depletion (or accumulation) of bacterioplankton, dissolved organic matter and inorganic nutrients in small open framework cavities (50–70 l) at 15 m depth on the coral reef along Curaçao, Netherlands Antilles. The bacterioplankton removal rate in cavities increased with increasing water exchange rates up to a threshold of 0.0045 s⁻¹, reaching values of 50–100 mg C m⁻² total interior cavity surface area (CSA) per day. Beyond the threshold, bacterioplankton removal dropped. The cryptic community is apparently adapted to the average water exchange in these cavities (0.0041 s⁻¹). Dissolved inorganic nitrogen (DIN), nitrate + nitrite (NO _x ) in particular, accumulated in cavity water and the accumulation decreased with increasing water exchange. Net NO _x effluxes exceeded net DIN effluxes from cavities (average efflux rate of 1.9 mmol NO _x vs. 0.8 mmol DIN m⁻² interior CSA per day). The difference is ascribed to net ammonium losses (NH₄) in cavities at reef concentrations >0.025 μM NH₄, possibly due to enhanced nitrification. Dissolved inorganic phosphate accumulated in cavities, but was not related to water exchange. The cryptic biota in cavities depend on water exchange for optimization of consumption of bacterioplankton and removal of inorganic nitrogen. Coral cavities are an evident sink of bacterioplankton and a source of NO _x and PO ₄ ³⁻ .     

Seasonal nitrogen assimilation and carbon fixation in a fjordic sea loch

Carbon (C) fixation and nitrogen (N) assimilation rates havebeen estimated from ¹⁴C and ¹⁵N techniques for a 12 month periodin a Scottish sea loch. The maximum rate of nitrogen assimilated(29.92 mmol N m^–2 day^–1) was in April at the mostseaward station; similar high rates were experienced duringMay at the other stations. Carbon fixation rates were maximal(488–4047 mg C m^–2day^–1) at the time of highphytoplankton biomass (maximum 8.3 mg m^–3 chlorophylla) during May, whilst nitrate concentrations remained >0.7µ.mol l^–1. C:N assimilation ratios suggest nitrogenlimitation only during the peak of the spring bloom, althoughat times nitrogen (nitrate and ammonium) concentration fellto 0.2 µmol l^–1 in the following months. The verticalstability of the water column, influenced by tidal and riverineflushing, varied along the axis of the loch, resulting in markeddifferences between sampling stations. Although ammonium waspreferentially assimilated by phytoplankton, >50% of productionwas supported by nitrate uptake and only during the summer monthswas the assimilation of ammonium quantitatively important.     

Influence of elevated CO2 and nitrogen nutrition on photosynthesis and nitrate photo-assimilation in maize (Zea mays L.)

Measurements of CO₂ and O₂ gas exchange and chlorophyll a fluorescence were used to test the hypothesis that elevated atmospheric CO₂ inhibits nitrate (NO₃ ^– ) photo‐assimilation in the C₄ plant, maize (Zea mays L.). The assimilatory quotient (AQ), the ratio of net CO₂ assimilation to net O₂ evolution, decreases as NO₃ ^– photo‐assimilation increases so that the difference in AQ between the ammonium‐ and nitrate‐fed plants (ΔAQ) provided an in planta estimate of NO₃ ^– photo‐assimilation. In fully expanded maize leaves, NO₃ ^– photo‐assimilation was detectable only under high light and was not affected by CO₂ treatments. Furthermore, CO₂ assimilation and O₂ evolution were higher under NO₃ ^– than ammonia (NH₄⁺) regardless of CO₂ levels. In conclusion, NO₃ ^– photo‐assimilation in maize primarily occurred at high light when reducing equivalents were presumably not limiting. Nitrate photo‐assimilation enhanced C₄ photosynthesis, and in contrast to C₃ plants, elevated CO₂ did not inhibit foliar NO₃^– photo‐assimilation.