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1.
The utilization of NO 3− by green algae growing photoautotrophically under air, which are growth conditions close to their more habitual situations in nature, is associated with the excretion of NO 2− and NH 4+ to the culture medium. The entire process is promoted by blue light and depends on photosynthetically active radiation for the required reducing equivalents. The stimulation of NO 3− utilization and of its associated NO 2− and NH 4+ excretions saturated at very low quantum fluxes of blue light (15 microequivalents per square meter per second) in Chlamydomonas reinhardii cells sparged with CO 2-free air and irradiated with 50 microequivalents per square meter per second background red light. The wavelength dependence data of this stimulation correlated closely with the in situ photoactivation of nitrate reductase and also with the light induced increase in its biosynthesis and/or assembly. These results indicate that the photoregulation of inorganic N metabolism in C. reinhardii is mainly due to the blue light modulation of nitrate reductase. Although flavins are the most suitable candidates to act as physiological photoreceptors, the wavelength dependence data only show a major peak in the blue region between 400 and 500 nanometers. 相似文献
2.
The blue light dependent utilization of nitrate by green algae under common air and high irradiances, besides its assimilatory nature, is associated with the release of NO 2
– and NH 4
+ to the culture medium. If the CO 2 content of the sparging air was increased up to 2%, previously excreted NO 2
– and NH 4
+ were rapidly assimilated. When under air and high irradiances the cell density in the culture reached values corresponding to 25 g Ch 1.ml -1, no further growth was observed and the highest values of NO 3
– consumption and NO 2
– and NH 4
+ release were attained. Besides low CO 2 tensions, increasing NO 3
– concentrations in the medium stimulated the release of NO 3
– and NH 4
+. Under CO 2-free air the consumption of NO 3
– and the release of NO 2
– and NH 4
+ on a total N bases were almost stoichiometric and their rates saturated at much lower irradiances than under air. Under CO 2-free air high rates of NO 2
– release were only observed under the blue radiations that were effectively absorbed by photosynthetically active pigments, i.e. 460 nm, but not under 404 and 630 nm radiations. However, the simultaneous illumination of the cells with 404 and 630 nm monochromatic light showed a remarkable synergistic effect on NO 2
– release.The results are discussed in terms of the close relationship between C and N metabolism, the photosynthetic reducing power required to convert NO
inf3
sup±
-N into R – NH 2-N and the blue light activation of nitrate reductase. 相似文献
3.
The effect of nitrogen form (NH 4-N, NH 4-N + NO 3−, NO 3−) on nitrate reductase activity in roots and shoots of maize ( Zea mays L. cv INRA 508) seedlings was studied. Nitrate reductase activity in leaves was consistent with the well known fact that NO 3− increases, and NH 4+ and amide-N decrease, nitrate reductase activity. Nitrate reductase activity in the roots, however, could not be explained by the root content of NO 3−, NH 4-N, and amide-N. In roots, nitrate reductase activity in vitro was correlated with the rate of nitrate reduction in vivo. Inasmuch as nitrate reduction results in the production of OH − and stimulates the synthesis of organic anions, it was postulated that nitrate reductase activity of roots is stimulated by the released OH − or by the synthesized organic anions rather than by nitrate itself. Addition of HCO 3− to nutrient solution of maize seedlings resulted in a significant increase of the nitrate reductase activity in the roots. As HCO 3−, like OH −, increases pH and promotes the synthesis of organic anions, this provides circumstantial evidence that alkaline conditions and/or organic anions have a more direct impact on nitrate reductase activity than do NO 3−, NH 4-N, and amide-N. 相似文献
4.
Chlamydomonas reinhardii cells, after a period of dark anaerobic adaptation, evolve H 2 not only in the dark but also in the light. Our results show that high irradiances impair prolonged H 2 evolution, while under low irradiances or darkness H 2 evolution proceeds for more than 50 hours. NO 3− and NO 2− suppress H 2 evolution both in the dark or under low irradiance. Apparently the cells prefer these oxidized nitrogen sources to protons as electron acceptors, since both NO 3− and NO 2− become reduced to NH 4+, which is excreted to the culture medium in high amounts. H 2 evolution started once these oxidized anions were largely depleted from the medium. Moreover, H 2 evolution was consistently associated with NH 4+ excretion even if NH 4+ was already present in high amounts in the medium. This observation indicates that the cells utilize not only their carbohydrate but also their protein reserves as sources of reducing power for H 2 evolution. This conclusion was supported by the observation that when nitrogen-starved cells were made anaerobic in a nitrogen-free medium, they not only evolved H 2 at very high rates but excreted concomitantly NH 4+ up to concentrations in the millimolar range. 相似文献
5.
Mass spectrometric analysis shows that assimilation of inorganic nitrogen (NH 4+, NO 2−, NO 3−) by N-limited cells of Selenastrum minutum (Naeg.) Collins results in a stimulation of tricarboxylic acid cycle (TCA cycle) CO 2 release in both the light and dark. In a previous study we have shown that TCA cycle reductant generated during NH 4+ assimilation is oxidized via the cytochrome electron transport chain, resulting in an increase in respiratory O 2 consumption during photosynthesis (HG Weger, DG Birch, IR Elrifi, DH Turpin [1988] Plant Physiol 86: 688-692). NO 3− and NO 2− assimilation resulted in a larger stimulation of TCA cycle CO 2 release than did NH 4+, but a much smaller stimulation of mitochondrial O 2 consumption. NH 4+ 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. NO 3− and NO 2− 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 NH 4+, NO 3− and NO 2− 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 NO 3− and NO 2− assimilation, suggesting an alternative sink for TCA cycle generated reductant. Evaluation of changes in gross O 2 consumption during NO 3− and NO 2− assimilation suggest that TCA cycle reductant was exported to the chloroplast during photosynthesis and used to support NO 3− and NO 2− reduction. 相似文献
6.
We examined nitrate assimilation and root gas fluxes in a wild-type barley ( Hordeum vulgare L. cv Steptoe), a mutant ( nar1a) deficient in NADH nitrate reductase, and a mutant ( nar1a; nar7w) deficient in both NADH and NAD(P)H nitrate reductases. Estimates of in vivo nitrate assimilation from excised roots and whole plants indicated that the nar1a mutation influences assimilation only in the shoot and that exposure to NO 3− induced shoot nitrate reduction more slowly than root nitrate reduction in all three genotypes. When plants that had been deprived of nitrogen for several days were exposed to ammonium, root carbon dioxide evolution and oxygen consumption increased markedly, but respiratory quotient—the ratio of carbon dioxide evolved to oxygen consumed—did not change. A shift from ammonium to nitrate nutrition stimulated root carbon dioxide evolution slightly and inhibited oxygen consumption in the wild type and nar1a mutant, but had negligible effects on root gas fluxes in the nar1a; nar7w mutant. These results indicate that, under NH 4+ nutrition, 14% of root carbon catabolism is coupled to NH 4+ absorption and assimilation and that, under NO 3− nutrition, 5% of root carbon catabolism is coupled to NO 3− absorption, 15% to NO 3− assimilation, and 3% to NH 4+ assimilation. The additional energy requirements of NO 3− assimilation appear to diminish root mitochondrial electron transport. Thus, the energy requirements of NH 4+ and NO 3− absorption and assimilation constitute a significant portion of root respiration. 相似文献
7.
Ricinus communis L. plants were grown in nutrient solutions in which N was supplied as NO 3− or NH 4+, the solutions being maintained at pH 5.5. In NO 3−-fed plants excess nutrient anion over cation uptake was equivalent to net OH − efflux, and the total charge from NO 3− and SO 42− reduction equated to the sum of organic anion accumulation plus net OH − efflux. In NH 4+-fed plants a large H + efflux was recorded in close agreement with excess cation over anion uptake. This H + efflux equated to the sum of net cation (NH 4+ minus SO 42−) assimilation plus organic anion accumulation. In vivo nitrate reductase assays revealed that the roots may have the capacity to reduce just under half of the total NO 3− that is taken up and reduced in NO 3−-fed plants. Organic anion concentration in these plants was much higher in the shoots than in the roots. In NH 4+-fed plants absorbed NH 4+ was almost exclusively assimilated in the roots. These plants were considerably lower in organic anions than NO 3−-fed plants, but had equal concentrations in shoots and roots. Xylem and phloem saps were collected from plants exposed to both N sources and analyzed for all major contributing ionic and nitrogenous compounds. The results obtained were used to assist in interpreting the ion uptake, assimilation, and accumulation data in terms of shoot/root pH regulation and cycling of nutrients. 相似文献
8.
The photoreversible nature of the regulation of nitrate reductase is one of the most interesting features of this enzyme. As well as other chemicals, NH 2OH reversibly inactivates the reduced form of nitrate reductase from Ankistrodesmus braunii. From the partial activities of the enzyme, only terminal nitrate reductase is affected by NH 2OH. To demonstrate that the terminal activity was readily inactivted by NH 2OH, the necessary reductants of the terminal part of the enzyme had to be cleared of dithionite since this compound reacts chemically with NH 2OH. Photoreduced flavins and electrochemically reduced methyl viologen sustain very effective inactivation of terminal nitrate reductase activity, even if the enzyme was previously deprived of its NADH-dehydrogenase activity. The early inhibition of nitrate reductase by NH 2OH appears to be competitive versus NO 3−. Since NO 3−, as well as cyanate, carbamyl phosphate and azide (competitive inhibitors of nitrate reductase versus NO 3−), protect the enzyme from NH 2OH inactivation, it is suggested that NH 2OH binds to the nitrate active site. The NH 2OH-inactivated enzyme was photoreactivated in the presence of flavins, although slower than when the enzyme was previously inactivated with CN −. NH 2OH and NADH concentrations required for full inactivation of nitrate reductase appear to be low enough to potentially consider this inactivation process of physiological significance. 相似文献
9.
Dissimilatory reduction of NO 2− to N 2O and NH 4+ by a soil Citrobacter sp. was studied in an attempt to elucidate the physiological and ecological significance of N 2O production by this mechanism. In batch cultures with defined media, NO 2− reduction to NH 4+ was favored by high glucose and low NO 3− concentrations. Nitrous oxide production was greatest at high glucose and intermediate NO 3− concentrations. With succinate as the energy source, little or no NO 2− was reduced to NH 4+ but N 2O was produced. Resting cell suspensions reduced NO 2− simultaneously to N 2O and free extracellular NH 4+. Chloramphenicol prevented the induction of N 2O-producing activity. The Km for NO 2− reduction to N 2O was estimated to be 0.9 mM NO 2−, yet the apparent Km for overall NO 2− reduction was considerably lower, no greater than 0.04 mM NO 2−. Activities for N 2O and NH 4+ production increased markedly after depletion of NO 3− from the media. Amendment with NO 3− inhibited N 2O and NH 4+ production by molybdate-grown cells but not by tungstate-grown cells. Sulfite inhibited production of NH 4+ but not of N 2O. In a related experiment, three Escherichia coli mutants lacking NADH-dependent nitrite reductase produced N 2O at rates equal to the wild type. These observations suggest that N 2O is produced enzymatically but not by the same enzyme system responsible for dissimilatory reduction of NO 2− to NH 4+. 相似文献
10.
The influence of NH 4+, in the external medium, on fluxes of NO 3− and K + were investigated using barley ( Hordeum vulgare cv Betzes) plants. NH 4+ was without effect on NO 3− ( 36ClO 3−) influx whereas inhibition of net uptake appeared to be a function of previous NO 3− provision. Plants grown at 10 micromolar NO 3− were sensitive to external NH 4+ when uptake was measured in 100 micromolar NO 3−. By contrast, NO 3− uptake (from 100 micromolar NO 3−) by plants previously grown at this concentration was not reduced by NH 4+ treatment. Plants pretreated for 2 days with 5 millimolar NO 3− showed net efflux of NO 3− when roots were transferred to 100 micromolar NO 3−. This efflux was stimulated in the presence of NH 4+. NH 4+ also stimulated NO 3− efflux from plants pretreated with relatively low nitrate concentrations. It is proposed that short term effects on net uptake of NO 3− occur via effects upon efflux. By contrast to the situation for NO 3−, net K + uptake and influx of 36Rb +-labeled K + was inhibited by NH 4+ regardless of the nutrient history of the plants. Inhibition of net K + uptake reached its maximum value within 2 minutes of NH 4+ addition. It is concluded that the latter ion exerts a direct effect upon K + influx. 相似文献
11.
The effects of nitrogen source NO 3− or NH 4+ on nitrogen metabolism during the first 2 weeks of germination of the rice seedling ( Oryza sativa L., var. IR22) grown in nutrient solution containing 40 μg/ml N were studied. Total, soluble protein, and free amino N levels were higher in the NH 4+-grown seedling, particularly during the 1st week of germination. Asparagine accounted for most of the difference in free amino acid level, in both the root and the shoot. Nitrate and nitrite reductase activities were present mainly in the shoot and were higher in the NO 3−-grown seedling, whereas the activity of glutamate dehydrogenase and glutamine synthetase in the root tended to be lower than that of the NH 4+-grown seedling during the 1st week of germination. Glycolate oxidase and catalase activities were present mainly in the shoot. Maximum activity of the above five enzymes occurred 7 to 10 days after germination. Differences in the zymograms of nitrate reductase, glutamate dehydrogenase, and catalase were mainly between shoot and root and not from N source. Nitrite reductase bands were observed only in plants grown in plants grown in NO 3−. 相似文献
12.
Aquaspirillum magnetotacticum MS-1 grew microaerobically but not anaerobically with NO 3− or NH 4+ as the sole nitrogen source. Nevertheless, cell yields varied directly with NO 3− concentration under microaerobic conditions. Products of NO 3− reduction included NH 4+, N 2O, NO, and N 2. NO 2− and NH 2OH, each toxic to cells at 0.2 mM, were not detected as products of cells growing on NO 3−. NO 3− reduction to NH 4+ was completely repressed by the addition of 2 mM NH 4+ to the growth medium, whereas NO 3− reduction to N 2O or to N 2 was not. C 2H 2 completely inhibited N 2O reduction to N 2 by growing cells. These results indicate that A. magnetotacticum is a microaerophilic denitrifier that is versatile in its nitrogen metabolism, concomitantly reducing NO 3− by assimilatory and dissimilatory means. This bacterium appears to be the first described denitrifier with an absolute requirement for O 2. The process of NO 3− reduction appears well adapted for avoiding accumulation of several nitrogenous intermediates that are toxic to cells. 相似文献
13.
The effect of exogenous NH 4+ on NO 3− uptake and in vivo NO 3− reductase activity (NRA) in roots of Phaseolus vulgaris L. cv Witte Krombek was studied before, during, and after the apparent induction of root NRA and NO 3− uptake. Pretreatment with NH 4Cl (0.15-50 millimolar) affected neither the time pattern nor the steady state rate of NO 3− uptake. When NH4+ was given at the start of NO3− nutrition, the time pattern of NO3− uptake was the same as in plants receiving no NH4+. After 6 hours, however, the NO3− uptake rate (NUR) and root NRA were inhibited by NH4+ to a maximum of 45% and 60%, respectively. The response of the NUR of NO3−-induced plants depended on the NH4Cl concentration. Below 1 millimolar NH4+, the NUR declined immediately and some restoration occurred in the second hour. In the third hour, the NUR became constant. In contrast, NH4+ at 2 millimolar and above caused a rapid and transient stimulation of NO3− uptake, followed again by a decrease in the first, a recovery in the second, and a steady state in the third hour. Maximal inhibition of steady state NUR was 50%. With NO3−-induced plants, root NRA responded less and more slowly to NH4+ than did NUR. Methionine sulfoximine and azaserine, inhibitors of glutamine synthetase and glutamate synthase, respectively, relieved the NH4+ inhibition of the NUR of NO3−-induced plants. We conclude that repression of the NUR by NH4+ depends on NH4+ assimilation. The repression by NH4+ was least at the lowest and highest NH4+ levels tested (0.04 and 25 millimolar). 相似文献
14.
Nitrogen-14 and nitrogen-15 nuclear magnetic resonance (NMR) spectra were recorded for freshly dissected buds of Picea glauca and for buds grown for 3, 6 and 9 weeks on shoot-forming medium. Resonances for Glu (and other αNH 2 groups), Pro, Ala, and the side chain groups in Gln, Arg, Orn, and γ-aminobutyric acid could be detected in in vivo15N NMR spectra. Peaks for α-amino groups, Pro, NO 3− and NH 4+ could also be identified in 14N NMR spectra. Perfusion experiments performed for up to 20 hours in the NMR spectrometer showed that 15N-labeled NH 4+ and NO 3− are first incorporated into the amide group of Gln and then in the αNH 2 pool. Subsequently, it also emerges in Ala and Arg. These data suggest that the glutamine synthetase/ glutamate synthase pathway functions under these conditions. The assimilation of NH 4+ is much faster than that of NO 3−. Consequently after 10 days of growth more than 70% of the newly synthesized internal free amino acid pool derives its nitrogen from NH 4+ rather than NO 3−. If NH 4+ is omitted from the medium, no NO 3− is taken up during 9 weeks and the buds support limited growth by utilizing their endogenous amino acid pools. It is concluded that NH 4+ and NO 3− are both required for the induction of nitrate- and nitrite reductase. 相似文献
15.
Soybean ( Glycine max [L.] Merr.) seeds were imbibed and germinated with or without NO 3−, tungstate, and norflurazon (San 9789). Norflurazon is a herbicide which causes photobleaching of chlorophyll by inhibiting carotenoid synthesis and which impairs normal chloroplast development. After 3 days in the dark, seedlings were placed in white light to induce extractable nitrate reductase activity. The induction of maximal nitrate reductase activity in greening cotyledons did not require NO 3− and was not inhibited by tungstate. Induction of nitrate reductase activity in norflurazon-treated cotyledons had an absolute requirement for NO 3− and was completely inhibited by tungstate. Nitrate was not detected in seeds or seedlings which had not been treated with NO 3−. The optimum pH for cotyledon nitrate reductase activity from norflurazon-treated seedlings was at pH 7.5, and near that for root nitrate reductase activity, whereas the optimum pH for nitrate reductase activity from greening cotyledons was pH 6.5. Induction of root nitrate reductase activity was also inhibited by tungstate and was dependent on the presence of NO 3−, further indicating that the isoform of nitrate reductase induced in norflurazon-treated cotyledons is the same or similar to that found in roots. Nitrate reductases with and without a NO 3− requirement for light induction appear to be present in developing leaves. In vivo kinetics (light induction and dark decay rates) and in vitro kinetics (Arrhenius energies of activation and NADH:NADPH specificities) of nitrate reductases with and without a NO 3− requirement for induction were quite different. Km values for NO 3− were identical for both nitrate reductases. 相似文献
16.
Biochar produced by pyrolysis of biomass can be used to counter nitrogen (N) pollution. The present study investigated the effects of feedstock and temperature on characteristics of biochars and their adsorption ability for ammonium N (NH 4
+-N) and nitrate N (NO 3
−-N). Twelve biochars were produced from wheat-straw (W-BC), corn-straw (C-BC) and peanut-shell (P-BC) at pyrolysis temperatures of 400, 500, 600 and 700°C. Biochar physical and chemical properties were determined and the biochars were used for N sorption experiments. The results showed that biochar yield and contents of N, hydrogen and oxygen decreased as pyrolysis temperature increased from 400°C to 700°C, whereas contents of ash, pH and carbon increased with greater pyrolysis temperature. All biochars could sorb substantial amounts of NH 4
+-N, and the sorption characteristics were well fitted to the Freundlich isotherm model. The ability of biochars to adsorb NH 4
+-N followed: C-BC>P-BC>W-BC, and the adsorption amount decreased with higher pyrolysis temperature. The ability of C-BC to sorb NH 4
+-N was the highest because it had the largest cation exchange capacity (CEC) among all biochars (e.g., C-BC400 with a CEC of 38.3 cmol kg −1 adsorbed 2.3 mg NH 4
+-N g −1 in solutions with 50 mg NH 4
+ L −1). Compared with NH 4
+-N, none of NO 3
−-N was adsorbed to biochars at different NO 3
− concentrations. Instead, some NO 3
−-N was even released from the biochar materials. We conclude that biochars can be used under conditions where NH 4
+-N (or NH 3) pollution is a concern, but further research is needed in terms of applying biochars to reduce NO 3
−-N pollution. 相似文献
17.
Aerobic and anaerobic groundwater continuous-flow microcosms were designed to study nitrate reduction by the indigenous bacteria in intact saturated soil cores from a sandy aquifer with a concentration of 3.8 mg of NO 3−-N liter −1. Traces of 15NO 3− were added to filter-sterilized groundwater by using a Darcy flux of 4 cm day −1. Both assimilatory and dissimilatory reduction rates were estimated from analyses of 15N 2, 15N 2O, 15NH 4+, and 15N-labeled protein amino acids by capillary gas chromatography-mass spectrometry. N 2 and N 2O were separated on a megabore fused-silica column and quantified by electron impact-selected ion monitoring. NO 3− and NH 4+ were analyzed as pentafluorobenzoyl amides by multiple-ion monitoring and protein amino acids as their N-heptafluorobutyryl isobutyl ester derivatives by negative ion-chemical ionization. The numbers of bacteria and their [ methyl- 3H]thymidine incorporation rates were simultaneously measured. Nitrate was completely reduced in the microcosms at a rate of about 250 ng g −1 day −1. Of this nitrate, 80 to 90% was converted by aerobic denitrification to N 2, whereas only 35% was denitrified in the anaerobic microcosm, where more than 50% of NO 3− was reduced to NH 4+. Assimilatory reduction was recorded only in the aerobic microcosm, where N appeared in alanine in the cells. The nitrate reduction rates estimated for the aquifer material were low in comparison with rates in eutrophic lakes and coastal sediments but sufficiently high to remove nitrate from an uncontaminated aquifer of the kind examined in less than 1 month. 相似文献
18.
The effect of nitrate and ammonium on the extractable activity of two enzymes of assimilatory sulfate reduction, ATP sulfurylase (EC 2.7.7.4) and adenosine 5′-phosphosulfate sulfotransferase (APSSTase), was examined in Lemna minor L. cultivated under steady state conditions. Nitrate reductase (EC 1.6.6.1) was measured for comparison. Low nitrate concentrations (0.2 and 0.04 millimolar) caused a decrease in the specific activity of all three enzymes measured. Twenty-four hours after transfer to medium without a nitrogen source, the specific activity of APSSTase and nitrate reductase was at less than 30% of the original level, whereas ATP sulfurylase was still at about 80%. NH 4+ added to the nutrient solution caused a 50 to 100% increase in the specific activity of APSSTase within 24 hours, followed by a slow decrease. After 72 hours with NH 4+, the specific activity was still 25% higher than originally. During the same period, the extractable protein increased by 30% on a fresh weight basis, and total protein by 55 to 60%. Nitrate reductase activity decreased to less than 5%. After omission of NH 4+ from the nutrient solution extractable APSSTase activity rapidly decreased to the level of cultures with NO 3− as a nitrogen source. Using [ 35S]SO 42− as a sulfur source, an increased incorporation of label into the protein fraction could be detected when NH 4+ was added to the nutrient solution. This indicated that more sulfate was assimilated and used for protein synthesis. The higher extractable activity of APSSTase with NH 4+ may be a regulatory mechanism involved in the formation of sufficient sulfur amino acids during a period of increased protein synthesis. 相似文献
19.
Changes in the concentrations of NH 4+ and amides during the growth of suspension cultures of rose ( Rosa cv. Paul's Scarlet) cells were examined. When cells were grown in medium possessing only NO 3− as a nitrogen source, the concentrations of NH 4+ and amides increased to 4.0 × 10 −1 and 5.9 micromoles per gram fresh weight, respectively. The amounts of both constituents declined during the later stages of growth. When a trace amount of NH 4+ was added to the NO 3− base starting medium, the concentration of NH 4+ in the cells was increased to 7.0 × 10 −1 micromoles per gram fresh weight. 相似文献
20.
Enhanced nitrogen (N) availability is one of the main drivers of biodiversity loss and degradation of ecosystem functions. However, in very nutrient-poor ecosystems, enhanced N input can, in the short-term, promote diversity. Mediterranean Basin ecosystems are nutrient-limited biodiversity hotspots, but no information is available on their medium- or long-term responses to enhanced N input. Since 2007, we have been manipulating the form and dose of available N in a Mediterranean Basin maquis in south-western Europe that has low ambient N deposition (<4 kg N ha −1 yr −1) and low soil N content (0.1%). N availability was modified by the addition of 40 kg N ha −1 yr −1 as a 1∶1 NH 4Cl to (NH 4) 2SO 4 mixture, and 40 and 80 kg N ha −1 yr −1 as NH 4NO 3. Over the following 5 years, the impacts on plant composition and diversity (richness and evenness) and some ecosystem characteristics (soil extractable N and organic matter, aboveground biomass and % of bare soil) were assessed. Plant species richness increased with enhanced N input and was more related to ammonium than to nitrate. Exposure to 40 kg NH 4
+-N ha −1 yr −1 (alone and with nitrate) enhanced plant richness, but did not increase aboveground biomass; soil extractable N even increased under 80 kg NH 4NO 3-N ha −1 yr −1 and the % of bare soil increased under 40 kg NH 4
+-N ha −1 yr −1. The treatment containing less ammonium, 40 kg NH 4NO 3-N ha −1 yr −1, did not enhance plant diversity but promoted aboveground biomass and reduced the % of bare soil. Data suggest that enhanced NH y availability affects the structure of the maquis, which may promote soil erosion and N leakage, whereas enhanced NO x availability leads to biomass accumulation which may increase the fire risk. These observations are relevant for land use management in biodiverse and fragmented ecosystems such as the maquis, especially in conservation areas. 相似文献
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