Nitrate nutrition ofDeschampsia flexuosa (L.) Trin. in relation to nitrogen deposition in Sweden

Summary Current and maximally induced nitrate reductase activity (NRA), total-N, nitrate, K, P, Ca, Mg, Mo and sucrose in leaves ofDeschampsia flexuosa was measured three times during the vegetation period in forests along a deposition gradient (150 km) in south Sweden, in north Sweden where the nitrogen deposition is considerably lower, and at heavily N-fertilized plots. In addition, the interaction between nitrogen nutrition and light was studied along transects from clearings into forest in both south and north Sweden. Plants from sites with high nitrogen deposition had elevated current NRA compared to plants from less polluted sites, indicating high levels of available soil nitrate at the former. Current NRA and total N concentration in grass from sites with high deposition resembled those found at heavily N-fertilized plots. Under such circumstances, the ratio current NRA: maximally induced NRA as well as the concentration of nitrate was high, while the concentration of sucrose was low. This suggests that the grass at these sites was already utilizing a large portion of its capacity to assimilate nitrate. Light was found to play an important role in the assimilation of nitrate; leaf concentration of sucrose was found to be negatively correlated with both nitrate and total N. Consequently, grass growing under dense canopies in south Sweden is not able to dilute N by increasing growth. The diminished capacity of the grass to assimilate nitrate will increase leaching losses of N from forests approaching N saturation.     

Soil N chemistry in oak forests along a nitrogen deposition gradient

Anthropogenic N deposition may change soil conditions in forest ecosystems as demonstrated in many studies of coniferous forests, whereas results from deciduous forests are relatively scarce. Therefore the influence of N deposition on several variables was studied in situ in 45 oak-dominated deciduous forests along a N deposition gradient in southern Sweden, where the deposition ranged from 10 to 20 kg N ha⁻¹ year⁻¹. Locally estimated NO ₋ ³ deposition, as measured with ion-exchange resins (IER) on the soil surface, and grass N concentration (%) were positively correlated with earlier modelled regional N deposition. Furthermore, the δ¹⁵N values of grass and uppermost soil layers were negatively correlated with earlier modelled N deposition. The data on soil NO ₋ ³ , measured with IER in the soil, and grass N concentration suggest increased soil N availability as a result of N deposition. The δ¹⁵N values of grass and uppermost soil layers indicate increased nitrification rates in high N deposition sites, but no large downward movements of NO ₋ ³ in these soils. Only a few sites had NO ₋ ³ concentrations exceeding 1 mg N l⁻¹ in soil solution at 50 cm depth, which showed that N deposition to these acid oak-dominated forests has not yet resulted in extensive leaching of N. The δ¹⁵N enrichment factor was the variable best correlated with NO ₋ ³ concentrations at 50 cm and is thus a variable that potentially may be used to predict leaching of NO ₋ ³ from forest soils.     

Seasonal nitrogen and carbon concentrations in white,brown and woody fine roots of sugar maple (Acer saccharum Marsh)

Small diameter (<1.0-mm) Acer saccharum Marsh roots were separated into white, brown and woody development state classes and analyzed for total N and C concentrations in April, July and October of 1988. White roots had greater concentrations of N and C than either brown or woody roots at each sampling date, and the N concentration of brown roots was consistently greater than that of woody roots. There were no temporal changes in N concentrations in any of the roots. C was slightly elevated in mid-summer in all three classes of roots. The data suggest the possible existence of an N translocation mechanism in ageing and developing fine roots. More research should be undertaken to establish the mechanisms of N loss in developing fine roots.     

Intact amino acid uptake by northern hardwood and conifer trees

Empirical and modeling studies of the N cycle in temperate forests of eastern North America have focused on the mechanisms regulating the production of inorganic N, and assumed that only inorganic forms of N are available for plant growth. Recent isotope studies in field conditions suggest that amino acid capture is a widespread ecological phenomenon, although northern temperate forests have yet to be studied. We quantified fine root biomass and applied tracer-level quantities of U–¹³C₂–¹⁵N-glycine, ¹⁵NH₄ ⁺ and ¹⁵NO₃ ⁻ in two stands, one dominated by sugar maple and white ash, the other dominated by red oak, beech, and hemlock, to assess the importance of amino acids to the N nutrition of northeastern US forests. Significant enrichment of ¹³C in fine roots 2 and 5 h following tracer application indicated intact glycine uptake in both stands. Glycine accounted for up to 77% of total N uptake in the oak–beech–hemlock stand, a stand that produces recalcitrant litter, cycles N slowly and has a thick, amino acid-rich organic horizon. By contrast, glycine accounted for only 20% of total N uptake in the sugar maple and white ash stand, a stand characterized by labile litter and rapid rates of amino acid production and turnover resulting in high rates of mineralization and nitrification. This study shows that amino acid uptake is an important process occurring in two widespread, northeastern US temperate forest types with widely differing rates of N cycling.     

The fate of 15N-labelled nitrate additions to a northern hardwood forest in eastern Maine,USA

We followed the movements of ¹⁵N-labelled nitrate additions into biomass and soil pools of experimental plots (15×15 m each) in a mid-successional beech-maple-birch-spruce forest in order to identify sinks for nitrate inputs to a forest ecosystem. Replicate plots (n=3) were spray-irrigated with either 28 or 56 kg N ha^–1 year^–1 using ¹⁵N-labelled nitric acid solutions (¹⁵N = 344 ) during four successive growing seasons (April–October). The ¹⁵N contents of foliage, bolewood, forests floor and mineral soil (0–5 cm) increased during the course of treatments. Mass balance calculations showed that one-fourth to one-third of the nitrate applied to forest plots was assimilated into and retained by above ground plant tissues and surface soil horizons at both rates of nitrate application. Plant and microbial assimilation were of approximately equal importance in retaining nitrate additions to this forest. Nitrate use among tree species varied, however, with red spruce showing lower rates of nitrate assimilation into foliage and bolewood than American beech and other deciduous species.     

Temporal patterns of inorganic nitrogen uptake by mature sugar maple (Acer saccharum Marsh.) and red spruce (Picea rubens Sarg.) trees using two common approaches

Background: Plant uptake of nitrogen influences many ecosystem processes, yet uptake by trees in northern forests of the United States has not been quantified throughout the growing season.

Aims: To measure NH₄ ⁺ and NO₃ ^? uptake by mature sugar maple (Acer saccharum) and red spruce (Picea rubens) trees during the early, mid and late growing season.

Methods: At Hubbard Brook Experimental Forest, New Hampshire, we used two approaches to measure nitrogen uptake capacity by mature trees: an in situ depletion method using intact roots and an ex situ ¹⁵N tracer method using excised roots.

Results: NH₄ ⁺ uptake was greater than NO₃ ^? for both methods and tree species (P < 0.05). NH₄ ⁺ uptake was lowest during the early growing season, while NO₃ ^? uptake was lowest during the late growing season. Measured rates of NH₄ ⁺ uptake were 2–3 orders of magnitude greater using the in situ depletion method compared with the ex situ ¹⁵N tracer method.

Conclusions: These results demonstrate seasonal differences in nitrogen uptake by two dominant tree species in a northern forest and show that the method employed can significantly impact measured rates of uptake, which could have implications for understanding the magnitude of plant nitrogen uptake and for cross-study comparisons of this process.     

The ability of several high arctic plant species to utilize nitrate nitrogen under field conditions

The ability to utilize NO _inf3 ^sup– in seven high arctic plant species from Truelove Lowland, Devon Island, Canada was investigated, using an in vivo assay of maximum potential nitrate reductase (NR) activity and applications of ¹⁵N. Plant species were selected on the basis of being characteristic of nutrient-poor and nutrient-rich habitats. In all species leaves were the dominant site of NR activity. Root NR activity was negligible in all species except Saxifraga cernua. NO _inf3 ^sup– availability per se did not appear to limit NR activity of the species typically found on nutrient-poor sites (Dryas integrifolia, Saxifraga oppositifolia, and Salix arctica), or in Cerastium alpinum, as leaf NR activities remained low, even after NO _inf3 ^sup– addition. ¹⁵NO _inf3 ^sup– uptake was limited in D. integrifolia and Salix arctica. However, the lack of field induction of NR activity in C. alpinum and Saxifraga oppositifolia was not due to restricted nitrate uptake, as ¹⁵NO _inf3 ^sup– labelled NO _inf3 ^sup– entered the roots and shoots of both species. Leaf NR activity rates were low in three of the species typical of nutrient-rich habitats (O. digyna, P. radicatum and Saxifraga cernua), sampled from a site containing low soil NO _inf3 ^sup– . Additions of NO _inf3 ^sup– significantly increased leaf NR activity in these latter species, suggesting that potential NR activity was limited by the availability of NO _inf3 ^sup– . ¹⁵N labelled NO _inf3 ^sup– was taken up by O. digyna. P. radicatum and Saxifraga cernua. Although two species (D. integrifolia and Salix arctica) showed little utilization of NO _inf3 ^sup– , we concluded that five of the seven selected high arctic plant species (C. alpinum, O. digyna, P. radicatum, Saxifraga cernua and Saxifraga oppositifolia) do have the potential to utilize NO _inf3 ^sup– as a nitrogen source under field conditions, with the highest potential to utilize NO _inf3 ^sup– occurring in three of the species typically found on fertile habitats.     

Unusual seasonal patterns and inferred processes of nitrogen retention in forested headwaters of the Upper Susquehanna River

Atmospheric deposition contributes a large fraction of the annual nitrogen (N) input to the basin of the Susquehanna River, a river that provides two-thirds of the annual N load to the Chesapeake Bay. Yet, there are few measurements of the retention of atmospheric N in the Upper Susquehanna’s forested headwaters. We characterized the amount, form (nitrate, ammonium, and dissolved organic nitrogen), isotopic composition (δ¹⁵N- and δ¹⁸O-nitrate), and seasonality of stream N over 2 years for 7–13 catchments. We expected high rates of N retention and seasonal nitrate patterns typical of other seasonally snow-covered catchments: dormant season maxima and growing season minima. Coarse estimates of N export indicated high rates of inorganic N retention (>95%), yet streams had unexpected seasonal nitrate patterns, with summer peaks (14–96 μmol L⁻¹), October crashes (<1 μmol L⁻¹), and modest rebounds during the dormant season (<1–20 μmol L⁻¹). Stream δ¹⁸O-nitrate values indicated microbial nitrification as the primary source of stream nitrate, although snowmelt or other atmospheric source contributed up to 47% of stream nitrate in some March samples. The autumn nitrate crash coincided with leaffall, likely due to in-stream heterotrophic uptake of N. Hypothesized sources of the summer nitrate peaks include: delayed release of nitrate previously flushed to groundwater, weathering of geologic N, and summer increases in net nitrate production. Measurements of shale δ¹⁵N and soil-, well-, and streamwater nitrate within one catchment point toward a summer increase in soil net nitrification as the driver of this pattern. Rather than seasonal plant demand, processes governing the seasonal production, retention, and transport of nitrate in soils may drive nitrate seasonality in this and many other systems.     

The effect of nitrogen refeeding on starved cells of Platymonas striata Butcher

A rapid uptake of nitrogen was observed in nitrogen-starved cells of Platymonas striata after refeeding with ammonium or nitrate ions. This was followed by a net loss of nitrogen per cell. Cells initially grown in and then starved in a regime of continuous light showed greater increases in average cell nitrogen on refeeding with ammonium or nitrate ions than did cells initially grown in and then starved in a regime of alternating light and darkness. A particulate subcellular location was observed for nitrate reductase (EC 1.6.6.1) in broken cell suspensions prepared by sonication. Nitrite reductase (EC 1.6.6.4) was located in the soluble fraction of these cell suspensions. Broken cell preparations displayed a lowered nitrate reductase activity as compared with the particulate component of these preparations. This was shown not to be due to heat-stable inhibitors present in the soluble phase of the cell. It appeared to be an artefact produced by the high nitrite reductase activity of the broken cell preparations, which removed much of the nitrite as it was formed. Nitrogen starvation of nitrate-grown cultures produced cellular increases in nitrate reductase and nitrite reductase activities which were further increased after the addition of nitrate. The results are discussed.Abbreviations ASP2 complete culture medium - ASP2 INF medium lacking in inorganic nitrogen - ASP2 NF medium lacking all nitrogen - NAR nitrate reductase - NIR nitrite reductase - EDTA Ethylenediaminetetracetic acid - PVP Polyvinylpyrollidone, M.W. 44,000     

Chronic nitrate additions dramatically increase the export of carbon and nitrogen from northern hardwood ecosystems

A long-term field experiment was initiated to simulate chronic atmospheric N deposition, a widespread phenomenon in industrial regions of the world. Eight years of experimental nitrate ( ) additions (3 g -N m^–2 per year) to four different northern hardwood forests located along a 500 km geographic gradient dramatically increased leaching losses of -N, dissolved organic carbon (DOC), and dissolved organic nitrogen (DON). During the last two water years, the average increase in solution -N and DON leaching from the -amended plots was 2.2 g N m^–2, equivalent to 72% of the annual experimental N addition. Results indicate that atmospheric N deposition may rapidly saturate some northern hardwood ecosystems across an entire biome in the upper Great Lakes Region of the USA. Changes in soil C and N cycling induced by chronic N deposition have the potential in this landscape to significantly alter the flux of DOC and DON from upland to aquatic ecosystems. Michigan Gradient study site characteristics are similar to those of European forests most susceptible to N saturation.     

Simulated nitrogen deposition affects community structure of arbuscular mycorrhizal fungi in northern hardwood forests

Our previous investigation found elevated nitrogen deposition caused declines in abundance of arbuscular mycorrhizal fungi (AMF) associated with forest trees, but little is known about how nitrogen affects the AMF community composition and structure within forest ecosystems. We hypothesized that N deposition would lead to significant changes in the AMF community structure. We studied the diversity and community structure of AMF in northern hardwood forests after more than 12 years of simulated nitrogen deposition. We performed molecular analyses on maple (Acer spp.) roots targeting the 18S rDNA region using the fungal‐specific primers AM1 and NS31. PCR products were cloned and identified using restriction fragment length polymorphism (RFLP) and sequencing. N addition significantly altered the AMF community structure, and Glomus group A dominated the AMF community. Some Glomus operational taxonomic units (OTUs) responded negatively to N inputs, whereas other Glomus OTUs and an Acaulospora OTU responded positively to N inputs. The observed effect on community structure implies that AMF species associated with maples differ in their response to elevated nitrogen. Given that functional diversity exists among AMF species and that N deposition has been shown to select less beneficial fungi in some ecosystems, this change in community structure could have implications for the functioning of this type of ecosystem.     

Mineralization of organically bound nitrogen in soil as influenced by plant growth and fertilization

Summary A loam soil containing an organic fraction labelled with¹⁵N was used for pot experiments with spring barley, rye-grass and clover. The organically bound labelled N was mineralized at a rate corresponding to a half-life of about 9 years. Fertilization with 106 and 424 kgN/ha of unlabelled N in the form of KNO₃ significantly increased uptake of labelled N from the soil in barley and the first harvest of rye-grass crops. The fertilized plants removed all the labelled NH₄ and NO₃ present in the soil, whereas the unfertilized plants removed only about 80%. The second, third and fourth harvests of the unfertilized rye-grass took up more labelled N than the fertilized rye-grass. The total uptake in the four harvests was similar whether the plants were fertilized or not. Application of KCl to barley plants in amounts equivalent to that of KNO₃ resulted in a small but insignificant increase in uptake of labelled N. The uptake of labelled N in the first harvest of clover which was not fertilized but inoculated with Rhizobium was similar to that of the fully fertilized rye-grass indicating that the biological fixation of N had the same effect as addition of N-fertilizer. N uptake in the following harvests was lower and the total uptake by four harvests of clover was similar to that of rye-grass. There was no indication that fertilization with KNO₃ accelerated the mineralization of the organically bound labelled N. The observed apparent ‘priming effect’ of the fertilizer on the uptake of labelled N was compensated by subsequent crops and harvests, and it seems to arise from a more thorough search of the soil volume by a better developed root system of the fertilized plants.     

Root contribution to nitrate reduction in barley seedlings (Hordeum vulgare L.)

Summary The leaf and root nitrate reductase activities were measured in 7 day-old barley seedlings by anoxic nitrite accumulation in darkness, during 48h after the transfer from a N-starved medium to a 1.5 mM K¹⁵NO₃ medium. Thisin situ nitrate reduction was compared with the¹⁵N incorporation in the reduced N fraction of the whole seedlings.The nitrate reduction integrated fromin situ measurements was lower than the reduced¹⁵N accumulation. The rootin situ nitrate reductase activity seemed to account for only the third of the real root nitrate reduction, which may have been responsible for the overall underestimation. This discrepancy was partly explained by the ability of the root to reduce nitrite in an anoxic environment.These results suggest that, after correction of thein situ estimation of the nitrate reduction. the roots contribute to about 50% of the total assimilation.     

Anthropogenic nitrogen deposition in boreal forests has a minor impact on the global carbon cycle

It is proposed that increases in anthropogenic reactive nitrogen (N_r) deposition may cause temperate and boreal forests to sequester a globally significant quantity of carbon (C); however, long‐term data from boreal forests describing how C sequestration responds to realistic levels of chronic N_r deposition are scarce. Using a long‐term (14‐year) stand‐scale (0.1 ha) N addition experiment (three levels: 0, 12.5, and 50 kg N ha⁻¹ yr⁻¹) in the boreal zone of northern Sweden, we evaluated how chronic N additions altered N uptake and biomass of understory communities, and whether changes in understory communities explained N uptake and C sequestration by trees. We hypothesized that understory communities (i.e. mosses and shrubs) serve as important sinks for low‐level N additions, with the strength of these sinks weakening as chronic N addition rates increase, due to shifts in species composition. We further hypothesized that trees would exhibit nonlinear increases in N acquisition, and subsequent C sequestration as N addition rates increased, due to a weakening understory N sink. Our data showed that understory biomass was reduced by 50% in response to the high N addition treatment, mainly due to reduced moss biomass. A ¹⁵N labeling experiment showed that feather mosses acquired the largest fraction of applied label, with this fraction decreasing as the chronic N addition level increased. Contrary to our hypothesis, the proportion of label taken up by trees was equal (ca. 8%) across all three N addition treatments. The relationship between N addition and C sequestration in all vegetation pools combined was linear, and had a slope of 16 kg C kg⁻¹ N. While canopy retention of N_r deposition may cause C sequestration rates to be slightly different than this estimate, our data suggest that a minor quantity of annual anthropogenic CO₂ emissions are sequestered into boreal forests as a result of N_r deposition.     

Nitrogen fixation and metabolism by groundwater-dependent perennial plants in a hyperarid desert

The Central Asian Taklamakan desert is characterized by a hyperarid climate with less than 50 mm annual precipitation but a permanent shallow groundwater table. The perched groundwater (2–16 m) could present a reliable and constant source of nitrogen throughout the growing season and help overcome temporal nitrogen limitations that are common in arid environments. We investigated the importance of groundwater and nitrogen fixation in the nitrogen metabolism of desert plants by assessing the possible forms and availability of soil N and atmospheric N and the seasonal variation in concentration as well as isotopic composition of plant N. Water availability was experimentally modified in the desert foreland through simulated flooding to estimate the contribution of surface water and temporally increased soil moisture for nutrient uptake and plant–water relations. The natural vegetation of the Taklamakan desert is dominated by plants with high foliar nitrogen concentrations (2–3% DM) and leaf nitrate reductase activity (NRA) (0.2–1 mol NO₂^– g^–1 FW h^–1). There is little evidence that nitrogen is a limiting resource as all perennial plants exhibited fast rates of growth. The extremely dry soil conditions preclude all but minor contributions of soil N to total plant N so that groundwater is suggested as the dominant source of N with concentrations of 100 M NO₃^–. Flood irrigation had little beneficial effect on nitrogen metabolism and growth, further confirming the dependence on groundwater. Nitrogen fixation was determined by the ¹⁵N natural abundance method and was a significant component of the N-requirement of the legume Alhagi, the average contribution of biologically fixed nitrogen in Alhagi was 54.8%. But nitrogen fixing plants had little ecological advantage owing to the more or less constant supply of N available from groundwater. From our data we conclude that the perennial species investigated have adapted to the environmental conditions through development of root systems that access groundwater to satisfy demands for both water and nutrients. This is an ecologically favourable strategy since only groundwater is a predictable and stable resource.     

Biochemical and morphological stratification inAcer saccharum root callus

Summary Root callus of sugar maple (Acer saccharum, Marsh.) exhibits differential metabolic activities and cellular morphologies at different depths. A thin layer of acid phosphataseactive cells encloses a second layer of cells that reduce nitroblue tetrazolium (NBT). Mitosis occurs within the cells of this NBT reducing layer (which is one-third of the total depth of the callus), and the cells beneath it show acid phosphatase activity. Morphologically the cells at the outer surface appear tubular and loosely attached, with intact nuclei and cytoplasm. The cells of the NBT region are isodiametric and possess a large vacuole. The cells below the NBT region show, progressive degeneration with depth by loss of nuclei, cytoplasm, and cell wall integrity. There is new cell growth only from the NBT region when aseptically cut callus sections are placed on new medium, regardless of whether or not this region is in direct contact with the agar. Several hypotheses to explain sugar maple callus organization are discussed. This paper is Vermont Agricultural Experiment, Station Journal No. 457.     

Remobilization of labelled nitrogen from vegetative tissues to pods of mungbean

Summary Remobilization of¹⁵N from vegetative tissue of mungbean (Vigna radiata (L.) Wilczek) into pods was measured during the reproductive phase of growth. Plant tissue was labelled with¹⁵N during vegetative development. Experiments were conducted in the field at two sites. At one site the soil provided cowpeas with most of their N but at the other site N fixation provided most of the N. Remobilized N from vegetative tissue to pods occurred soon after they began to develop. The quantity of the labelled N ultimately remobilized to the pods amounted to 50% for one cultivar (Tx33) at the high soil N site and 70% at the low N site. For the other cultivar (Tx13) the values were 25% and 30%, respectively. The two cultivars performed very differently with respect to partitioning of N into pods and the rate of N fixation. Even though more N was accumulated in the shoots of the high N fixing cultivar (Tx13) less total N was contained in the pods.     

Relationships between external nitrate availability,nitrate uptake and expression of nitrate reductase in roots of barley grown in N-limited split-root cultures

Despite the large number of studies of nitrate metabolism in plants, it remains undetermined to what extent this key plant system is controlled by overall plant N nutrition on the one hand, and by the nitrate ion itself on the other hand. To investigate these questions, V _max for nitrate uptake (high-affinity range), and nitrate reductase (NR) mRNA and activity, were measured in roots of N-limited barley (Hordeum vulgare L. cv. Golf) grown under conditions of constant relative addition of nitrate, with the seminal roots split between two culture compartments. The total amount of nitrate added per unit time (0.09·d^-1) was distributed between the two root parts (subroots) in ratios of 1000, 982, 955, 9010, 8020, and 5050. These nitrate-addition ratios resulted in nitrate fluxes ranging from 0 to 23 mol nitrate·g^-1 DW root·h^-1, while the external nitrate concentrations varied between 0 and 1.2 M. The apparent V _max for net nitrate uptake showed saturation-type responses to nitrate flux maintained during preceding growth. The flux resulting in half-maximal induction of nitrate uptake was approximately 4 mol nitrate·g^-1 DW root·h^-1, corresponding to an external nitrate concentration of 0.7 M. The activity of NR and levels of NR mRNA did not saturate within the range of nitrate fluxes studied. None of the parameters studied saturated with respect to the steady-state external nitrate concentration. At the zero nitrate addition — the 0%-root — initial uptake activity as determined in short-term ¹⁵N-labelling experiments was insignificant, and NR activity and NR mRNA were not detectable. However, nitrate uptake was rapidly induced, showing that the 0%-root had retained the capacity to respond to nitrate. These results suggest that local nitrate availability has a significant impact on the nitrate uptake and reducing systems of a split-root part when the total plant nitrate nutrition is held constant and limiting.Abbreviation NR nitrate reductase This work was supported by the Lars Hierta Memory Foundation, the Royal Swedish Academy of Sciences, and by the Swedish Natural Science Research Council via project grants (to C.-M.L. and B.I.) and visiting scientist grant (to W.H.C.). We thank Mrs. Ellen Campbell for technical advice, and Mrs. Judith V. Purves, Long Ashton Research Station, Long Ashton, UK, for analyses of ¹⁵N-labelling in tissue samples.     

Phytoplankton nitrogen demand and the significance of internal and external nitrogen sources in a large shallow lake (Lake Balaton,Hungary)

Since the middle of 1990s the trend of Lake Balaton towards an increasingly trophic status has been reversed, but N₂-fixing cyanobacteria are occasionally dominant, endangering water quality in summer. The sources of nitrogen and its uptake by growing phytoplankton were therefore studied. Experiments were carried out on samples collected from the middle of the Eastern (Siófok) and Western (Keszthely) basins between February and October 2001. Ammonium, urea and nitrate uptake and ammonium regeneration were measured in the upper 5-cm layer of sediment using the ¹⁵N-technique. Ammonium was determined by an improved microdiffusion assay. N₂ fixation rates were measured by the acetylene-reduction method. Ammonium regeneration rates in the sediment were similar in the two basins. They were relatively low in winter (0.13 and 0.16 μg N cm^?3 day^?1 in the Eastern and Western basin, respectively), increased slowly in the spring (0.38 and 0.45 μg N cm^?3 day^?1) and peaked in late summer (0.82 and 1.29 μg N cm^?3 day^?1, respectively). Ammonium uptake was predominant in spring in the Eastern basin and in summer in the Western basin, coincident with the cyanobacterial bloom. The amount of N₂ fixed was less than one third of the internal load during summer when external N loading was insignificant. Potentially, the phytoplankton N demand could be supported entirely by the internal N load via ammonium regeneration in the water column and sediment. However, the quantity of N from ammonium regeneration in the upper layer of sediment combined with that from the water column would limit the standing phytoplankton crop in spring in both basins and in late summer in the Western basin, especially when the algal biomass increases suddenly.     

Relationship between nitrogen fixation and nitrate metabolism in the Nodularia strains M1 and M2

Nitrogen fixation and nitrate-reduction activities were determined in photoautotrophic cultures of two wild-type strains of cyanobacterium Nodularia, spp. M1 and M2. Air could support growth of the two strains at a similar rate in the presence or absence of exogenous nitrate, ammonium and/or bicarbonate. Nitrogenase activity in air-grown cultures varied with culture age, and totally disappeared after 6 h of darkness. Recovery took place upon culture re-illumination. Ammonium at a concentration of 1 mM resulted in the total disappearance of nitrogenase activity and of heterocysts. In contrast, 20 mM nitrate hardly affected nitrogenase activity and heterocyst formation after ten generations. Under the same conditions, either ammonium or nitrate completely abolished nitrogenase activity and heterocyst formation in Anabaena sp. PCC 7119, a typical heterocystous strain. The inefficiency of nitrate in inhibiting nitrogen fixation in Nodularia M1 and M2 seemed to be caused by a low nitrate-reductase activity, and not by an impairment of nitrate-uptake activity. On the other hand, the presence of nitrate was not required for uptake activity to be expressed in Nodularia.Abbreviation NR nitrate reductase We thank C. Fernández-Cabrera (Consejo Superior de Investigaciones Científicas, CSIC, Madrid, Spain) for technical assistance, and Dr. G. Pérez-Silva (CSIC) for his collaboration in the Anabaena NR assays. This work was supported by grants from Spanish CI-CyT (PB 87-0204 and PB 92-0497).