Seasonal changes in nitrate use by three woody species: the importance of the leaf-expansion period

Seasonal changes in plant NO₃ ⁻-N use were investigated by measuring leaf nitrate reductase activity (NRA), leaf N concentration, and leaf expansion in one evergreen woody species (Quercus glauca Thunb.) and two deciduous woody species [Acer palmatum Thunb. and Zelkova serrata (Thunb.) Makino]. Leaf N concentration was highest at the beginning of leaf expansion and decreased during the expansion process to a steady state at the point of full leaf expansion in all species. The leaf NRA of all species was very low at the beginning of leaf expansion, followed by a rapid increase and subsequent decrease. The highest leaf NRA was observed in the middle of the leaf-expansion period, and the lowest leaf NRA occurred in summer for all species. Significant positive correlations were detected between leaf NRA and leaf expansion rates, while leaf N concentrations were negatively correlated with leaf area. In the evergreen Q. glauca, the N concentration in current buds increased before leaves opened; concurrently, the N concentration in 1-year-old leaves decreased by 25%. Our results show that the leaf-expansion period is the most important period for NO₃ ⁻-N assimilation by broadleaf tree species, and that decreases in leaf N concentration through the leaf-expansion period are at least partly compensated for by newly assimilated NO₃ ⁻-N in current leaves.     

Chronic Atmospheric NO<Stack><Subscript>3</Subscript><Superscript>−</Superscript></Stack> Deposition Does Not Induce NO<Stack><Subscript>3</Subscript><Superscript>−</Superscript></Stack> Use by <Emphasis Type="Italic">Acer saccharum</Emphasis> Marsh.

The ability of an ecosystem to retain anthropogenic nitrogen (N) deposition is dependent upon plant and soil sinks for N, the strengths of which may be altered by chronic atmospheric N deposition. Sugar maple (Acer saccharum Marsh.), the dominant overstory tree in northern hardwood forests of the Lake States region, has a limited capacity to take up and assimilate NO₃⁻. However, it is uncertain whether long-term exposure to NO₃⁻ deposition might induce NO₃⁻ uptake by this ecologically important overstory tree. Here, we investigate whether 10 years of experimental NO₃⁻ deposition (30 kg N ha⁻¹ y⁻¹) could induce NO₃⁻ uptake and assimilation in overstory sugar maple (approximately 90 years old), which would enable this species to function as a direct sink for atmospheric NO₃⁻ deposition. Kinetic parameters for NH₄⁺ and NO₃⁻ uptake in fine roots, as well as leaf and root NO₃⁻ reductase activity, were measured under conditions of ambient and experimental NO₃⁻ deposition in four sugar maple-dominated stands spanning the geographic distribution of northern hardwood forests in the Upper Lake States. Chronic NO₃⁻ deposition did not alter the V _max or K _m for NO₃⁻ and NH₄⁺ uptake nor did it influence NO₃⁻ reductase activity in leaves and fine roots. Moreover, the mean V _max for NH₄⁺ uptake (5.15 μmol ¹⁵N g⁻¹ h⁻¹) was eight times greater than the V _max for NO₃⁻ uptake (0.63 μmol ¹⁵N g⁻¹ h⁻¹), indicating a much greater physiological capacity for NH₄⁺ uptake in this species. Additionally, NO₃⁻ reductase activity was lower than most values for woody plants previously reported in the literature, further indicating a low physiological potential for NO₃⁻ assimilation in sugar maple. Our results demonstrate that chronic NO₃⁻ deposition has not induced the physiological capacity for NO₃⁻ uptake and assimilation by sugar maple, making this dominant species an unlikely direct sink for anthropogenic NO₃⁻ deposition.     

Functional diversity in an Amazonian rainforest of French Guyana: a dual isotope approach (δ15N and δ13C)

Functional aspects of biodiversity were investigated in a lowland tropical rainforest in French Guyana (5°2′N, annual precipitation 2200 mm). We assessed leaf δ¹⁵N as a presumptive indicator of symbiotic N₂ fixation, and leaf and wood cellulose δ¹³C as an indicator of leaf intrinsic water-use efficiency (CO₂ assimilation rate/leaf conductance for water vapour) in dominant trees of 21 species selected for their representativeness in the forest cover, their ecological strategy (pioneers or late successional stage species, shade tolerance) or their potential ability for N₂ fixation. Similar measurements were made in trees of native species growing in a nearby plantation after severe perturbation (clear cutting, mechanical soil disturbance). Bulk soil δ¹⁵N was spatially quite uniform in the forest (range 3–5‰), whereas average leaf δ¹⁵N ranged from −0.3‰ to 3.5‰ in the different species. Three species only, Diplotropis purpurea, Recordoxylon speciosum (Fabaceae), and Sclerolobium melinonii (Caesalpiniaceae), had root bacterial nodules, which was also associated with leaf N concentrations higher than 20 mg g⁻¹. Although nodulated trees displayed significantly lower leaf δ¹⁵N values than non-nodulated trees, leaf δ¹⁵N did not prove a straightforward indicator of symbiotic fixation, since there was a clear overlap of δ¹⁵N values for nodulated and non-nodulated species at the lower end of the δ¹⁵N range. Perturbation did not markedly affect the difference δ¹⁵N_soil−δ¹⁵N_leaf, and thus the isotopic data provide no evidence of an alteration in the different N acquisition patterns. Extremely large interspecific differences in sunlit leaf δ¹³C were observed in the forest (average values from −31.4 to −26.7‰), corresponding to intrinsic water-use efficiencies (ratio CO₂ assimilation rate/leaf conductance for water vapour) varying over a threefold range. Wood cellulose δ¹³C was positively related to total leaf δ¹³C, the former values being 2–3‰ higher than the latter ones. Leaf δ¹³C was not related to leaf δ¹⁵N at either intraspecific or interspecific levels. δ¹³C of sunlit leaves was highest in shade hemitolerant emergent species and was lower in heliophilic, but also in shade-tolerant species. For a given species, leaf δ¹³C did not differ between the pristine forest and the disturbed plantation conditions. Our results are not in accord with the concept of existence of functional types of species characterized by common suites of traits underlying niche differentiation; rather, they support the hypothesis that each trait leads to a separate grouping of species. Received: 18 August 1997 / Accepted: 14 April 1998     

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.     

Microbial Cycling of C and N in Northern Hardwood Forests Receiving Chronic Atmospheric NO 3 − Deposition

Sugar maple (Acer saccharum Marsh.)-dominated northern hardwood forests in the upper Lakes States region appear to be particularly sensitive to chronic atmospheric NO₃⁻ deposition. Experimental NO₃⁻ deposition (3 g NO₃⁻ N m⁻² y⁻¹) has significantly reduced soil respiration and increased the export of DOC/DON and NO₃⁻ across the region. Here, we evaluate the possibility that diminished microbial activity in mineral soil was responsible for these ecosystem-level responses to NO₃⁻ deposition. To test this alternative, we measured microbial biomass, respiration, and N transformations in the mineral soil of four northern hardwood stands that have received 9 years of experimental NO₃⁻ deposition. Microbial biomass, microbial respiration, and daily rates of gross and net N transformations were not changed by NO₃⁻ deposition. We also observed no effect of NO₃⁻ deposition on annual rates of net N mineralization. However, NO₃⁻ deposition significantly increased (27%) annual net nitrification, a response that resulted from rapid microbial NO₃⁻ assimilation, the subsequent turnover of NH₄⁺, and increased substrate availability for this process. Nonetheless, greater rates of net nitrification were insufficient to produce the 10-fold observed increase in NO₃⁻ export, suggesting that much of the exported NO₃⁻ resulted directly from the NO₃⁻ deposition treatment. Results suggest that declines in soil respiration and increases in DOC/DON export cannot be attributed to NO₃⁻-induced physiological changes in mineral soil microbial activity. Given the lack of response we have observed in mineral soil, our results point to the potential importance of microbial communities in forest floor, including both saprotrophs and mycorrhizae, in mediating ecosystem-level responses to chronic NO₃⁻ deposition in Lake States northern hardwood forests.     

Soil-mixing effects on inorganic nitrogen production and consumption in forest and shrubland soils

Soils that are physically disturbed are often reported to show net nitrification and NO₃⁻ loss. To investigate the response of soil N cycling rates to soil mixing, we assayed gross rates of mineralization, nitrification, NH₄⁺ consumption, and NO₃⁻ consumption in a suite of soils from eleven woody plant communities in Oregon, New Mexico, and Utah. Results suggest that the common response of net NO₃⁻ flux from disturbed soils is not a straightforward response of increased gross nitrification, but instead may be due to the balance of several factors. While mineralization and NH₄⁺ assimilation were higher in mixed than intact cores, NO₃⁻ consumption declined. Mean net nitrification was 0.12 mg N kg⁻¹ d⁻¹ in disturbed cores, which was significantly higher than in intact cores (−0.19 mg N kg⁻¹ d⁻¹). However, higher net nitrification rates in disturbed soils were due to the suppression of NO₃⁻ consumption, rather than an increase in nitrification. Our results suggest that at least in the short term, disturbance may significantly increase NO₃⁻ flux at the ecosystem level, and that N cycling rates measured in core studies employing mixed soils may not be representative of rates in undisturbed soils.     

Leaf nitrogen dioxide uptake coupling apoplastic chemistry, carbon/sulfur assimilation, and plant nitrogen status

Emission and plant uptake of atmospheric nitrogen oxides (NO + NO₂) significantly influence regional climate change by regulating the oxidative chemistry of the lower atmosphere, species composition and the recycling of carbon and nutrients, etc. Plant uptake of nitrogen dioxide (NO₂) is concentration-dependent and species-specific, and covaries with environmental factors. An important factor determining NO₂ influx into leaves is the replenishment of the substomatal cavity. The apoplastic chemistry of the substomatal cavity plays crucial roles in NO₂ deposition rates and the tolerance to NO₂, involving the reactions between NO₂ and apoplastic antioxidants, NO₂-responsive germin-like proteins, apoplastic acidification, and nitrite-dependent NO synthesis, etc. Moreover, leaf apoplast is a favorable site for the colonization by microbes, which disturbs nitrogen metabolism of host plants. For most plant species, NO₂ assimilation in a leaf primarily depends on the nitrate (NO₃ ⁻) assimilation pathway. NO₂–N assimilation is coupled with carbon and sulfur (sulfate and SO₂) assimilation as indicated by the mutual needs for metabolic intermediates (or metabolites) and the NO₂-caused changes of key metabolic enzymes such as phosphoenolpyruvate carboxylase (PEPc) and adenosine 5′-phosphosulfate sulfotransferase, organic acids, and photorespiration. Moreover, arbuscular mycorrhizal (AM) colonization improves the tolerance of host plants to NO₂ by enhancing the efficiency of nutrient absorption and translocation and influencing foliar chemistry. Further progress is proposed to gain a better understanding of the coordination between NO₂–N, S and C assimilation, especially the investigation of metabolic checkpoints, and the effects of photorespiratory nitrogen cycle, diverse PEPc and the metabolites such as cysteine, O-acetylserine (OAS) and glutathione.     

The influence of temperature and nitrate on vegetative growth and nitrogen accumulation by nodulated soybeans

Summary Inoculated soybeans [Glycine max (L.) Merrill] were grown in controlled environments to evaluate the relationship between temperature and applied NO₃−N on growth rates, N accumulation, and acetylene reduction activity during the vegetative growth stage. Soybeans were grown at day/night temperatures of 22/18 and 26/22°C in sand culture with daily applications of 21.4 mM (high) and 2.1 mM (low) NO₃−N in a complete nutrient solution for durations of 14, 21, and 42 days after emergence and with an N-free solution. Dry matter and N accumulation were greater at 26/22 than 22/18°C. In general, both increased as the level and duration of applied NO₃−N was increased. These increases were attributable to an abbreviation in the interval between emergence and onset of rapid growth. The presence and assimilation of NO₃−N, even at the high level, did not inhibit development of functional nodules. Neither mass nor acetylene reduction activity of nodules was reduced by high NO₃−N; however, the root mass was increased by NO₃−N more than the nodule mass. There was an interaction between temperature and NO₃−N on specific nodule activity as measured by acetylene reduction. The specific nodule activity was unaffected by NO₃−N at 22/18°C, but at 26/22°C the specific activity was lower in the absence of NO₃−N than when NO₃−N was present. Apparently, rapid early growth at 26/22°C depleted cotyledonary reserves of N before nodules became active and, thereafter, the plants were unable to develop adequate leaf area to support nodule development and functioning. This result has implications in N fertilization of late-planted soybeans. Paper number 6637 of the Journal Series of the North Carolina Agricultural Research Service, Raleigh, North Carolina, 27650. The research was supported in part by a grant from the North Carolina Soybean Producers Association and by USDA-SEA-CR grant 701-15-26.     

Nitrate-dependent control of root architecture and N nutrition are altered by a plant growth-promoting Phyllobacterium sp

Both root architecture and plant N nutrition are altered by inoculation with the plant growth-promoting rhizobacteria (PGPR) Phyllobacterium strain STM196. It is known that NO₃⁻ and N metabolites can act as regulatory signals on root development and N transporters. In this study, we investigate the possible interrelated effects on root development and N transport. We show that the inhibition of Arabidopsis lateral root growth by high external NO₃⁻ is overridden by Phyllobacterium inoculation. However, the leaf NO₃⁻ pool remained unchanged in inoculated plants. By contrast, the Gln root pool was reduced in inoculated plants. Unexpectedly, NO₃⁻ influx and the expression levels of AtNRT1.1 and AtNRT2.1 genes coding for root NO₃⁻ transporters were also decreased after 8 days of Phyllobacterium inoculation. Although the mechanisms by which PGPR exert their positive effects remain unknown, our data show that they can optimize plant development independently from N supply, thus alleviating the regulatory mechanisms that operate in axenic conditions. In addition, we found that Phyllobacterium sp. elicited a very strong induction of AtNRT2.5 and AtNRT2.6, both genes preferentially expressed in the shoots whose functions are unknown.     

NO<Subscript>3</Subscript><Superscript>−</Superscript>-Driven SO<Subscript>4</Subscript><Superscript>2−</Superscript> Production in Freshwater Ecosystems: Implications for N and S Cycling

Massive anthropogenic acceleration of the global nitrogen (N) cycle has stimulated interest in understanding the fate of excess N loading to aquatic ecosystems. Nitrate (NO₃ ⁻) is traditionally thought to be removed mainly by microbial respiratory denitrification coupled to carbon (C) oxidation, or through biomass assimilation. Alternatively, chemolithoautotrophic bacterial metabolism may remove NO₃ ⁻ by coupling its reduction with the oxidation of sulfide to sulfate (SO₄ ²⁻). The NO₃ ⁻ may be reduced to N₂ or to NH₄ ⁺, a form of dissimilatory nitrate reduction to ammonium (DNRA). The objectives of this study were to investigate the importance of S oxidation as a NO₃ ⁻ removal process across diverse freshwater streams, lakes, and wetlands in southwestern Michigan (USA). Simultaneous NO₃ ⁻ removal and SO₄ ²⁻ production were observed in situ using modified “push-pull” methods in nine streams, nine wetlands, and three lakes. The measured SO₄ ²⁻ production can account for a significant fraction (25–40%) of the overall NO₃ ⁻ removal. Addition of ¹⁵NO₃ ⁻ and measurement of ¹⁵NH₄ ⁺ production using the push–pull method revealed that DNRA was a potentially important process of NO₃ ⁻ removal, particularly in wetland sediments. Enrichment cultures suggest that Thiomicrospira denitrificans may be one of the organisms responsible for this metabolism. These results indicate that NO₃ ⁻-driven SO₄ ²⁻ production could be widespread and biogeochemically important in freshwater sediments. Removal of NO₃ ⁻ by DNRA may not ameliorate problems such as eutrophication because the N remains bio-available. Additionally, if sulfur (S) pollution enhances NO₃ ⁻ removal in freshwaters, then controls on N processing in landscapes subject to S and N pollution are more complex than previously appreciated. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Isolation and characterization of nitrite-reductase-deficient mutants of Chlorella sorokiniana (strain 211-8k)

 A method is presented to isolate mutants of Chlorella sorokiniana with defects in NO₃ ⁻ metabolism. Three nitrite-reductase (NIR; E.C.1.7.7.1)-deficient mutants were obtained from 500 pinpoint-colony-forming clones. The final screening was performed using NO₃ ⁻, NO₂ ⁻ or NH⁺ ₄ as N-source. The mutants isolated absorb NO₃ ⁻ with rates close to those measured for the wild type and they excrete NO₂ ⁻ into the medium. The ratio between NO₃ ⁻ uptake and NO₂ ⁻ excretion was 1:1. The sensitivity of NO₃ ⁻ uptake to NH⁺ ₄ was reduced in the mutant strains as it was in the N-starved wild type of Chlorella. Nitrate reductase (NR; EC 1.6.6.1) expression and NR activity were slightly reduced compared to the wild type due to feedback regulation in the mutant strains. No NIR protein was found in the three mutants. However, NIR activity was obtained (50% of the wild-type) for one mutant strain. The NIR-deficient mutants and the already available NR-deficient mutants will be promising tools for investigations of the nitrate assimilation pathway on the molecular level and for studies searching for signaling of C and N metabolism by inorganic N-compounds. Received: 8 October 1999 / Accepted: 25 January 2000     

Can R*s Predict Invasion in Semi-arid Grasslands?

We estimated R*s and tested the applicability of R* theory on nonindigenous plant invasions in semi-arid rangeland. R* is the concentration of a resource that a species requires to survive in a habitat. R* theory predicts that a species with a lower R* for the most limiting resource will competitively displace a species with a higher R* under equilibrium conditions. In a greenhouse, annual sunflower (Helianthus annuus L.), bluebunch wheatgrass (Agropyron spicatum Pursh), and spotted knapweed (Centaurea maculosa Lam.) were grown in monoculture and 2- and 3-species mixtures for three growth periods in an attempt to reduce soil NO₃-N concentrations below each species’ R*. At the end of each growth period, aboveground biomass by species and soil plant available nitrogen were sampled. Decreasing biomass coupled with decreasing soil plant available nitrogen was used to quantify R*s for the three species. R*s for annual sunflower, bluebunch wheatgrass, and spotted knapweed were estimated to be 0.6±0.16 ppm NO₃⁻, less than 0.05 ppm NO₃⁻, and 0.6±0.13 ppm NO₃⁻, respectively. Estimated R*s did not predict the outcome of competition among species. To successfully predict plant community dynamics on semi-arid rangeland with and without the presence of a nonindigenous invasive species, a more comprehensive model that includes mechanisms in addition to competition may have to be considered. We speculate that R* theory may prove most useful for predicting the outcome of competition within functional groups.     

Role of cytokinin in enhanced productivity of maize supplied with NH4 + and NO3 −

Supplying both N forms (NH₄ ⁺+NO₃ ⁻) to the maize (Zea mays L.) plant can optimize productivity by enhancing reproductive development. However, the physiological factors responsible for this enhancement have not been elucidated, and may include the supply of cytokinin, a growth-regulating substance. Therefore, field and gravel hydroponic studies were conducted to examine the effect of N form (NH₄ ⁺+NO₃ ⁻ versus predominantly NO₃ ⁻) and exogenous cytokinin treatment (six foliar applications of 22 μM 6-benzylaminopurine (BAP) during vegetative growth versus untreated) on productivity and yield of maize. For untreated plants, NH₄ ⁺+NO₃ ⁻ nutrition increased grain yield by 11% and whole shoot N content by 6% compared with predominantly NO₃ ⁻. Cytokinin application to NO₃ ⁻-grown field plants increased grain yield to that of NH₄ ⁺+NO₃ ⁻-grown plants, which was the result of enhanced dry matter partitioning to the grain and decreased kernel abortion. Likewise, hydroponically grown maize supplied with NH₄ ⁺+NO₃ ⁻ doubled anthesis earshoot weight, and enhanced the partitioning of dry matter to the shoot. NH₄ ⁺+NO₃ ⁻ nutrition also increased earshoot N content by 200%, and whole shoot N accumulation by 25%. During vegetative growth, NH₄ ⁺+NO₃ ⁻ plants had higher concentrations of endogenous cytokinins zeatin and zeatin riboside in root tips than NO₃ ⁻-grown plants. Based on these data, we suggest that the enhanced earshoot and grain production of plants supplied with NH₄ ⁺+NO₃ ⁻ may be partly associated with an increased endogenous cytokinin supply.     

Nitrogen dynamics and soil nitrate retention in a <Emphasis Type="Italic">Coffea arabica</Emphasis>—<Emphasis Type="Italic">Eucalyptus deglupta</Emphasis> agroforestry system in Southern Costa Rica

Nitrogen fertilization is a key factor for coffee production but creates a risk of water contamination through nitrate (NO₃⁻) leaching in heavily fertilized plantations under high rainfall. The inclusion of fast growing timber trees in these coffee plantations may increase total biomass and reduce nutrient leaching. Potential controls of N loss were measured in an unshaded coffee (Coffea arabica L.) plot and in an adjacent coffee plot shaded with the timber species Eucalyptus deglupta Blume (110 trees ha⁻¹), established on an Acrisol that received 180 kg N ha⁻¹ as ammonium-nitrate and 2,700 mm yr⁻¹ rainfall. Results of the one year study showed that these trees had little effect on the N budget although some N fluxes were modified. Soil N mineralization and nitrification rates in the 0–20 cm soil layer were similar in both systems (≈280 kg N ha⁻¹ yr⁻¹). N export in coffee harvest (2002) was 34 and 25 kg N ha⁻¹ yr⁻¹ in unshaded and shaded coffee, and N accumulation in permanent biomass and litter was 25 and 45 kg N ha⁻¹ yr⁻¹, respectively. The losses in surface runoff (≈0.8 kg mineral N ha⁻¹ yr⁻¹) and N₂O emissions (1.9 kg N ha⁻¹ yr⁻¹) were low in both cases. Lysimeters located at 60, 120, and 200 cm depths in shaded coffee, detected average concentrations of 12.9, 6.1 and 1.2 mg NO₃⁻-N l⁻¹, respectively. Drainage was slightly reduced in the coffee-timber plantation. NO₃⁻ leaching at 200 cm depth was about 27 ± 10 and 16 ± 7 kg N ha⁻¹ yr⁻¹ in unshaded and shaded coffee, respectively. In both plots, very low NO₃⁻ concentrations in soil solution at 200 cm depth (and in groundwater) were apparently due to NO₃⁻ adsorption in the subsoil but the duration of this process is not presently known. In these conventional coffee plantations, fertilization and agroforestry practices must be refined to match plant needs and limit potential NO₃⁻ contamination of subsoil and shallow soil water.     

The kinetics of NH4 + and NO3 − uptake by Douglas fir from single N-solutions and from solutions containing both NH4 + and NO3 −

The kinetics of NH₄ ⁺ and NO₃ ⁻ uptake in young Douglas fir trees (Pseudotsuga menziesii [Mirb.] Franco) were studied in solutions, containing either one or both N species. Using solutions containing a single N species, the V_max of NH₄ ⁺ uptake was higher than that of NO₃ ⁻ uptake. The K_m of NH₄ ⁺ uptake and K_m of NO₃ ⁻ uptake differed not significantly. When both NH₄ ⁺ and NO₃ ⁻ were present, the V_max for NH₄ ⁺ uptake became slightly higher, and the K_m for NH₄ ⁺ uptake remained in the same order. Under these conditions the NO₃ ⁻ uptake was almost totally inhibited over the whole range of concentrations used (10–1000 μM total N). This inhibition by NH₄ ⁺ occurred during the first two hours after addition. ei]{gnA C}{fnBorstlap}     

Isotopic signature of nitrate in two contrasting watersheds of Brush Brook,Vermont, USA

We used the dual isotope method to study differences in nitrate export in two subwatersheds in Vermont, USA. Precipitation, soil water and streamwater samples were collected from two watersheds in Camels Hump State Forest, located within the Green Mountains of Vermont. These samples were analyzed for the δ¹⁵N and δ¹⁸O of NO₃⁻. The range of δ¹⁵N–NO₃⁻ values overlapped, with precipitation −4.5‰ to +2.0‰ (n = 14), soil solution −10.3‰ to +6.2‰ (n = 12) and streamwater +0.3‰ to +3.1‰ (n = 69). The δ¹⁸O of precipitation NO₃⁻ (mean 46.8 ± 11.5‰) was significantly different (P < 0.001) from that of the stream (mean 13.2 ± 4.3‰) and soil waters (mean 14.5 ± 4.2‰) even during snowmelt periods. Extracted soil solution and streamwater δ¹⁸O of NO₃⁻ were similar and within the established range of microbially produced NO₃⁻, demonstrating that NO₃⁻ was formed by microbial processes. The δ¹⁵N and δ¹⁸O of NO₃⁻ suggests that although the two tributaries have different seasonal NO₃⁻ concentrations, they have a similar NO₃⁻ source.     

Alleviation of nickel toxicity by ammonium supply to sunflower plants

Phytotoxicity of nickel (Ni) varies within plant species and cultivars as well as with the concentration of Ni in the rooting medium. Moreover, it is known that several nutrients can modify the plant response to excess Ni. Nitrogen can be absorbed by plants as different N forms and because N metabolism and Ni are closely related, a hydroponic experiment was conducted to study the effect of Ni toxicity on the growth, nutrient status of the different plant parts and leaf chlorophyll concentrations in sunflower plants (Helianthus annuus L.) cv Quipu grown with different forms of N supply. The plants were grown under controlled conditions for 35 days. Depending on the N source supplied, there were significant differences in the sensitivity of sunflower plants to excess Ni. Tolerance was lowest when grown with NO₃ ⁻ alone. A high Ni and NO₃ ⁻ as the only N source resulted in reduced dry weight and significant decreases in nutrient concentration. Plants supplied with a mixture of NO₃ ⁻ and NH₄ ⁺ absorbed in the presence of Ni in solution about three times less Ni than those supplied with NO₃ ⁻ alone. Consequently, there were great differences in Ni concentrations between treatments. With a N nutrition of 100% NO₃ ⁻-N, Ni supply led to severe growth inhibition. Just contrary, simultaneous supply of NO₃ ⁻ and NH₄ ⁺ not only reduced Ni toxicity, but growth was even stimulated by Ni if supplied to plants fed with NO₃ ⁻ and NH₄ ⁺. This indicates the significant role of the N form supplied in the behaviour of Ni toxicity in sunflower plants. This revised version was published online in June 2006 with corrections to the Cover Date.     

Functional role of DNRA and nitrite reduction in a pristine south Chilean <Emphasis Type="Italic">Nothofagus</Emphasis> forest

Nitrite (NO₂ ⁻) is an intermediate in a variety of soil N cycling processes. However, NO₂ ⁻ dynamics are often not included in studies that explore the N cycle in soil. Within the presented study, nitrite dynamics were investigated in a Nothofagus betuloides forest on an Andisol in southern Chile. We carried out a ¹⁵N tracing study with six ¹⁵N labeling treatments, including combinations of NO₃ ⁻, NH₄ ⁺ and NO₂ ⁻. Gross N transformation rates were quantified with a ¹⁵N tracing model in combination with a Markov chain Monte Carlo optimization routine. Our results indicate the occurrence of functional links between (1) NH₄ ⁺ oxidation, the main process for NO₂ ⁻ production (nitritation), and NO₂ ⁻ reduction, and (2) oxidation of soil organic N, the dominant NO₃ ⁻ production process in this soil, and dissimilatory NO₃ ⁻ reduction to NH₄ ⁺ (DNRA). The production of NH₄ ⁺ via DNRA was approximately ten times higher than direct mineralization from recalcitrant soil organic matter. Moreover, the rate of DNRA was several magnitudes higher than the rate of other NO₃ ⁻ reducing processes, indicating that DNRA is able to outcompete denitrification, which is most likely not an important process in this ecosystem. These functional links are most likely adaptations of the microbial community to the prevailing pedo-climatic conditions of this Nothofagus ecosystem.     

Water relations, gas exchange characteristics, and the level of some metabolites in two cultivars of spring wheat under different N regimes

Twenty eight-day old plants of two spring wheat cultivars differing in salinity tolerance were subjected to varying levels of nitrogen (56, 112, and 224 mg N·kg⁻¹ soil) for 42 days. Both cultivars performed differently under varying soil N levels in terms of growth, and grain yield and yield components. Nitrogen levels, 112 and 224 mg·kg⁻¹ soil, caused maximal growth in Sarsabz and Barani-83, respectively. Cv Sarsabz maintained higher leaf water and turgor potentials, but lower leaf osmotic potential than those of Barani-83 at all external N regimes. Sarsabz had higher Chl a, Chl b and carotenoids contents in leaves than those in Barani-83 at 56 and 112 mg N·kg⁻¹ soil. Sarsabz had higher contents of leaf soluble proteins, soluble sugars, and free amino acids than those in Barani-83 at all external N levels. In Barani-83 net CO₂ assimilation rate remained almost unchanged, whereas in Sarsabz it decreased consistently with increase in external N level. The better growth performance of Sarsabaz as compared to Barani-83 under varying soil N levels except 224 mg N·kg⁻¹ soil was associated with maintenance of high leaf turgor potential but not with net CO₂ assimilation rate.