Evaluation of the stem injection technique and subsequent15 N partitioning in red alder crowns

Red alder (Alnus rubra Bong.) trees were labeled with¹⁵ NO^- ₃ or¹⁵ NH⁺ ₄ using the stem-injection method. Leaves were sampled 3 and 15 months subsequent to injection within several crown positions, including top, bottom, proximal, medial, and distal. Stem injection of both¹⁵ NH⁺ ₄ and¹⁵ NO^- ₃ at levels approaching 1% of crown N effectively labeled red alder trees. Although more variable, ¹⁵NO^- ₃ may have been more efficient in initial labeling. The distribution of ¹⁵N in the crown was uniform 3 months after labeling, but was diluted in the distal and top positions by the following year. In both years there was a clear increase in total N concentration toward the periphery of the tree. This increase became more pronounced with increasing crown size and crown closure. Crown position with respect to light availability may be the most important determinant in N allocation in red alder foliage.     

Growth,chemical composition,and nitrate reductase activity of Rumex species in relation to form and level of N supply

Growth, chemical composition, and nitrate reductase activity (NRA) of hydroponically cultured Rumex crispus, R. palustris, R. acetosa, and R. maritimus were studied in relation to form (NH₄ ⁺, NO₃ ^-, or both) and level of N supply (4 mM N, and zero-N following a period of 4mM N). A distinct preference for either NH₄ ⁺ or NO₃ ^- could not be established. All species were characterized by a very efficient uptake and utilization of N, irrespective of N source, as evident from high concentrations of organic N in the tissues and concurrent excessive accumulations of free NO₃ ^- and free NH₄ ⁺. Especially the accumulation of free NH₄ ⁺ was unusually large. Generally, relative growth rate (RGR) was highest with a combination of NH₄ ⁺ and NO₃ ^-. Compared to mixed N supply, RGR of NO₃ ^-- and NH₄ ⁺-grown plants declined on average 3% and 9%, respectively. Lowest RGR with NH₄ ⁺ supply probably resulted from direct or indirect toxicity effects associated with high NH₄ ⁺ and/or low Ca²⁺ contents of tissues. NRA in NO₃ ^- and NH₄NO₃ plants was very similar with maxima in the leaves of ca 40 μmol NO₂ ^- g^-1 DW h^-1. ‘Basal’ NRA levels in shoot tissues of NH₄ ⁺ plants appeared relatively high with maxima in the leaves of ca 20 μmol NO₂ ^- g^-1 DW h^-1. Carboxylate to organic N ratios, (C-A)/N_org, on a whole plant basis varied from 0.2 in NH₄ ⁺ plants to 0.9 in NO₃ ^- plants. After withdrawal of N, all accumulated NO₃ ^- and NH₄ ⁺ was assimilated into organic N and the organic N redistributed on a large scale. NRA rapidly declined to similar low levels, irrespective of previous N source. Shoot/root ratios of -N plants were 50–80% lower than those from +N plants. In comparison with +N, RGR of -N plants did not decline to a large extent, decreasing by only 15% in -NH₄ ⁺ plants due to very high initial organic-N contents. N-deprived plants all exhibited an excess cation over anion uptake (net proton efflux), and whole-plant (C-A)/N_org ratios increased to values around unity. Possible difficulties in interpreting the (C-A)/N_org ratio and NRA of plants in their natural habitats are briefly discussed.     

Inter-varietal differences in xylem exudate composition and growth under contrasting forms of N supply in cucumber

Variations in the inorganic and organic composition of xylem exudate, growth and N content under contrasting forms of N supply in three cucumber cultivars (Hyclos, Medusa and Victory) were studied in glasshouse conditions. The plants were grown hydroponically with two NO₃ ^-:NH₄ ⁺ ratios (100:0 and 60:40).The xylem sap of Medusa grown with both N sources displayed an increase of organic N and carboxylate concentrations and a decrease of cations, inorganic anions and carbohydrates compared with that of those grown with NO₃ ^- alone, showing a higher growth and N content in tissues and thus better utilization of N supplied as NO₃ ^- and NH₄ ⁺. Mixed N nutrition in Hyclos caused the greatest amounts of NO₃ ^- and NH₄ ⁺ in xylem sap, lower root weight and N levels in the leaves, while its root was unable to generate an adequate supply of organic N compounds. Despite the levels of cations, inorganic and organic anions were reduced by the NH₄ ⁺ supplied to Victory, the ionic balance in the xylem sap, growth and N content remained similar to that of those supplied with NO₃ ^- alone. Finally, the cucumber cultivars studied here, responded differently to the form of N supplied, it may partly be due to their ability of assimilating N in the roots and partly to the form in which the N is translocated to the shoot.     

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

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

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

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

The role of N-remobilisation and the uptake of NH4+ and NO3- by Lolium perenne L. in laminae growth following defoliation under field conditions

Several studies have previously shown that shoot removal of forage species, either by cutting or herbivore grazing, results in a large decline in N uptake (60%) and/or N₂ fixation (80%). The source of N used for initial shoot growth following defoliation relies mainly on mobilisation of N reserves from tissues remaining after defoliation. To date, most studies investigating N-mobilisation have been conducted, with isolated plants grown in controlled conditions. The objectives of this study were for Lolium perenne L., grown in a dense canopy in field conditions, to determine: 1) the contribution of N-mobilisation, NH₄ ⁺ uptake and NO₃ ^- uptake to growing shoots after defoliation, and 2) the contribution of the high (HATS) and low (LATS) affinity transport systems to the total plant uptake of NH₄ ⁺ and NO₃ ^-. During the first seven days following defoliation, decreases in biomass and N-content of roots (34% and 47%, respectively) and to a lesser extent stubble (18% and 43%, respectively) were observed, concomitant with mobilisation of N to shoots. The proportion and origin of N used by shoots (derived from reserves or uptake) was similar to data reported for isolated plants. Both HATS and LATS contributed to the total root uptake of NH₄ ⁺ and NO₃ ^-. The V_max of both the NH₄ ⁺ and NO₃ ^- HATS increased as a function of time after defoliation, and both HATS systems were saturated by substrate concentrations in the soil at all times. The capacity of the LATS was reduced as soil NO₃ ^- and NH₄ ⁺ concentrations decreased following defoliation. Data from ¹⁵N uptake by field-grown plants, and uptake rates of NH₄ ⁺ and NO₃ ^- estimated by excised root bioassays, were significantly correlated, though uptake was over-estimated by the later method. The results are discussed in terms of putative mechanisms for regulating N uptake following severe defoliation. This revised version was published online in June 2006 with corrections to the Cover Date.     

Nitrification and Denitrification in Lake and Estuarine Sediments Measured by the 15N Dilution Technique and Isotope Pairing

The transformation of nitrogen compounds in lake and estuarine sediments incubated in the dark was analyzed in a continuous-flowthrough system. The inflowing water contained ¹⁵NO₃^-, and by determination of the isotopic composition of the N₂, NO₃^-, and NH₄⁺ pools in the outflowing water, it was possible to quantify the following reactions: total NO₃^- uptake, denitrification based on NO₃^- from the overlying water, nitrification, coupled nitrification-denitrification, and N mineralization. In sediment cores from both lake and estuarine environments, benthic microphytes assimilated NO₃^- and NH₄⁺ for a period of 25 to 60 h after darkening. Under steady-state conditions in the dark, denitrification of NO₃^- originating from the overlying water accounted for 91 to 171 μmol m^-2 h^-1 in the lake sediments and for 131 to 182 μmol m^-2 h^-1 in the estuarine sediments, corresponding to approximately 100% of the total NO₃^- uptake for both sediments. It seems that high NO₃^- uptake by benthic microphytes in the initial dark period may have been misinterpreted in earlier investigations as dissimilatory reduction to ammonium. The rates of coupled nitrification-denitrification within the sediments contributed to 10% of the total denitrification at steady state in the dark, and total nitrification was only twice as high as the coupled process.     

Nitrogen uptake and assimilation in Enteromorpha intestinalis (L.) Link (Chlorophyta): using N to determine preference during simultaneous pulses of nitrate and ammonium

We investigated the ability of Enteromorpha intestinalis (L.) Link to take up pulses of different species of nitrogen simultaneously, as this would be an important mechanism to enhance bloom ability in estuaries. Uptake rates and preference for NH₄⁺ or NO₃⁻ following 1, 3, 6, 9, 12 or 24 h of exposure to either ¹⁵NH₄NO₃ or NH₄¹⁵NO₃ were determined by disappearance of N from the medium. Differences in assimilation rates for NH₄⁺ or NO₃⁻ were quantified by the accumulation of NH₄⁺, NO₃⁻, and atom % ¹⁵N in the algal tissue. NH₄⁺ concentration was reduced more quickly than water NO₃⁻ concentration. Water column NH₄⁺ concentration after the longest time interval was reduced from 300 to 50 μM. Water NO₃⁻ was reduced from 300 to 150 μM. The presence of ¹⁵N or ¹⁴N had no effect on uptake of either NH₄⁺ or NO₃⁻. ¹⁵N was removed from the water at an almost identical rate and magnitude as ¹⁴N. Differences in accumulation of ¹⁵NH₄⁺ and ¹⁵NO₃⁻ in the tissue reflected disappearance from the water; ¹⁵N from NH₄⁺ accumulated faster and reached an atom % twice that of ¹⁵N from NO₃⁻. This outcome suggested that when NH₄⁺ and NO₃⁻ were supplied in equal concentrations, more NH₄⁺ was taken up and assimilated. The ability to take up high concentrations of NH₄⁺, and NO₃⁻ simultaneously is important for bloom-forming species of estuarine macroalgae subject to multiple nutrient species from various sources.     

Determination of 15N Abundance in Nanogram Pools of NO3- and NO2- by Denitrification Bioassay and Mass Spectrometry

Suspensions of two strains of Pseudomonas aeruginosa (ON12 and ON12-1) were used to reduce NO₃^- and NO₂^-, respectively, to N₂O. The evolved N₂O was quantified by gas chromatography with electron capture detection, and the ¹⁵N abundance was determined by mass spectrometry with a special inlet system and triple-collector detection. Sample gas containing unknown N₂O pools as small as 0.5 ng of N was analyzed by use of a spike technique, in which a reference gas of N₂O of natural ¹⁵N abundance was added to obtain enough total N for the mass spectrometer. In NO₃^- or NO₂^- pools, the ¹⁵N abundance could be determined in samples as small as approximately 3.5 ng of N. No cross-contamination took place between the NO₃^- and NO₂^- pools. The excellent separation of NO₃^- and NO₂^- pools, small sample size required, and low contamination risk during N₂O analysis offer great advantages in isotope studies of inorganic N transformations by, e.g., nitrifying or denitrifying bacteria in the environment.     

Changing nitrogen source on the yield and nitrogen content of soybeans

Summary Soybean plants were grown in nutrient culture solutions containing 150 ppm of N either as an equal concentration of NH₄ ⁺ or NO₃ ^–, or all NO₃ ^–. At the R2 stage of growth for some plants, the N form was changed to either all NO₃ ^– or all NH₄ ⁺, but at the same total N concentration as before. Highest seed yield was obtained with all NO₃ ^– over the entire growth period, the poorest when the N form was switched from an equal ratio of NH₄ ⁺ and NO₃ ^– to all NH₄ ⁺ at the R2 stage. Kjeldahl N concentrations in the plant leaves and seed were highest when NH₄ ⁺ was part or all of the N source in the nutrient solution. These results may partially explain why the literature is inconsistent on the effect of added fertilizer N on soybean seed yield, and may pose a problem in using leaf Kjeldahl N concentration to determine plant N sufficiency.     

Selective Inhibition of Ammonium Oxidation and Nitrification-Linked N2O Formation by Methyl Fluoride and Dimethyl Ether

Methyl fluoride (CH₃F) and dimethyl ether (DME) inhibited nitrification in washed-cell suspensions of Nitrosomonas europaea and in a variety of oxygenated soils and sediments. Headspace additions of CH₃F (10% [vol/vol]) and DME (25% [vol/vol]) fully inhibited NO₂^- and N₂O production from NH₄⁺ in incubations of N. europaea, while lower concentrations of these gases resulted in partial inhibition. Oxidation of hydroxylamine (NH₂OH) by N. europaea and oxidation of NO₂^- by a Nitrobacter sp. were unaffected by CH₃F or DME. In nitrifying soils, CH₃F and DME inhibited N₂O production. In field experiments with surface flux chambers and intact cores, CH₃F reduced the release of N₂O from soils to the atmosphere by 20- to 30-fold. Inhibition by CH₃F also resulted in decreased NO₃^- + NO₂^- levels and increased NH₄⁺ levels in soils. CH₃F did not affect patterns of dissimilatory nitrate reduction to ammonia in cell suspensions of a nitrate-respiring bacterium, nor did it affect N₂O metabolism in denitrifying soils. CH₃F and DME will be useful in discriminating N₂O production via nitrification and denitrification when both processes occur and in decoupling these processes by blocking NO₂^- and NO₃^- production.     

Effect of nitrogen (NH4 NO3) supply on absorption of ammonium and nitrate by conventional and hybrid rice during reproductive growth

Although NH₄ ⁺ has generally been accepted as the preferred N source for fertilising rice, some workers have concluded tha NO₃ ^- is as effective as NH₄ ⁺. The present glasshouse study exmined the relative uptake of NH₄ ⁺ and NO₃ ^- from solution and cultures containing 5–120 mg N/L supplied as NH₄NO₃ by a hybrid rice (India) and a conventional rice cultivar (Japonica). At all levels of N supply, the hybrid rice had higher leaf area and higher rates of uptake of total N than the conventional cultivar. Net photosynthesis rates were similar for both cultivars at the highest rates of N supply, but were lower at 5–40 mg N/L for the hybrid cultivar than for the conventional cultivar. At all levels of N supply, the conventional rice cultivar absorbed more NH₄ ⁺ than NO₃ ^-. In contrast, the hybrid rice absorbed more NH₄ ⁺ than NO₃ ^- at the low levels of N supply (5–40 mg N/L), but more NO₃ ^- than NH₄ ⁺ at the high levels of at 80 and 120 mg N/L. It is concluded that the uptake of N by rice is under genetic control and also dependent on levels of N supply. Thus the appropriate form of N fertiliser for rice may depend on cultivar and rates of N supply.     

Carbon and nitrogen limitations on soybean seedling development

Carbon and nitrogen limitations on symbiotically grown soybean seedlings (Glycine max [L.] Merr.) were assessed by providing 0.0, 1.0, or 8.0 millimolar NH₄NO₃ and 320 or 1,000 microliters CO₂/liter for 22 days after planting. Maximum development of the Rhizobium-soybean symbiosis, as determined by acetylene reduction, was measured in the presence of 1.0 millimolar NH₄NO₃ under both levels of CO₂. Raising NH₄NO₃ from 0.0 to 8.0 millimolar under 320 microliters CO₂/liter increased plant dry weight by 251% and Kjeldahl N content by 287% at 22 days after planting. Increasing NH₄NO₃ from 1.0 to 8.0 millimolar under 320 microliters CO₂/liter increased total dry weight and Kjeldahl N by 100 and 168%, respectively, on day 22. Raising CO₂ from 320 to 1,000 microliters CO₂/liter during the same period had no significant effect on Kjeldahl N content of plants grown with 0.0 or 1.0 millimolar NH₄NO₃. The maximum CO₂ treatment effects were observed in plants supplied with 8.0 millimolar NH₄NO₃, where dry weight and Kjeldahl N content were increased 64% and 20%, respectively. An increase in shoot CO₂-exchange rate associated with the CO₂-enrichment treatment was reflected in a significant increase in leaf dry weight and starch content for plants grown with 1,000 microliters CO₂/liter under all combined N treatments. These data show directly that seedling growth in symbiotically grown soybeans was limited primarily by N availability. The failure of the CO₂-enrichment treatment to increase total plant N significantly in Rhizobium-dependent plants indicates that root nodule development and functioning in such plants was not limited by photosynthate production.     

Differential effects of mineral and organic N sources,and of ectomycorrhizal infection by Hebeloma cylindrosporum,on growth and N utilization in Pinus pinaster

The effect of ectomycorrhizal association of Pinus pinaster with Hebeloma cylindrosporum was investigated in relation to the nitrogen source supplied as mineral (NH₄⁺ or NO₃^?) or organic N (L ‐glutamate) and at 5 mol m^?3. Plants were grown for 14 and 16 weeks with mineral and organic N, respectively, and samples were collected during the last 6 weeks of culture. Total fungal biomass was estimated using glucosamine amount and its viability was assessed using the glucosamine to ergosterol ratio. Non‐mycorrhizal plants grew better with NH₄⁺ than with NO₃^? and grew very slowly when supplied with L ‐glutamate. The presence of the fungus decreased the growth of the host plant with mineral N whereas it increased it with L ‐glutamate. Whatever the N source, most of the living fungal biomass was associated with the roots, whereas the main part of the total biomass was assayed outside the root. The form of mineral N did not significantly affect N accumulation rates over the 42 d in control plants. In mycorrhizal plants grown on either N source, the fungal tissues developing outside of the root were always the main N sink. The ectomycorrhizal association did not change ¹⁵NH₄⁺ uptake rate by roots, suggesting that the growth decrease of the host‐plant was related to the carbon cost for fungal growth and N assimilation rather than to a direct effect on NH₄⁺ acquisition. In contrast, in NO₃^?‐grown plants, in addition to draining carbon for NO₃^? reduction the fungus competed with the root for NO₃^? uptake. With NH₄⁺ or NO₃^? feeding, although mycorrhizal association improved N accumulation in shoots, we concluded that it was unlikely that the fungus had supplied the plant with N. In L ‐glutamate‐grown plants, the presence of the fungus increased the proportion of glutamine in the xylem sap and improved both N nutrition and the growth rate of the host plant.     

氮素类型和剂量对寒温带针叶林土壤N2O排放的影响

大气氮沉降输入会增加森林生态系统氮素有效性,进而改变土壤N_2O产生与排放,然而有关不同氮素离子(氧化态NO_3~--N与还原态NH_4~+-N)沉降对土壤N_2O排放的影响知之甚少。以大兴安岭寒温带针叶林为研究对象,构建了3种类型(NH_4Cl、KNO_3、NH_4NO_3)和4个施氮水平(0、10、20、40 kg N hm~(-2)a~(-1))的增氮控制试验,利用流动化学分析仪和静态箱-气相色谱法4次/月测定凋落物层和矿质层土壤无机氮含量、土壤-大气界面N_2O净交换通量以及相关环境因子,分析施氮类型和剂量对土壤氮素有效性、土壤N_2O通量的影响探讨氮素富集条件下土壤N_2O通量的环境驱动机制。结果表明:施氮类型和剂量均显著影响土壤无机氮含量,土壤NH_4~+-N的积累效应显著高于NO_3~--N。施氮一致增加寒温带针叶林土壤N_2O排放,NH_4NO_3促进效应最为明显,增幅为442%-677%,高于全球平均水平(134%)。土壤N_2O通量与土壤温度、凋落物层NH_4~+-N含量正相关,且随着施氮水平增加而增加。结果表明大气氮沉降短期内不会导致寒温带针叶林土壤NO_3~--N大量流失,但会显著促进土壤N_2O的排放。此外,外源性NH_4~+和NO_3~-输入对土壤N_2O排放的促进作用具有协同效应,在未来森林生态系统氮循环和氮平衡研究中应该区分对待。     

The influence of NO3 - and NH4 + nutrition on the carbon and nitrogen partitioning characteristics of wheat (Triticum aestivum L.) and maize (Zea mays L.) plants

The carbon and nitrogen partitioning characteristics of wheat (Triticum aestivum L.) and maize (Zea mays L.) grown hydroponically at a constant pH on either 4 mM or 12 mM NO₃ ^- or NH₄ ⁺ nutrition were investigated using either ¹⁴C or ¹⁵N techniques. Greater allocation of ¹⁴C to amino-N fractions occurred at the expense of allocation of ¹⁴C to carbohydrate fractions in NH₄ ⁺-compared to NO₃ ^--fed plants. The [¹⁴C]carbohydrate:[¹⁴C]amino-N ratios were 1.5-fold and 2.0-fold greater in shoots and roots respectively of 12 mM NO₃ ^--compared to 12 mM NH₄ ⁺-fed wheat. In both 4 mM and 12 mM N-fed maize the [¹⁴C]carbohydrate:[¹⁴C]amino-N ratios were approximately 1.7-fold and 2.0-fold greater in shoots and roots respectively of NO₃ ^--compared to NH₄ ⁺-fed plants. Similar results were observed in roots of wheat and maize grown in split-root culture with one root-half in NO₃ ^--and the other in NH₄ ⁺-containing nutrient media. Thus the allocation of carbon to the amino-N fractions occurred at the expense of carbohydrate fractions, particularly within the root. Allocation of ¹⁴N and ¹⁵N within separate sets of plants confirmed that NH₄ ^--fed plants accumulated more amino-N compounds than NO₃ ^--fed plants. Wheat roots supplied with ¹⁵NH₄ ⁺ for 8 h were found to accumulate ¹⁵NH₄ ⁺ (8.5 g ¹⁵N g^-1 h^-1) whereas in maize roots very little ¹⁵NH₄ ⁺ accumulated (1.5 g ¹⁵N g^-1 h^-1)It is proposed that the observed accumulation of ¹⁵NH₄ ⁺ in wheat roots in these experiments is the result of limited availability of carbon within the roots of the wheat plants for the detoxification of NH₄ ⁺, in contrast to the situation in maize. Higher photosynthetic capacity and lower shoot: root ratios of the C₄ maize plants ensure greater carbon availability to the root than in the C₃ wheat plants. These differences in carbon and nitrogen partitioning between NO₃ ^--and NH₄ ⁺-fed wheat and maize could be responsible for different responses of wheat and maize root growth to NO₃ ^- and NH₄ ⁺ nutrition.     

A N and N Nuclear Magnetic Resonance Study of Nitrogen Metabolism in Shoot-Forming Cultures of White Spruce (Picea glauca) Buds

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 αNH₂ 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, NO₃⁻ and NH₄⁺ could also be identified in ¹⁴N NMR spectra. Perfusion experiments performed for up to 20 hours in the NMR spectrometer showed that ¹⁵N-labeled NH₄⁺ and NO₃⁻ are first incorporated into the amide group of Gln and then in the αNH₂ 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 NH₄⁺ is much faster than that of NO₃⁻. Consequently after 10 days of growth more than 70% of the newly synthesized internal free amino acid pool derives its nitrogen from NH₄⁺ rather than NO₃⁻. If NH₄⁺ is omitted from the medium, no NO₃⁻ is taken up during 9 weeks and the buds support limited growth by utilizing their endogenous amino acid pools. It is concluded that NH₄⁺ and NO₃⁻ are both required for the induction of nitrate- and nitrite reductase.     

Alpine plants show species-level differences in the uptake of organic and inorganic nitrogen

As an estimate of species-level differences in the capacity to take up different forms of N, we measured plant uptake of ¹⁵N-NH₄ ⁺, ¹⁵N-NO₃ ^– and ¹⁵N, [1]-¹³C glycine within a set of herbaceous species collected from three alpine community types. Plants grown from cuttings in the greenhouse showed similar growth responses to the three forms of N but varied in the capacity to take up NH₄ ⁺, NO₃ ^– and glycine. Glycine uptake ranged from approximately 42% to greater than 100% of NH₄ ⁺ uptake; however, four out of nine species showed significantly greater uptake of either NH₄ ⁺ or NO₃ ^– than of glycine. Relative concentrations of exchangeable N at the sites of plant collection did not correspond with patterns of N uptake among species; instead, species from the same community varied widely in the capacity to take up NH₄ ⁺, NO₃ ^–, and glycine, suggesting the potential for differentiation among species in resource (N) use.     

EXPERIMENTAL EVIDENCE SUPPORTS THE USE OF δ15N CONTENT OF THE OPPORTUNISTIC GREEN MACROALGA ENTEROMORPHA INTESTINALIS (CHLOROPHYTA) TO DETERMINE NITROGEN SOURCES TO ESTUARIES1

One assumption underlying the use of stable nitrogen isotopes (δ¹⁵N) for determining nitrogen (N) sources is that the δ¹⁵N of primary producers reflects N sources in a predictable manner. To test this assumption, we conducted two experiments. First, we varied δ¹⁵N with constant concentration of NO₃^? or NH₄⁺ to determine whether either nutrient is preferentially selected by the macroalga Enteromorpha intestinalis (L. Link) and if isotopic ratio affected selectivity. Tissue δ¹⁵N increased with δ¹⁵N supplied for both NO₃^? and NH₄⁺ but sequestering of ¹⁵NH₄⁺ was more rapid than for ¹⁵NO₃^?; in addition, some evidence suggested that high relative abundance of ¹⁵N may have decreased NO₃^? uptake. Second, we held δ¹⁵N constant and varied concentrations of either NO₃^? or NH₄⁺ to determine whether fractionation is concentration dependent. Uptake of N was described by a Michaelis‐Menten equation for both NO₃^? and NH₄⁺, with higher V_max and K_1/2 for NH₄⁺ than for NO₃^?. There was no relationship between N concentration and tissue δ¹⁵N for either NO₃^? or NH₄⁺; therefore, no selection for ¹⁴N over ¹⁵N occurred. This study demonstrated that accumulation of ¹⁵N in macroalgal tissue was predictable over a range of water δ¹⁵N values and N concentrations, suggesting that E. intestinalis may be used to assess the availability of N sources to estuarine and coastal communities. However, caution must be used when interpreting tissue δ¹⁵N depending on the primary inorganic form of N available.