Preference For NH4 Research Articles

Nitrogen acquisition strategy and its effects on invasiveness of a subtropical invasive plant.

Preference and plasticity in nitrogen (N) form uptake are the main strategies with which plants absorb soil N. However, little effort has been made to explore effects of N form acquisition strategies, especially the plasticity, on invasiveness of exotic plants, although many studies have determined the effects of N levels (e.g. N deposition). To address this problem, we studied the differences in N form acquisition strategies between the invasive plant Solidago canadensis and its co-occurring native plant Artemisia lavandulaefolia, effects of soil N environments, and the relationship between N form acquisition strategy of S. canadensis and its invasiveness using a 15N-labeling technique in three habitats at four field sites. Total biomass, root biomass, and the uptakes of soil dissolved inorganic N (DIN) per quadrat were higher for the invasive relative to the native species in all three habitats. The invader always preferred dominant soil N forms: NH4 + in habitats with NH4 + as the dominant DIN and NO3 - in habitats with NO3 - as the dominant DIN, while A. lavandulaefolia consistently preferred NO3 - in all habitats. Plasticity in N form uptake was higher in the invasive relative to the native species, especially in the farmland. Plant N form acquisition strategy was influenced by both DIN levels and the proportions of different N forms (NO3 -/NH4 +) as judged by their negative effects on the proportional contributions of NH4 + to plant N (f NH4 +) and the preference for NH4 + (β NH4 +). In addition, total biomass was positively associated with f NH4 + or β NH4 + for S. canadensis, while negatively for A. lavandulaefolia. Interestingly, the species may prefer to absorb NH4 + when soil DIN and/or NO3 -/NH4 + ratio were low, and root to shoot ratio may be affected by plant nutrient status per se, rather than by soil nutrient availability. Our results indicate that the superior N form acquisition strategy of the invader contributes to its higher N uptake, and therefore to its invasiveness in different habitats, improving our understanding of invasiveness of exotic plants in diverse habitats in terms of utilization of different N forms.

Heterotrophic ammonium oxidation is not active in acidic paddy soils

In acidic paddy soils where crops with an ammonium (NH4+) preference are grown, ammonia (NH3) is largely transformed to NH4+, and organic nitrogen (N) is often too low to support organic nitrification, heterotrophic nitrifiers may have a preference for NH4+. If so, acidic paddy soils are likely a hotspot of heterotrophic NH4+ oxidation (OHNH4). To reveal if OHNH4 is active in acidic paddy soils, we investigated the existence of acetylene (C2H2)-resistant NH4+ oxidation at pH 5.4, 5.3, and 5.0 in paddy soils with 60% water holding capacity (WHC), using 15N tracing and nitrification inhibition techniques (1.0% C2H2). As expected, C2H2-resistant NH4+ oxidation (0.0088 ± 0.0012 mg N kg−1 soil d−1) was detected in soils with a pH of 5.0. In the presence of 1.0% C2H2, increased 15N in the NO3− pool under 15NH4NO3 labelling suggested the existence of C2H2-resistant NH4+ oxidizers. Through a concentration gradient experiment of C2H2 (0, 0.01%, 0.1%, and 1.0%) on the pH 5.0 soil, 0.01% C2H2 was found to be sufficient for complete inhibition of autotrophic nitrification, suggesting that NH4+ oxidation in samples receiving 0.1% or 1.0% C2H2 is unlikely driven by autotrophs. The trend toward higher rates when C2H2 increased from 0.01% to 0.1% suggested that, C2H2-resistant NH4+ oxidizers in paddy soils are likely carbon (C)-limited. The trend toward lower rates when C2H2 increased from 0.1% to 1.0% implied an inhibition in the high concentration. At 0.1% C2H2, the trend toward lower rates when soil moisture increased from 60%WHC to 100%WHC indicated that C2H2-resistant NH4+/NH3 oxidizers prefer drier conditions, and OHNH4 will be less active at flooded conditions. Since there is evidence of organic and inorganic N oxidation by heterotrophs are mutually exclusive, we thus used a 15N tracing model to separate organic nitrification from inorganic nitrification to clarify that low OHNH4 in samples was irrelevant to heterotrophic nitrification of organic N (OHORG). In summary, isotopic evidence has been provided to suggest that OHNH4, although with a low rate, has the potential to proceed in acidic paddy soils. In which, low OHNH4 is unrelated to OHORG, and an exogenous C source is needed to activate heterotrophic NH4+/NH3 oxidizers.

'Preferential' ammonium uptake by rice does not always turn into higher N recovery of fertilizer sources under water-saving irrigation

Ammonium (NH4+) has been reported as the preferred mineral N source (a preference for NH4+ uptake over NO3-) for rice (Oryza sativa L.), the predominant supply of NH4+ has been hypothesized as a means of increasing the N recovery efficiency (NRE). However, the contribution of NH4+ preference to NRE is unclear under different irrigation regimes. To evaluate the NRE of rice as affected by varied N supply forms, rice (cv. 'Suijing 18') was grown in a pot experiment with 15N-NH4+ or 15N-NO3- applied to flooded and water-saving irrigation regimes. At harvest, plant biomass, N accumulation, and the fate of 15N-labeled N in the plant-soil system were assessed. Ammonium-N application enhanced rice shoot growth compared with nitrate-N under both irrigation regimes. This enhancement by NH4+ was further promoted under water-saving irrigation. In contrast, nitrate-N enhanced rice root growth, more fertilizer-derived N residues in the soil compared with those of NH4+ treated, regardless of the irrigation regimes. Ammonium enhanced the NRE of rice compared with NO3- treated under flooded irrigation. However, the so-called 'preference' for NH4+ by rice did not always turn into high N recovery efficiency, and there was no difference in NRE of rice between varied N supply forms under water-saving irrigation. Even if NH4+ is 'preferred' by the rice, the N cycle in the paddy soil is complex and dynamic, especially under water-saving irrigation which resulted in a rapid conversion of the applied 15NH4+ to NO3-, and hence less opportunities for rice to take up the applied 15NH4+. Therefore, the nitrogen preference of plants is not the only critical factor in selecting a proper nitrogen fertilizer source for rice, it is better to take the match of plant N preference, N fertilizer type, and irrigation regime into consideration to optimize irrigation and fertilization management and increase the NRE of plants.

Nitrogen signals and their ecological significance for seed germination of ten psammophilous plant species from European dry acidic grasslands.

The presented study evaluated effects of potassium nitrate (KNO3), ammonium nitrate (NH4 NO3) and ammonium chloride (NH4Cl) on the germination-related characteristics of 10 species from European dry acidic grasslands. Germination was studied under controlled laboratory conditions. The seeds were subjected to KNO3, NH4 NO3 and NH4Cl in four doses (1, 10, 50 and 100 mM) and to distilled water. Final germination percentage, index of germination velocity and index of germination synchrony were determined. Content of nitrogen in the soil probed from the site of seeds collection was also analyzed. Significant effects of type of the nitrogen compounds and their concentrations were observed. High concentrations of nitrogen-containing salts inhibited completion of germination in almost all species. Helichrysum arenarium and Hypericum perforatum showed preference for NH4+ over NO3‒, whereas Arnoseris minima, Alyssum montanum, Jasione montana and Spergula morisonii showed preference for NO3‒ over NH4+. Centaurea scabiosa, C. stoebe and Hypochaeris radicata had no preference and wide tolerance to the type of nitrogen-containing compound. Echium vulgare showed differential response hard for interpretation. A. montanum and J. montana showed stenotopic behavior in terms of nitrogen-related conditions. It is proposed that nitrogen-rich soil gaps favor establishment of more nitro-tolerant plant species (e.g. C. scabiosa, C. stoebe and H. radicata) as compared to nitrogen-poor ones.

Case of a stronger capability of maize seedlings to use ammonium being responsible for the higher 15N recovery efficiency of ammonium compared with nitrate

Matching a crop’s nitrogen (N) preference with the specific form of N is critical to enhance N recovery efficiency (NRE). Many studies show that maize (Zea mays L.) has a preference for NH4+ uptake over NO3−, but the contribution of this preference to NRE is unclear. This study evaluated the utilization of ammonium (NH4+) and nitrate (NO3−) by maize and its relationship with NRE. Maize (cv. ‘Zhengdan 958’) seedlings were grown in a soil culture experiment with 15NH4+ or 15NO3− applied to sterilized or non-sterilized soils, and in a hydroponic experiment supplied with NH4+ or NO3−. We recorded maize root and shoot growth and N uptake, and the distribution of 15N-labeled N in the soil–plant system. Ammonium application enhanced maize root and shoot growth and N uptake compared with those of NO3−-treated plants in both culture systems. This enhancement by NH4+ was further promoted in sterilized soil. More applied N remained in soil supplied with NO3− than with NH4+ after seedling harvest. The NRE of NH4+ in maize was significantly higher than that of NO3−, while a similar proportion of applied N was lost with NH4+ or NO3−supply. Ammonium supply enhances maize growth and NRE compared with NO3− under the present experimental condition, which may be attributable to the superior capability of maize to utilize NH4+.

Mud-entrained macroalgae utilise porewater and overlying water column nutrients to grow in a eutrophic intertidal estuary

Sediment porewater nutrients often occur at concentrations that are orders of magnitude higher than nutrients in overlying waters, and accordingly may subsidise growth of benthic macroalgal mats in estuarine ecosystems. The relative contribution of porewater nutrients is expected to be particularly important for macroalgae entrained in intertidal mudflat sediments, where access to water column nutrients is tidally constrained. In this study, filamentous Gracilaria chilensis thalli were simultaneously exposed to sediment and overlying water nutrient sources, labelled using 15N tracers (15NH4+ or 15NO3−) during a 5-day experiment. Dissolved inorganic N (DIN) uptake from porewater and overlying water accounted for 33 and 52%, respectively, of the N estimated as necessary to support the growth of G. chilensis, despite the two-fold lower DIN concentration of the overlying water and its periodic availability (8 h day−1). Of the total N assimilated by the plants, ~ 15% could not be accounted for, supporting the acquisition of other N forms in order to meet demand. We also found that regardless of background NH4+:NO3− ratios (i.e. 1:3 in overlying water and 12:1 in porewater), plants accumulated 15NH4+ significantly more readily than 15NO3−, indicating a preference for NH4+. This ability to utilise multiple sources and species of N relatively rapidly may partly explain the competitive success of entrained macroalgae relative to non-entrained species and historically abundant seagrass beds in these environments. These results underscore the significance of both internal nutrient loading and external inputs as important in sustaining opportunistic macroalgal blooms in shallow estuaries.

Uptake of dissolved inorganic and organic nitrogen by the benthic toxic dinoflagellate Ostreopsis cf. ovata

Environmental factors that shape dynamics of benthic toxic blooms are largely unknown. In particular, for the toxic dinoflagellate Ostreopsis cf. ovata, the importance of the availability of nutrients and the contribution of the inorganic and organic pools to growth need to be quantified in marine coastal environments. The present study aimed at characterizing N-uptake of dissolved inorganic and organic sources by O. cf. ovata cells, using the 15N-labelling technique. Experiments were conducted taking into account potential interactions between nutrient uptake systems as well as variations with the diel cycle. Uptake abilities of O. cf. ovata were parameterized for ammonium (NH4+), nitrate (NO3−) and N-urea, from the estimation of kinetic and inhibition parameters. In the range of 0 to 10μmolNL−1, kinetic curves showed a clear preference pattern following the ranking NH4+>NO3−>N-urea, where the preferential uptake of NH4+ relative to NO3− was accentuated by an inhibitory effect of NH4+ concentration on NO3− uptake capabilities. Conversely, under high nutrient concentrations, the preference for NH4+ relative to NO3− was largely reduced, probably because of the existence of a low-affinity high capacity inducible NO3− uptake system. Ability to take up nutrients in darkness could not be defined as a competitive advantage for O. cf. ovata. Species competitiveness can also be defined from nutrient uptake kinetic parameters. A strong affinity for NH4+ was observed for O. cf. ovata cells that may partly explain the success of this toxic species during the summer season in the Bay of Villefranche-sur-mer (France).

The effect of form and level of inorganic N on nitrogen use efficiency of sugarcane grown in pots

ABSTRACTTo-date, assessments of nitrogen use efficiency (NUE) of sugarcane have not included the contribution of its components, nitrogen uptake efficiency (NUpE) and nitrogen utilization efficiency (NUtE). This study determined these values, based on biomass and plant nitrogen (N) content, in two four-month-old pot-grown genotypes. The treatments included six N regimes, with nitrate (NO3—N) or ammonium (NH4+-N) supplied alone, or as NO3−-N for the first 6 weeks and then NH4+-N until harvest, each as 4 or 20 mM. Regardless of the N form, NUE was higher at four than at 20 mM due to significantly higher NUpE at low N supply. The results indicated that there was luxury N uptake and preference for NH4+-N nutrition, which resulted in the highest determined NUE. There were significant differences between genotypes in biomass, morphological growth parameters, N uptake, total plant N and NUE, the latter matching previously established sucrose yield-based NUE field rankings.

Calcium-Ammonium Selectivity of Two Benchmark Soils from Botswana as Assessed by Competing Semi-empirical Ion Exchange Equations

The effectiveness of lime-ammonium-nitrate (LAN) as a nitrogen (N) fertilizer in weathered soils depends on the respective selectivity for ammonium (NH4) and calcium (Ca) by the soils. The study assessed Ca2+/NH4+ exchange selectivity of two benchmark soils from Botswana and examined the soil fertility management implications. Surface horizons (0–20 cm) of Pellustert and Haplustalf were equilibrated with 50 ml stock solution containing variable concentrations of Ca2+ and NH4+. The Ca2+/NH4+ exchange data were fitted into the Vanselow (KV), Gaines and Thomas (KGT), Davies (KD), and the regular solution (KRS) equations. The selectivity coefficients for the Ca2+/NH4+ exchange reactions varied widely with the soil exchanger composition except for the relatively stable KRS. The selectivity coefficients indicated strong preference for NH4+ to Ca2+. The thermodynamic exchange constant, Kex, was 5.75 ± 1.24 in the Pellustert, indicating preferential adsorption of NH4+, but not in the Haplustalf with Kex = 0.92 ± 0.27. The free energy for Ca2+/NH4+ exchange (ΔG°ex) was negative (−4.26 ± 0.59 kJ mol−1) in the Pellustert but slightly positive in the Haplustalf (0.34 ± 0.87 kJ mol−1). In conclusion, the soil-NH4 complex was more stable than soil-Ca complex in the Pellustert, indicating LAN as a N fertilizer would have greater potential effectiveness in the Pellustert than in the Haplustalf.

Potassium→ammonium exchange of two benchmark soils from Botswana and its implication for nitrogen economy of the soils

Ion exchange preferences for NH4+ and K+ by soil exchanger surface can greatly affect the NO3− leaching into groundwater and nitrogen-use efficiency in agricultural production. Since NH4+ and K+ salts are usually applied together as fertilizers, the binary K→NH4 exchange of two benchmark Botswana soils, Pellustert and a Haplustalf, was studied to determine the selectivity coefficients and the thermodynamic exchange constant with special reference to N economy. The Vanselow and the Gaines and Thomas coefficients indicated preference for NH4+ by the Pellustert and K by the Haplustalf across the exchanger phase composition. The equilibrium constant (Kex) was 1.807 for the Pellustert and 0.174 for the Haplustalf. The exchange free energy (ΔGex0) was −1.467 kJ mol−1 for the Pellustert and 4.334 kJ mol−1 for the Haplustalf. Negative ΔGex0 for the Pellustert is consistent with preference for NH4+ to K+ in contrast to positive ΔGex0 for the Haplustalf. The greater stability of NH4X than KX complex in the Pellustert, and KX than NH4X in the Haplustalf, would mean increased residence time of NH4+ in the Pellustert than the Haplustalf. The implication of short residence time of NH4+ in soil is rapid nitrification, thereby leading to NO3−−N leaching losses and possible groundwater contamination.

Influence of Humic Acid on Interaction of Ammonium and Potassium Ions on Clay Minerals

Interaction of ammonium (NH+4) and potassium (K+) is typical in field soils. However, the effects of organic matter on interaction of NH+ and K+ have not been thoroughly investigated. In this study, we examined the changes in major physicochemical properties of three clay minerals (kaolinite, illite, and montmorillonite) after humic acid (HA) coating and evaluated the influences of these changes on the interaction of NH+4 and K+ on clay minerals using batch experiments. After HA coating, the cation exchange capacity (CEC) and specific surface area (SSA) of montmorillonite decreased significantly, while little decrease in CEC and SSA occurred in illite and only a slight increase in CEC was found in kaolinite. Humic acid coating significantly increased cation adsorption and preference for NH+4, and this effect was more obvious on clay minerals with a lower CEC. Results of Fourier transform infrared spectrometry analysis showed that HA coating promoted the formation of H-bonds between the adsorbed NH+ and the organo-mineral complexes. HA coating increased cation fixation capacity on montmorillonite and kaolinite, but the opposite occurred on illite. In addition, HA coating increased the competitiveness of NH+ on fixation sites. These results showed that HA coating affected both the nature of clay mineral surfaces and the reactions of NH+4 and K+ with clay minerals, which might influence the availability of nutrient cations to plants in field soils amended with organic matter.

A sting in the tail of flexible molecules: spectroscopic and energetic challenges in the case of p-aminophenethylamine

The neurotransmitter analogue p-aminophenethylamine (APEA) illustrates many of the pitfalls and challenges associated with spectroscopic and conformational analysis of flexible molecules. The combined experimental-theoretical study presented here resolves a long-standing controversy over its conformational energetic preferences. Jet-cooled resonance enhanced two-photon ionisation (R2PI) and IR-UV ion depletion techniques enabled conformer-specific IR spectra in the NH-CH stretch region to be measured for four distinct conformers of APEA. Comparison of spectra with theoretical calculations (including MP2, M06-2X and B3LYP with aug-cc-pVTZ basis sets) allows the two most populated conformers to be unambiguously identified as those having a gauche arrangement of the side chain, which facilitates an NH···π type hydrogen bond. The other two observed conformers are assigned to structures with an anti-side chain. A fifth gauche conformer, predicted to be least stable, is not observed. Comparison with published conformer specific IR spectra of tyramine (Makara et al., J. Phys. Chem. A, 2008, 112, 13463-13469) and Raman spectra of phenylethylamine (Golan et al., J. Chem. Phys., 2009, 131, 024305) reveals an entirely consistent pattern of spectral signatures associated with the four specific conformations of the ethylamine side chain evident in APEA, and aids assignment of the associated CH and NH stretch fundamentals, some of which have very weak IR intensities. Extensive calculations of the relative energetic trends of the five conformers have been carried out. In comparison to the highest level of theory considered, CCSD(T)-F12b/cc-pVDZ-F12, MP2 overestimate the energy difference, whereas DFT significantly underestimates the energetic preference for NH···π stabilised gauche conformers, although inclusion of dispersion (M06-2X, B3LYP-D3) improves the DFT results.

Saturation of NO3−uptake in prairie streams as a function of acute and chronic N exposure

We conducted a series of stepwise NO3− additions to investigate the response of NO3− uptake to short-term (acute) changes in N concentration in 3 prairie streams. Observed NO3− uptake rates increased with short-term elevations in NO3− concentration and were consistent with linear and Michaelis–Menten kinetics models. We compiled these data with uptake rates from additional published studies to calculate robust estimates of N uptake kinetics for prairie streams. Half-saturation coefficients based on compiled data were 6.7 µg/L for NH4+ and 67 µg/L for NO3−-N. This difference in half-saturation coefficients suggests that NH4+ is more efficiently assimilated than NO3−, indicating a preference for NH4+ as an N source. Similarly, ambient concentrations of NH4+ and NO3− were less than their respective half-saturation coefficients, and aerial uptake rates were generally <5% of the maximum, suggesting severe limitation of N uptake at ambient conditions. The observed pattern of uptake kinetics suggests that physiological constraints limit biotic N uptake in these low-N streams and contrasts with the pattern of uptake observed in streams with chronically elevated ambient NO3− concentrations.

Growth responses of the perennial legume Sesbania sesban to NH 4 and NO 3 nutrition and effects on root nodulation

Preference for NH 4 + or NO 3 − nutrition by the perennial legume Sesbania sesban (L.) Merr. was assessed by supplying plants with NH 4 + and NO 3 − alone or mixed at equal concentrations (0.5 mM) in hydroponic culture. In addition, growth responses of S. sesban to NH 4 + and NO 3 − nutrition and the effects on root nodulation and nutrient and mineral composition of the plant tissues were evaluated in a hydroponic setup at a range of external concentration of NH 4 + and NO 3 − (0, 0.1, 0.2, 0.5, 2 and 5 mM). Seedlings of S. sesban grew equally well when supplied with either NH 4 + or NO 3 − alone or mixed and had high relative growth rates (RGRs) ranging between 0.19 and 0.21 d −1. When larger plants of S. sesban were supplied with NH 4 + or NO 3 − alone, the RGRs and shoot elongation rates were not affected by the external concentration of inorganic N. At external N concentrations up to 0.5 mM nodulation occurred and contributed to the N nutrition through fixation of gaseous N 2 from the atmosphere. For both NH 4 + and NO 3 −-fed plants the N concentration in the plant tissues, particularly water-extractable NO 3 −, increased at high supply concentrations, and concentrations of mineral cations generally decreased. It is concluded that S. sesban can grow without an external inorganic N supply by fixing atmospheric N 2 gas via root nodules. Also, S. sesban grows well on both NH 4 + and NO 3 − as the external N source and the plant can tolerate relatively high concentrations of NH 4 +. This wide ecological amplitude concerning N nutrition makes S. sesban very useful as a N 2-fixing fallow crop in N deficient areas and also a candidate species for use in constructed wetland systems for the treatment of NH 4 + rich waters.

Nitrogen uptake by four tree species of the Catskill Mountains, New York: Implications for forest N dynamics

Watersheds of the Catskill Mountains, New York have marked differences in nitrogen (N) dynamics among dominant tree species stands. Our objectives were to study how tree species vary in N uptake to better understand the basis for the observed variation in these forested watersheds. We conducted a 15N tracer greenhouse study to determine NH4 + and NO3 − uptake of American beech (Fagus grandifolia Ehrh.), eastern hemlock (Tsuga Canadensis L.), red oak (Quercus rubra L.) and sugar maple (Acer saccharum Marsh.) seedlings. Seedlings and their native soil were collected in November 1997, over-wintered and allowed to break dormancy in spring 1998. Half of the seedlings of each tree species received 15NH4-NO3 to examine NH4 + uptake and the other half received NH4-15NO3 to examine NO3 − uptake. Plants were harvested 4 days following 15N addition. Tree species varied in their preference for NH4 + and NO3 −. Sugar maple and eastern hemlock seedlings took up more NH4 + than NO3 − per unit plant biomass, while beech was the only species to take up more NO3 − than NH4 +. Red oak took up more NH4 + than NO3 − into roots, stems and leaves, but the difference between the two forms of N was not statistically significant. These results demonstrate that tree species of the Catskill Mountains vary in their capacity to take up NH4 + and NO3 −. Coupled with stand-level studies of N dynamics, this variation can help explain some of the patterns of forested watershed N retention and loss in the Catskill Mountains shown in our field investigations (Templer, 2001; Templer et al., in press Ecosystems).

Nitrogen uptake and assimilation in Enteromorpha intestinalis (L.) Link (Chlorophyta): using 15N 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 NH 4 + or NO 3 − following 1, 3, 6, 9, 12 or 24 h of exposure to either 15NH 4NO 3 or NH 4 15NO 3 were determined by disappearance of N from the medium. Differences in assimilation rates for NH 4 + or NO 3 − were quantified by the accumulation of NH 4 +, NO 3 −, and atom % 15N in the algal tissue. NH 4 + concentration was reduced more quickly than water NO 3 − concentration. Water column NH 4 + concentration after the longest time interval was reduced from 300 to 50 μM. Water NO 3 − was reduced from 300 to 150 μM. The presence of 15N or 14N had no effect on uptake of either NH 4 + or NO 3 −. 15N was removed from the water at an almost identical rate and magnitude as 14N. Differences in accumulation of 15NH 4 + and 15NO 3 − in the tissue reflected disappearance from the water; 15N from NH 4 + accumulated faster and reached an atom % twice that of 15N from NO 3 −. This outcome suggested that when NH 4 + and NO 3 − were supplied in equal concentrations, more NH 4 + was taken up and assimilated. The ability to take up high concentrations of NH 4 +, and NO 3 − simultaneously is important for bloom-forming species of estuarine macroalgae subject to multiple nutrient species from various sources.

Characterization of the affinity for nitrogen, uptake kinetics, and environmental relationships for Prorocentrum minimum in natural blooms and laboratory cultures

During the late spring and early summer of 1998, an extensive bloom of the dinoflagellate Prorocentrum minimum (>93% of phytoplankton cell density) developed in several tributaries of the Chesapeake Bay, USA. In January 1999, a bloom of mixed dinoflagellates ( Heterocapsa rotundata, H. triquetra and P. minimum, with P. minimum forming 21% of total phytoplankton cells and 39% of the total biovolume) developed in the mesohaline Neuse Estuary, North Carolina, USA. During these blooms, experiments were carried out to characterize the nitrogen uptake kinetics of these assemblages with 15 N isotopic techniques. Four nitrogenous substrates (NO 3 −, NH 4 +, urea, and a mixed amino acids substrate) were used to determine uptake rate and substrate preference. Rates of nitrogen uptake were also measured in P. minimum cultures grown on varying growth nitrogen substrates. The calculated kinetic parameters determined for the P. minimum-dominated field assemblages and the cultures indicated a preference for NH 4 +. NH 4 + was also the primary nitrogen source supporting the blooms. In addition, a high affinity for urea was also found, and urea contributed significantly to the Neuse Estuary bloom. Furthermore, results showed that the regulation of uptake for each of the substrates was different: strong positive relationships between affinity and temperature were found for NH 4 + and amino acids, while a negative response was found for NO 3 −, and very little response to temperature was noted for urea. These differences suggest that a diversity of nitrogen uptake mechanisms may aid the development and maintenance of P. minimum blooms.

Kinetics of ammonium and nitrate uptake by eucalypt roots and associated proton fluxes measured using ion selective microelectrodes.

Ion-selective microelectrodes were used non-invasively to measure the concentration dependence of NH4+ and NO3- fluxes around the roots of intact solution-cultured Eucalyptus nitens (Deane & Maiden) Maiden. In addition, NH4+ and H+ fluxes were measured simultaneously at a range of NH4+ concentrations, and NO3- and H+ fluxes were measured simultaneously at a range of NO3- concentrations. Nitrogen concentrations ranged from 10-250 μM, i.e. in the range corresponding to the high affinity transport system (HATS). Both NH4+ and NO3- fluxes exhibited saturating Michaelis-Menten-style kinetics. The Km was 16 μM for NH4+ and 18 μM for NO3-. Values of Vmax were 53 nmol m-2 s-1 for NH4+ and 37 nmol m-2 s-1 for NO3-. Proton fluxes were highly correlated with NH4+ and NO3- fluxes, but the relationships were different. Proton efflux increased with increasing NH4+ concentration and mirrored the changing NH4+ fluxes. The ratio between NH4+ and H+ fluxes was 1 : -1.6. Proton influx was evident with initial exposure to NO3-, with the flux stoichiometry for NO3- : H+ being 1 : 1.4. Subsequent increases in NO3- concentration caused a gradual increase in H+ efflux such that the flux stoichiometry for NO3- : H+ became 1 : -0.8. The presence of 100 μM NH4+ greatly reduced NO3- fluxes and caused a large and constant H+ efflux. These results are evidence that E. nitens has a preference for NH4+ as a source of N, and that the fluxes of NH4+ and NO3- are quantitatively linked to H+ flux.

Soil nitrogen form and plant nitrogen uptake along a boreal forest productivity gradient.

We present results from a study of soil solution concentrations of ammonium (NH4+), nitrate (NO3-), and amino acid N over one growing season along a local 90-m-long plant productivity gradient in a boreal forest. Three forest types are found along the gradient: an ericaceous dwarf-shrub type between 0 and 40m, a low-herb type between 40 and 80m, and a tall-herb type at 90m. Soil sampling of the mor layer was performed in June, July, August and October in the three forest types. In addition, plant uptake of NH4+, NO3- and the amino acid glycine was investigated. A mixture of the three N forms was injected into the soil; one N form at a time was labeled with 15N, and in the case of glycine also with 13C. In the dwarf-shrub forest, where plant productivity was low, the soil N pool was strongly dominated by amino acid N. There, plants took up more NH4+ than NO3-. Glycine uptake did not differ significantly from either NH4+ or NO3- uptake. Along the gradient, soil concentrations of NH4+ and NO3- increased, as did plant productivity. In the low-herb forest NH4+ comprised a major portion of the soil N pool, and plants took up more NH4+ than NO3- or glycine. In the tall-herb forest, NO3- was as abundant as NH4+, and together these two N forms dominated the soil N pool. Here, plants took up nearly equal amounts of NO3- and NH4+, and this uptake exceeded that of glycine severalfold. Apart from the overall preference for NH4+ that plants exhibited throughout the gradient, the results show a correlation between soil concentrations of amino acids and NO3- and plant preferences for these N forms.

Maitotoxin (MTX) activates a nonselective cation channel in Xenopus laevis oocytes.

Maitotoxin (MTX) may exert its toxic effect by activating ion conductances and has been shown to elicit a fertilization-like response in Xenopus laevis oocytes. In the present study we investigated the electrophysiological response of stage V-VI Xenopus oocytes to MTX using the two-microelectrode voltage-clamp technique. Membrane voltage (Vm) measurements demonstrated that MTX (50 pM to 1 nM) depolarized the oocytes from -49+/-7 to -14+/-1 mV. Subsequent replacement of bath Na+ by the impermeant cation NMDG (N-methyl-d-glucamine) shifted Vm from -14+/-1 to -53+/-5 mV (n=29). This indicates that MTX activates a cation conductance. Indeed, current measurements at a holding potential of -60 or -100 mV showed that within 10 s of MTX application an inward current component developed which was largely abolished by extracellular Na+ removal. After a 1-min application of 1 nM MTX the NMDG-sensitive current increased more than 100-fold from 0.14+/-0.03 microA to a peak value of 21+/-3 microA (n=11). The effect of MTX was concentration dependent with an EC50 of about 250 pM but only slowly reversible. Ion substitution experiments indicated that the stimulated conductance was nonselective for monovalent cations with a slight preference for NH4+ (2.1) > K+ (1.5) > Na+ (1.0) > Li+ (0.7). Regarding divalent cations, a complex biphasic response to extracellular Na+ replacement by Ca2+ was observed, which suggests that the stimulated channels may have a small Ca2+ permeability but that exposure to high extracellular Ca2+ enhances recovery from MTX stimulation. No significant conductance for Mn2+ was observed. Application of 1 mM benzamil, 1 mM amiloride, or 100 microM 1-(beta-[3-(4-Methoxyphenyl)-propoxy]-4-methoxyphenethyl)-1H-imidazol e hydrochloride (SK&F 96365) reduced the MTX-stimulated inward current by 81%, 62%, or 65%, respectively. Gd3+ had an inhibitory effect of 29% and 38% at concentrations of 10 microM or 100 microM, respectively. Flufenamic acid, niflumic acid, (RS)-(3,4-dihydro-6, 7-dimethoxyisoquinoline-1-gamma1)-2-phenyl-N,N-di-[2-(2,3, 4-trimethoxyphenyl)-ethyl]-acetamide (LOE908), and 3', 5'-dichlorodiphenylamine-2-carboxylic acid (DCDPC), known blockers of other nonselective cation channels, had no significant effect. We conclude that MTX activates a nonselective cation conductance in Xenopus oocytes. The underlying channels may be involved in changes in Vm that occur during the early stages of fertilization.