Measured and modelled effects of temperature,dissolved oxygen and nutrient concentration on sediment-water nutrient exchange

Empirical models of sediment-water fluxes of NH₄ ⁺, NO₃ ^– were and PO₄ ^3– were formed based on published reports. The models were revised and parameters evaluated based on laboratory incubations of sediments collected from Gunston Cove, VA. Observed fluxes ranged from — 18 (sediments uptake) to 276 (sediment release) mg NH₄ ⁺ m^–2 day^–1, –17 to –509 mg NO₃ ^– m^–2 day^–1, and –16.4 to 8.9 mg PO₄ ^3– m^–2 day^–1. The model and observations indicated release of NH₄ ⁺ was enhanced by high temperature and by low DO. Uptake of NO₃ ^– was enhanced primarily by high NO₃ ^– concentration and to a lesser extent by high temperature and by low DO. Direction of PO₄ ^3– flux depended on concentration in the water. Release was enhanced by low DO. No effect of temperature on PO₄ ^3– flux was observed.     

The influence of nitrogen concentration and ammonium/nitrate ratio on N-uptake,mineral composition and yield of citrus

In short-term water culture experiments with different ¹⁵N labeled ammonium or nitrate concentrations, citrus seedlings absorbed NH₄ ⁺ at a higher rate than NO₃ ^–. Maximum NO₃ ^– uptake by the whole plant occurred at 120 mg L^–1 NO₃ ^–-N, whereas NH₄ ⁺ absorption was saturated at 240 mg L^–1 NH₄ ⁺-N. ¹⁵NH₄ ⁺ accumulated in roots and to a lesser degree in both leaves and stems. However, ¹⁵NO₃ ^– was mostly partitioned between leaves and roots.Adding increasing amounts of unlabeled NH₄ ⁺ (15–60 mg L^–1 N) to nutrient solutions containing 120 mg L^–1 N as ¹⁵N labeled nitrate reduced ¹⁵NO₃ ^– uptake. Maximum inhibition of ¹⁵NO₃ ^– uptake was about 55% at 2.14 mM NH₄ ⁺ (30 mg L^–1 NH₄ ⁺-N) and it did not increase any further at higher NH₄ ⁺ proportions.In a long-term experiment, the effects of concentration and source of added N (NO₃ ^– or NH₄ ⁺) on nutrient concentrations in leaves from plants grown in sand were evaluated. Leaf concentration of N, P, Mg, Fe and Cu were increased by NH₄ ⁺ versus NO₃ ^– nutrition, whereas the reverse was true for Ca, K, Zn and Mn.The effects of different NO₃ ^–-N:NH₄ ⁺-N ratios (100:0, 75:25, 50:50, 25:75 and 0:100) at 120 mg L^–1 total N on leaf nutrient concentrations, fruit yield and fruit characteristics were investigated in another long-term experiment with plants grown in sand cultures. Nitrogen concentrations in leaves were highest when plants were provided with either NO₃ ^– or NH₄ ⁺ as a sole source of N. Lowest N concentration in leaves was found with a 75:25 NO₃ ^–-N/NH₄ ⁺-N ratio. With increasing proportions of NH₄ ⁺ in the N supply, leaf nutrients such as P, Mg, Fe and Cu increased, whereas Ca, K, Mn and Zn decreased. Yield in number of fruits per tree was increased significantly by supplying all N as NH₄ ⁺, although fruit weight was reduced. The number of fruits per tree was lowest with the 75:25 NO₃ ^–-N:NH₄ ⁺-N ratio, but in this treatment fruits reached their highest weight. Rind thickness, juice acidity, and colour index of fruits decreased with increasing NH₄ ⁺ in the N supply, whereas the % pulp and maturity index increased. Percent of juice in fruits and total soluble solids were only slightly affected by NO₃ ^–:NH₄ ⁺ ratio.     

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.     

Seasonal variation of nitrogen transformations in the pelagial of selected nearshore waters of the Baltic Sea with emphasis on the particulate pool

Physical and chemical conditions, particulate matter and N-uptake were characterized at two sampling sites at the eastern German coast of the Baltic Sea (Pomeranian Bay) over the annual period of 1997 (February–November). The inshore sampling sites (5 m water depth) differed with respect to the potential influences of river run-off and salt water exchange (mean values of salinity: 7.05 and 8.72 PSU), respectively. The mean org-C_diss/org-C_part-ratios (4.9 and 12.6) fell in the same order of magnitude (1.0–12.6) as values of neighboring inshore waters, and increasing values reflect an enhancement of the trophic level. Beside differences of nitrogen concentrations (dissolved inorganic nitrogen: 1.8–23.8 and 0.9–9.9 mol l^–1), particulate nitrogen (4.30–41.01 and 2.69–9.08 mol l^–1) and absolute uptake of N-nutrients (mean sum of NH₄ ⁺, urea, NO₃ ^– uptake rates: 0.141 and 0.087 mol l^–1 h^–1), specific uptake of ¹⁵N-labelled nutrients (NH₄ ⁺, urea, NO₃ ^–) as well as the relationships between the measured variables characterize distinguishable inshore systems. The high variability at the shallow sampling sites prevents, however a simple resolution of the seasonal courses. Light dose could be identified as a potential key in order to describe long-term variations of N-uptake at the station with higher organic matter concentration (station KW), but phytoplankton development is better reflected in the seasonal course of N-uptake at the other station. Specific nitrogen uptake rates (NH₄ ⁺: 0.0009–0.0353 h^–1, urea: 0.0001–0.0137 h^–1, NO₃ ^–: 0.000004–0.0009 h^–1) and relative nitrogen preferences indicate extraordinary importance of reduced nitrogenous nutrients (NH₄ ⁺, urea) at both stations throughout the year.     

The seasonal dynamics of amino acids and other nutrients in Alaskan Arctic tundra soils

Past research strongly indicates the importance of amino acids in the N economy of the Arctic tundra, but little is known about the seasonal dynamics of amino acids in tundra soils. We repeatedly sampled soils from tussock, shrub, and wet sedge tundra communities in the summers of 2000 and 2001 and extracted them with water (H₂O) and potassium sulfate (K₂SO₄) to determine the seasonal dynamics of soil amino acids, ammonium (NH₄⁺), nitrate (NO₃^–), dissolved organic nitrogen (DON), dissolved organic carbon (DOC), and phosphate (PO₄^2–). In the H₂O extractions mean concentrations of total free amino acids (TFAA) were higher than NH₄⁺ in all soils but shrub. TFAA and NH₄⁺ were highest in wet sedge and tussock soils and lowest in shrub soil. The most predominant amino acids were alanine, arginine, glycine, serine, and threonine. None of the highest amino acids were significantly different than NH₄⁺ in any soil but shrub, in which NH₄⁺ was significantly higher than all of the highest individual amino acids. Mean NO₃^– concentrations were not significantly different from mean TFAA and NH₄⁺ concentrations in any soil but tussock, where NO₃^– was significantly higher than NH₄⁺. In all soils amino acid and NH₄⁺ concentrations dropped to barely detectable levels in the middle of July, suggesting intense competition for N at the height of the growing season. In all soils but tussock, amino acid and NH₄⁺ concentrations rebounded in August as the end of the Arctic growing season approached and plant N demand decreased. This pattern suggests that low N concentrations in tundra soils at the height of the growing season are likely the result of an increase in soil N uptake associated with the peak in plant growth, either directly by roots or indirectly by microbes fueled by increased root C inputs in mid-July. As N availability decreased in July, PO₄^2– concentrations in the K₂SO₄ extractions increased dramatically in all soils but shrub, where there was a comparable increase in PO₄^2– later in the growing season. Previous research suggests that these increases in PO₄^2– concentrations are due to the mineralization of organic phosphorus by phosphatase enzymes associated with soil microbes and plant roots, and that they may have been caused by an increase in organic P availability.     

Nitrogen relations of a low nitrate uptake inbred line of white clover (Trifolium repens L.)

The nitrogen relations of an inbred line of white clover (Trifolium repens L.) thought to exhibit an abnormally low capacity for NO₃ ^– uptake (line LNU) were compared with a line regarded as normal with respect to NO₃ ^– uptake (line NNU). Growth, nodulation, N₂ fixation and NO₃ ^– uptake were measured over 7 weeks in flowing solution culture (Experiment 1) by plants dependent for N acquisition on either (i) NO₃ ^– uptake, (ii) NO₃ ^– uptake +N₂ fixation, or (iii) N₂ fixation only. Effects of plant N status on the short-term uptake and translocation of ¹⁵NH₄ ⁺ and ¹⁵NO₃ ^– were also investigated (Experiment 2). Nitrate uptake per plant by –fix/+NO₃ ^– line LNU was 50% of uptake by line NNU over 35 days, and there were significant differences in specific uptake rates of NO₃ ^– between the lines over the first 24 days. The `low NO<sub>3</sub> <sup>–</sup> uptake' phenotype was indistinct under +fix/+NO<sub>3</sub> <sup>–</sup> treatment. Nitrate lowered specific rates of nitrogen fixation by line NNU but had no effect on line LNU. Only low N status line LNU plants had lower short-term rates of NH<sub>4</sub> <sup>+</sup> and NO<sub>3</sub> <sup>–</sup> uptake than line NNU. It is concluded that the `low NO₃ ^– uptake' phenotype of line LNU is inconsistently expressed. Circumstantial evidence points to increased NO₃ ^– efflux and decreased xylem translocation of NO₃ ^– as possible explanations for the lower NO₃ ^– uptake by line LNU.     

Plant-mediated processess to acquire nutrients: nitrogen uptake by rice plants

The ways in which root–soil interactions can control nutrient acquisition by plants is illustrated by reference to the N nutrition of rice. Model calculations and experiments are used to assess how uptake is affected by root properties and N transport through the soil. Measurements of the kinetics of N absorption and assimilation and their regulation, and of interactions between NH₄ ⁺ and NO₃ ^– nutrition, are described. It is shown that uptake of N from the soil–-as opposed to N in ricefield floodwater which can be absorbed very rapidly but is otherwise lost by gaseous emission–-will often be limited by root uptake properties. Rice roots are particularly efficient in absorbing and assimilating NO₃ ^–, and NH₄ ⁺ absorption and assimilation are stimulated by NO₃ ^–. The uptake of NO₃ ^– formed in the rice rhizosphere by root-released O₂ may be more important than previously thought, with beneficial consequences for rice growth. Other root-induced changes in the rice rhizosphere and their consequences are discussed.     

AMMONIUN AND NITRATE UPTAKE BY THE MARINE MACROPHYTES HYPNEA MUSVUFORMIS (RHODOPHYTA) AND MACROCYSTIS PYRIFERA (PHAEOPHYTA)1, 2

NH₄⁺ and NO₃^? uptake were measured by continuous sampling with an autoanalyzer. For Hypnea musciformis (Wulfen) Lamouroux, NO₃^?up take followed saturable kinetics (K₂=4.9 μg-at N t^?1, V_max= 2.85 μg- at N, g(wet)^?1. h^?1. The ammonium uptake data fit a trucatd hyperbola, i.e., saturation was not reach at the concentrations used. NO₃^? uptake was reduced one-half in the presence of NH₄⁺, but presence of NO₃^? had no effect on NH₄⁺ uptake. Darkness reduced both NO₃^? and NH₄⁺ uptake by one-third to one-half. For Macrocystis pyrufera (L) C. Agardh, NO₃^? uptake followed saturable kinetices: K₂=13.1 μg-at N. l^?1. V_max=3.05 μg-at N. g(wet)^?1. h^?1.NH₄⁺ uptake showed saturable kinetics at concentration below 22 μg-at N l -1 (K₂=5.3 μg-at N.1–1, V_max= 2.38 μg-at N G (wet)^?1.h^?1: at higher concentration uptake increased lincarly with concentrations. NO₃^?and NH₄⁺ were taken up simulataneously: presence of one form did not affect uptake of the other.     

Short Term Studies of Nitrate Uptake into Barley Plants Using Ion-Specific Electrodes and ClO(3): II. Regulation of NO(3) Efflux by NH(4)

The influence of NH₄⁺, in the external medium, on fluxes of NO₃⁻ and K⁺ were investigated using barley (Hordeum vulgare cv Betzes) plants. NH₄⁺ was without effect on NO₃⁻ (³⁶ClO₃⁻) influx whereas inhibition of net uptake appeared to be a function of previous NO₃⁻ provision. Plants grown at 10 micromolar NO₃⁻ were sensitive to external NH₄⁺ when uptake was measured in 100 micromolar NO₃⁻. By contrast, NO₃⁻ uptake (from 100 micromolar NO₃⁻) by plants previously grown at this concentration was not reduced by NH₄⁺ treatment. Plants pretreated for 2 days with 5 millimolar NO₃⁻ showed net efflux of NO₃⁻ when roots were transferred to 100 micromolar NO₃⁻. This efflux was stimulated in the presence of NH₄⁺. NH₄⁺ also stimulated NO₃⁻ efflux from plants pretreated with relatively low nitrate concentrations. It is proposed that short term effects on net uptake of NO₃⁻ occur via effects upon efflux. By contrast to the situation for NO₃⁻, net K⁺ uptake and influx of ³⁶Rb⁺-labeled K⁺ was inhibited by NH₄⁺ regardless of the nutrient history of the plants. Inhibition of net K⁺ uptake reached its maximum value within 2 minutes of NH₄⁺ addition. It is concluded that the latter ion exerts a direct effect upon K⁺ influx.     

Uptake and inhibition kinetics of nitrogen in Microcystis aeruginosa: Results from cultures and field assemblages collected in the San Francisco Bay Delta,CA

The nitrogen (N) uptake kinetic parameters for Microcystis field assemblages collected from the San Francisco Bay Delta (Delta) in 2012 and non-toxic and toxic laboratory culture strains of M. aeruginosa were assessed. The ¹⁵N tracer technique was used to investigate uptake of ammonium (NH₄⁺), nitrate (NO₃⁻), urea and glutamic acid over short-term incubations (0.5–1 h), and to study inhibition of NO₃⁻, NH₄⁺ and urea uptake by NH₄⁺, NO₃⁻ and NH₄⁺, respectively. This study demonstrates that Delta Microcystis can utilize different forms of inorganic and organic N, with the greatest capacity for NH₄⁺ uptake and the least for glutamic acid uptake, although N uptake did not always follow the classic Michaelis–Menten hyperbolic relationship at substrate concentrations up to 67 μmol N L⁻¹. Current ambient N concentrations in the Delta may be at sub-saturating levels for N uptake, indicating that if N loading (especially NH₄⁺) were to increase, Delta Microcystis assemblages have the potential for increased N uptake rates. Delta Microcystis had the highest specific affinity, α, for NH₄⁺ and the lowest for NO₃⁻. In culture, N uptake by non-toxic and toxic M. aeruginosa strains was much higher than from the field, but followed similar N utilization trends to those in the field. Neither strain showed severe inhibition of NO₃⁻ uptake by NH₄⁺ or inhibition of NH₄⁺ uptake on NO₃⁻, but both strains showed some inhibition of urea uptake by NH₄⁺.     

Clinoptilolite zeolite and cellulose amendments to reduce ammonia volatilization in a calcareous sandy soil

Leaching of nitrate (NO₃ ^–) below the root zone and gaseous losses of nitrogen (N) such as ammonia (NH₃) volatilization, are major mechanisms of N loss from agricultural soils. New techniques to minimize such losses are needed to maximize N uptake efficiency and minimize production costs and the risk of potential N contamination of ground and surface waters. The effects of cellulose (C), clinoptilolite zeolite (CZ), or a combination of both (C+CZ) on NH₃ volatilization and N transformation in a calcareous Riviera fine sand (loamy, siliceous, hyperthermic, Arenic Glossaqualf) from a citrus grove were investigated in a laboratory incubation study. Ammonia volatilization from NH₄NO₃ (AN), (NH₄)₂SO₄(AS), and urea (U) applied at 200 mg N kg^–1 soil decreased by 2.5-, 2.1- and 0.9-fold, respectively, with cellulose application at 15 g kg^–1 and by 4.4-, 2.9- and 3.0-fold, respectively, with CZ application at 15 g kg^–1 as compared with that from the respective sources without the amendments. Application of cellulose plus CZ (each at 15 g kg^–1) was the most effective in decreasing NH₃ volatilization. Application of cellulose increased the microbial biomass, which was responsible for immobilization of N, and thus decreased volatilization loss of NH₃–N. The effect of CZ, on the other hand, may be due to increased retention of NH₄ in the ion-exchange sites. The positive effect of interaction between cellulose and CZ amendment on microbial biomass was probably due to improved nutrient retention and availability to microorganisms in the soil. Thus, the amendments provide favorable conditions for microbial growth. These results indicate that soil amendment of CZ or CZ plus organic materials such as cellulose has great potential in reducing fertilizer N loss in sandy soils.     

Growth and solute composition of the salt-tolerant kallar grass [Leptochloa fusca (L.) Kunth] as affected by nitrogen source

A study was conducted to elucidate the effect of N form, either NH₄ ⁺ or NO₃ ^–, on growth and solute composition of the salt-tolerant kallar grass [Leptochloa fusca (L.) Kunth] grown under 10 mM or 100 mM NaCl in hydroponics. Shoot biomass was not affected by N form, whereas NH₄ ⁺ compared to NO₃ ^– nutrition caused an almost 4-fold reduction in the root biomass at both salinity levels. Under NH₄ ⁺ nutrition, salinity had no effect on the biomass yield, whereas under NO₃ ^– nutrition, increasing salinity from 10 mM to 100 mM caused 23% and 36% reduction in the root and shoot biomass, respectively. The reduced root growth under NH₄ ⁺ nutrition was not attributable to impaired shoot to root C allocation since N form did not affect the overall root sugar concentration and the starch concentration was even higher under NH₄ ⁺ compared to NO₃ ^– nutrition. The low NH₄ ⁺ (2 mM) and generally higher amino-N concentrations in NH₄ ⁺- compared to NO₃ ^–-fed plants indicated that the grass was able to effectively detoxify NH₄ ⁺. Salinity had no effect on Ca²⁺ and Mg²⁺ levels, whereas their concentration in shoots was lower under NH₄ ⁺ compared to NO₃ ^– nutrition (over 66% reduction in Ca²⁺; over 20% reduction in Mg²⁺), but without showing deficiency symptoms. Ammonium compared to NO₃ ^– nutrition did not inhibit K⁺ uptake, and the K⁺-Na⁺ selectivity either remained unaffected or it was higher under NH₄ ⁺ than under NO₃ ^– nutrition. Results suggested that while NH₄ ⁺ versus NO₃ ^– nutrition substantially reduced root growth, and also strongly modified anion concentrations and to a minor extent concentrations of divalent cations in shoots, it did not influence salt tolerance of kallar grass.     

Nitrogen uptake in needles of Scots pine (Pinus sylvestris L.) when exposed to gaseous ammonia and ammonium fertilizer in the soil

Young saplings of Pinus sylvestris L. were exposed to gaseous NH₃ at 53 or 105 g m^–3 for one year in open-top chambers. Saplings received ¹⁵N-labelled (NH₄)₂SO₄ via the soil. To examine the importance of foliar N uptake, changes in the concentration of total and labelled N in the needles were followed. Increase in needle biomass and N concentration were found in trees exposed to NH₃, confirming that atmospheric NH₃ acted as a N fertilizer. NH₃ had a greater and quicker effect than (NH₄)₂SO₄: compared with the growth in ambient air, the N concentration in the needles exposed to NH₃ had increased by 49% in four months, while the increase after highest N-fertilization (200 kg N ha^–1 y^–1) was only 8%. The small contribution of NH₄ ⁺ fertilization to the total N concentration was not due to a deficient N uptake: the ¹⁵N concentration in the needles increased significantly with time. On the other hand, NH₃ uptake in shoots may have a negative effect on the NH₄ ⁺ root uptake. The relation between plant N and atmospheric NH₃ concentration was non-linear and possible reasons for this observation are discussed. Fumigation with NH₃ significantly decreased the ratios of K/N and P/N, showing that fumigation disrupted the nutrient balance.     

Effects of different nitrogen forms and combination with foliar spraying with 6-benzylaminopurine on growth,transpiration, and water and potassium uptake and flow in tobacco

NH₄ ⁺-N can have inhibitory effects on plant growth. However, the mechanisms of these inhibitory effects are still poorly understood. In this study, effects of different N forms and a combination of ammonium + 6-benzylaminopurine (6-BA, a synthetic cytokinin) on growth, transpiration, uptake and flow of water and potassium in 88-days-old tobacco (Nicotiana tabacum L. K 326) plants were studied over a period of 12 days. Plants were supplied with equal amounts of N in different forms: NO₃ ^–, NH₄NO₃, NH₄ ⁺ or NH₄ ⁺+6-BA (foliar spraying every 2 days after onset of the treatments). For determining flows and partitioning upper, middle and lower strata of three leaves each were analysed. During the 12 days study period, 50% replacement of NO₃ ^–-N by NH₄ ⁺-N (NH₄NO₃) did not change growth, transpiration, uptake and flow of water and K⁺ compared with the NO₃ ^–-N treatment. However, NH₄ ⁺-N as the sole N-source caused: (i) a substantial decrease in dry weight gain to 42% and 46% of the NO₃ ^–-N and NH₄NO₃ treatments, respectively; (ii) a marked reduction in transpiration rate, due to reduced stomatal conductance, illustrated by more negative leaf carbon-isotope discrimination (¹³C) compared with the NO₃ ^– treatment, especially in upper leaves; (iii) a strong reduction both in total water uptake, and in the rate of water uptake by roots, likely due to a decrease in root hydraulic conductivity; (iv) a marked reduction of K⁺ uptake to 10%. Under NH₄ ⁺ nutrition the middle leaves accumulated 143%, and together with upper leaves 206% and the stem 227% of the K⁺ currently taken up, indicating massive mobilisation of K⁺ from lower leaves and even the roots. Phloem retranslocation of K⁺ from the shoot and cycling through the root contributed 67% to the xylem transport of K⁺, and this was 2.2 times more than concurrent uptake. Foliar 6-BA application could not suppress or reverse the inhibitory effects on growth, transpiration, uptake and flow of water and ions (K⁺) caused by NH₄ ⁺-N treatment, although positive effects by 6-BA application were observed, even when 6-BA (10^–8 M) was supplied in nutrient solution daily with watering. Possible roles of cytokinin to regulate growth and development of NH₄ ⁺-fed plants are discussed.     

Ammonium uptake in Lemna gibba G 1, related membrane potential changes, and inhibition of anion uptake

In N-starved (?N) fronds of Lemna gibba L. G 1, NH₄⁺ uptake rates were several-fold those of NO₃^?-supplied (+N) fronds. NO₃^?, uptake in +N-plants was slow and not inhibited by addition of NH₄⁺. However, in ?N-plants with higher NO₃^? and still higher NH₄⁺ uptake rates, addition of NH₄⁺ immediately reduced the NO₃^? uptake rates to about one third until the NH₄⁺ was consumed. The membrane potential (E_m) decreased immediately upon addition of NH₄⁺ in all fronds, but whereas depolarisation was moderate and transient in +N-plants, it was strong, up to 150 mV, in N-starved plants, where E_m remained at the level of the K⁺ diffusion potential (E_D) until NH₄⁺ was removed. In N-starved plants NH₄⁺ uptake and membrane depolarisation showed the same concentration dependence, except for an apparent linear component for uptake. Phosphate uptake was inhibited by NH₄⁺ similarly to NO₃^? uptake, but only in P- and N-starved plants, not after mere P starvation. Influx of NO₃^? and H₂PO ₄^? into the negatively charged cells of Lemna is mediated by anion/H⁺ cotransport, but NH₄⁺ influx can follow the electrochemical gradient. Its saturating component may reflect a carrier-mediated NH₄⁺ uniport, the linear component diffusion of NH₄⁺ or NH₃. Inhibition of anion/H⁺ cotransport by high NH₄⁺ influx rates may be due to loss of the proton-driving force, Δμ?H⁺, across the plasmalemma. Reversible inhibition by NH₄⁺ of the H⁺ extrusion pump may contribute to the finding that Δμ?H⁺ cannot be reconstituted in the presence of higher NH₄⁺ concentrations.     

Effects of inorganic nitrogen forms on growth, morphology, nitrogen uptake capacity and nutrient allocation of four tropical aquatic macrophytes (Salvinia cucullata, Ipomoea aquatica, Cyperus involucratus and Vetiveria zizanioides)

This study assesses the growth and morphological responses, nitrogen uptake and nutrient allocation in four aquatic macrophytes when supplied with different inorganic nitrogen treatments (1) NH₄⁺, (2) NO₃⁻, or (3) both NH₄⁺ and NO₃⁻. Two free-floating species (Salvinia cucullata Roxb. ex Bory and Ipomoea aquatica Forssk.) and two emergent species (Cyperus involucratus Rottb. and Vetiveria zizanioides (L.) Nash ex Small) were grown with these N treatments at equimolar concentrations (500 μM). Overall, the plants responded well to NH₄⁺. Growth as RGR was highest in S. cucullata (0.12 ± 0.003 d⁻¹) followed by I. aquatica (0.035 ± 0.002 d⁻¹), C. involucratus (0.03 ± 0.002 d⁻¹) and V. zizanioides (0.02 ± 0.003 d⁻¹). The NH₄⁺ uptake rate was significantly higher than the NO₃⁻ uptake rate. The free-floating species had higher nitrogen uptake rates than the emergent species. The N-uptake rate differed between plant species and seemed to be correlated to growth rate. All species had a high NO₃⁻ uptake rate when supplied with only NO₃⁻. It seems that the NO₃⁻ transporters in the plasma membrane of the root cells and nitrate reductase activity were induced by external NO₃⁻. Tissue mineral contents varied with species and tissue, but differences between treatments were generally small. We conclude, that the free-floating S. cucullata and I. aquatica are good candidate species for use in constructed wetland systems to remove N from polluted water. The rooted emergent plants can be used in subsurface flow constructed wetland systems as they grow well on any form of nitrogen and as they can develop a deep and dense root system.     

The influence of ammonium and chloride on potassium and nitrate absorption by barley roots depends on time of exposure and cultivar

Net uptakes of K⁺ and NO₃⁻ were monitored simultaneously and continuously for two barley (Hordeum vulgare) cultivars, Prato and Olli. The cultivars had similar rates of net K⁺ and NO₃⁻ uptake in the absence of NH₄⁺ or Cl⁻. Long-term exposure (over 6 hours) to media which contained equimolar mixtures of NH₄⁺, K⁺, Cl⁻, or NO₃⁻ affected the cultivars very differently: (a) the presence of NH₄⁺ as NH₄Cl stimulated net NO₃⁻ uptake in Prato barley but inhibited net NO₃⁻ uptake in Olli barley; (b) Cl⁻ inhibited net NO₃⁻ uptake in Prato but had little effect in Olli; and (c) NH₄⁺ as (NH₄)₂SO₄ inhibited net K⁺ uptake in Prato but had little effect in Olli. Moreover, the immediate response to the addition of an ion often varied significantly from the long-term response; for example, the addition of Cl⁻ initially inhibited net K⁺ uptake in Olli barley but, after a 4 hour exposure, it was stimulatory. For both cultivars, net NH₄⁺ and Cl⁻ uptake did not change significantly with time after these ions were added to the nutrient medium. These data indicate that, even within one species, there is a high degree of genotypic variation in the control of nutrient absorption.     

Removal of inorganic ions from wastewaters by immobilized microalgae

Anabaena doliolum and Chlorella vulgaris immobilized on chitosan were more efficient at removing NO₃ ^–, NO₂ ^p–, PO₄ ^3– and CR₂O₇ ^2– from wastewaters than cells immobilized on agar, alginate, carrageenan or even free cells. Carrageenan-immobilized cells, however, were better at removing NH₄ ⁺ and Ni²⁺. The PO₄ ^3– uptake capacity was significantly increased in cells starved of PO₄ ^3– for 24 h. Agar-immobilized cells, though having good metal and nutrient uptake efficiency, had only a slow growth rate. Chitosan is recommended as an algal support for wastewater detoxification.The authors are with the Laboratory of Algal Biology, Department of Botany, Banaras Hindu University, Varanasi-221005, India     

Response of Douglas fir seedlings to nitrate and ammonium nitrogen sources at different levels of pH and iron supply

Douglas fir seedlings were grown for two to three months in sand and soil cultures in a greenhouse to examine their growth response to nitrogen (N) source at different levels of pH and iron (Fe) supply. In the first two experiments nutrient solutions of known pH were automatically applied to the top of the sand cultures and allowed to run to waste from the bottom. Under these conditions seedlings made most growth on nitrate (NO₃–N) under acid (pH4) conditions, but most growth on ammonium (NH₄–N) under neutral (pH7) conditions. Calcium carbonate (CaCO₃) was used to create a range of pH conditions (from 4.0 to 7.2) in a peat and sand artificial soil. Over the pH range 4 to 6 NH₄–N or NO₃+NH₄–N produced larger seedlings than NO₃–N alone, but above pH6 growth on all N sources was depressed. Chemical analysis showed that seedling Ca concentration had increased and Fe concentration had decreased with increase in CaCO₃ application. Both Ca and Fe concentrations were higher in seedlings receiving NO₃–N than in those receiving NH₄ or NO₃+NH₄.In sub-irrigated sand cultures, Doughlas fir seedlings receiving NO₃–N were shown to respond to additions of Fe chelate, but seedlings receiving NH₄–N responded little to Fe chelate. At pH5 seedlings receiving NO₃–N did not grow as big as seedlings receiving NH₄–N in the absence of Fe chelate, but addition of Fe chelate resulted in NO₃-fed seedlings growing larger than NH₄-fed seedlings. The relationship between seedling Fe concentration and N nutrition is discussed.The relatively larger root dry weight and surface area of seedlings grown on NO₃–N, as compared to NH₄–N, in sand culture, was noted.     

Effects of supplied nitrogen form on growth and water uptake of French bean (Phaseolus vulgaris L.) plants

In order to investigate the effect of N form on dry matter (DM) formation and water uptake rate, French bean (Phaseolus vulgaris L. `Sotaxa') plants were grown with a split-root system. Three treatments were compared: sole nitrate (NO^– ₃) supply (NN), sole ammonium (NH⁺ ₄) supply (AA) and spatially separated supply of NO^– ₃ and NH⁺ ₄ (NA). The pH of the nutrient solutions was kept constant at 6.3 using a pH-stat system. 9 days after onset of the treatments, NN plants had higher root (36%) and shoot dry matter (11%) than AA plants. N form drastically influenced partitioning of assimilates: in the NA treatment, the root half exposed to NO^– ₃ revealed a 170% higher DM than the root half exposed to NH⁺ ₄. N form affected stable carbon-isotope discrimination () of leaf tissue. In leaves of plants which were supplied with NH⁺ ₄ (AA; NA) was significantly more negative (–29.4, –29.6) than in NN treatment (–28.2). We explain this effect by differences in stomatal conductance. We suppose that the significantly less negative of root tissue under NH⁺ ₄ supply is most probably related to higher PEP-case activity. The water uptake rate was higher in NN than in AA grown plants. This effect was found in both, short- and long-term experiments. In case of NA plants, the water uptake in the root part being exposed to NO^– ₃ was 104% higher than in those receiving NH⁺ ₄. At least in the case of the NA treatment we can exclude shoot growth effects as being responsible for differences in water uptake. We therefore assume that differences in root hydraulic conductivity are responsible for the observed effects.