Effect of nitrogen form and phosphorus source on the growth,nutrient uptake and rhizosphere soil property of Camellia sinensis L.

The effects of nitrogen form and phosphorus source on the growth, nutrient uptake and rhizosphere soil property of tea (Camellia sinensis L.) were investigated in a pot experiment. The experiment was performed with a compartmental cropping device, which enables the collection of rhizosphere soil at defined distances from the root of tea plant. Nitrogen was supplied as nitrate or ammonium in combination with soluble phosphorus as Ca(H₂PO₄)₂ or insoluble P as rock phosphate. The leaf dry matter production of tea was significantly greater in the treatments with NH₄ ⁺ than NO₃ ^-, whereas dry matter production of root and stem was not significantly affected. Addition of phosphorus as either source did not influence the dry matter production. The concentrations of K in root, Mg and Ca in both the shoot and root supplied with NO₃ ^- were significantly higher than in NH₄ ⁺ and influence of P sources was minor. On the contrary, Al and Mn concentrations were significantly larger in NH₄ ^--fed plants which could be attributed to remarkably increased availability of Al and Mn caused by acidification of the rhizosphere soil (the first 1-mm soil section from the root surface) with NH₄–N nutrition. The concentration of N in shoot was also significantly higher in NH₄- than in NO₃-fed plants, indicating higher use efficiency of NH₄–N. Whatever the phosphate source, rhizosphere pH declined in ammonium compared to in nitrate treatment. The pH decrease was much larger when no P or soluble P were applied and reached 0.85–1.30 units which extended to 3–5 mm away from the root surface. Exchangeable acidity, content of exchangeable Al and Mn were also considerably higher in the rhizosphere soils of NH₄ ⁺ fed tea plants. Significant amounts of P dissolved from rock phosphate accumulated in rhizosphere of NH₄ ⁺, not NO₃ ^-, suggesting that the dissolution of rock phosphate was induced by the proton excreted by tea root fed with ammonium. With soluble P addition, shoot and root P concentrations were greater in NH₄ ⁺ than in NO₃ ^- treatment and it appeared that this difference could not be sufficiently explained by the available P content in soil which was only slightly higher in NH₄ ⁺ treatment. With rock phosphate addition, the shoot and root P concentrations were hardly affected by nitrogen form, although the available P content was much higher and accumulated in the rhizosphere soil supplied with ammonium. The reason for this was discussed with regard to the inter-relationship of Al with P uptake. This revised version was published online in June 2006 with corrections to the Cover Date.     

Nitrogen form, availability, and mycorrhizal colonization affect biomass and nitrogen isotope patterns in Pinus sylvestris

Nitrogen (N) isotope patterns are useful for understanding carbon and nitrogen dynamics in mycorrhizal systems but questions remain about how different N forms, fungal symbionts, and N availabilities influence δ¹⁵N signatures. Here, we studied how biomass allocation and δ¹⁵N patterns in Pinus sylvestris L. cultures were affected by nitrogen supply rate (3% per day or 4% per day relative to the nitrogen already present), nitrogen form (ammonium versus nitrate), and mycorrhizal colonization by fungi with a greater (Laccaria laccata) or lesser (Suillus bovinus) ability to assimilate nitrate. Mycorrhizal (fungal) biomass was greater with ammonium than with nitrate nutrition for Suillus cultures but similar for Laccaria cultures. Total biomass was less with nitrate nutrition than with ammonium nutrition for nonmycorrhizal cultures and was less in mycorrhizal cultures than in nonmycorrhizal cultures. The sequestration of available N by mycorrhizal fungi limited plant N supply. This limitation and the higher energetic cost of nitrate reduction than ammonium assimilation appeared to control plant biomass accumulation. Colonization decreased foliar δ¹⁵N by 0.5 to 2.2‰ (nitrate) or 1.7 to 3.5‰ (ammonium) and increased root tip δ¹⁵N by 0 to 1‰ (nitrate) or 0.6 to 2.3‰ (ammonium). Root tip δ¹⁵N and fungal biomass on root tips were positively correlated in ammonium treatments (r ²?=?0.52) but not in nitrate treatments (r ²?=?0.00). Fungal biomass on root tips was enriched in ¹⁵N an estimated 6–8‰ relative to plant biomass in ammonium treatments. At high nitrate availability, Suillus colonization did not reduce plant δ¹⁵N. We conclude that: (1) transfer of ¹⁵N-depleted N from mycorrhizal fungi to plants produces low plant δ¹⁵N signatures and high root tip and fungal δ¹⁵N signatures; (2) limited nitrate reduction in fungi restricted transfer of ¹⁵N-depleted N to plants when nitrate is supplied and may account for many field observations of high plant δ¹⁵N under such conditions; (3) plants could transfer assimilated nitrogen to fungi at high nitrate supply but such transfer was without ¹⁵N fractionation. These factors probably control plant δ¹⁵N patterns across N availability gradients and were here incorporated into analytical equations for interpreting nitrogen isotope patterns in mycorrhizal fungi and plants.     

Comparison of nitrogen uptake in the roots and rhizomes of <Emphasis Type="Italic">Leymus chinensis</Emphasis>

Leymus chinensis (Trin.) Tzvel is a rhizomatous grass species in the Eastern Eurasian steppe zone that is often limited by low soil nitrogen availability. Although a previous study showed that the rhizomes of L. chinensis have the capacity to take up nitrogen, the importance of such uptake for nitrogen nutrition is unclear. Moreover, little is known regarding the inorganic nitrogen uptake kinetics of roots and rhizomes in response to nitrogen status. Here, we first found that ammonium is preferred over nitrate and glycine for L. chinensis growth. Using the ¹⁵N-labelling method, we found that the rate of ion influx into roots was approximately five-fold higher than into rhizomes under the same nitrogen content, and the ion influxes into roots and rhizomes under 0.05 mM N were greater than in the presence of 3 mM N, especially in the form of NH₄⁺. Using a non-invasive micro-test technique, we characterised the patterns of NH₄⁺ and NO₃^– fluxes in the root mature zone, root tip, rhizome mature zone, and rhizome tip following incubation in the solution with different N compounds and different N concentrations. These results suggest a dynamic balance between the uptake, utilisation, and excretion of nitrogen in L. chinensis.     

Utilization of Ammonium as a Nitrogen Source : Effects of Ambient Acidity on Growth and Nitrogen Accumulation by Soybean

Dry matter accumulation of plants utilizing NH₄⁺ as the sole nitrogen source generally is less than that of plants receiving NO₃⁻ unless acidity of the root-zone is controlled at a pH of about 6.0. To test the hypothesis that the reduction in growth is a consequence of nitrogen stress within the plant in response to effects of increased acidity during uptake of NH₄⁺ by roots, nonnodulated soybean plants (Glycine max [L.] Merr. cv Ransom) were grown for 24 days in flowing nutrient culture containing 1.0 millimolar NH₄⁺ as the nitrogen source. Acidities of the culture solutions were controlled at pH 6.1, 5.1, and 4.1 ± 0.1 by automatic additions of 0.01 n H₂SO₄ or Ca(OH)₂. Plants were sampled at intervals of 3 to 4 days for determination of dry matter and nitrogen accumulation. Rates of NH₄⁺ uptake per gram root dry weight were calculated from these data. Net CO₂ exchange rates per unit leaf area were measured on attached leaves by infrared gas analysis. When acidity of the culture solution was increased from pH 6.1 to 5.1, dry matter and nitrogen accumulation were reduced by about 40% within 14 days. Net CO₂ exchange rates per unit leaf area, however, were not affected, and the decreased growth was associated with a reduction in rates of appearance and expansion of new leaves. The uptake rates of NH₄⁺ per gram root were about 25% lower throughout the 24 days at pH 5.1 than at 6.1. A further increase in solution acidity from pH 5.1 to 4.1 resulted in cessation of net dry matter production and appearance of new leaves within 10 days. Net CO₂ exchange rates per unit leaf area declined rapidly until all viable leaves had abscised by 18 days. Uptake rates of NH₄⁺, which were initially about 50% lower at pH 4.1 than at 6.1, continued to decline with time of exposure until net uptake ceased at 10 days. Since these responses also are characteristic of the sequence of responses that occur during onset and progression of a nitrogen stress, they corroborate our hypothesis.     

Influence of ammonium on fine root development and rhizosphere pH of Douglas-fir seedlings in sand

Ammonium sulphate is a major component of the air pollutants deposited on forests in the Netherlands. Different amounts of NH₄ ⁺ were added to Douglas-fir seedlings grown in tall containers of sand, to study the influence of high concentrations of NH₄ ⁺ in the soil on the development of fine roots and the effects of nitrogen uptake on rhizosphere pH. At the end of this eight-month experiment part of the ammonium appeared to have nitrified into nitrate. High doses of ammonium negatively affected root length and root length per unit of dry matter (specific root length). Although Douglas fir shows a preferential ammonium uptake in nutrient solutions the increases in the pH of the rhizosphere in this experiment indicate that nitrogen was mostly taken up as nitrate. When the ammonium concentration in the soil is low, it cannot be taken up readily because of its low mobility in soil. Shoot growth was stimulated by high availability of nitrogen. The possible effects of high doses of ammonium on long-term forest vitality are discussed.     

Tree species effects on coupled cycles of carbon, nitrogen, and acidity in mineral soils at a common garden experiment

Forest biogeochemical cycles are shaped by effects of dominant tree species on soils, but the underlying mechanisms are not well understood. We investigated effects of temperate tree species on interactions among carbon (C), nitrogen (N), and acidity in mineral soils from an experiment with replicated monocultures of 14 tree species. To identify how trees affected these soil properties, we evaluated correlations among species-level characteristics (e.g. nutrient concentrations in leaf litter, wood, and roots), stand-level properties (e.g. nutrient fluxes through leaf litterfall, nutrient pools in stemwood), and components of soil C, N, and cation cycles. Total extractable acidity (acidity_tot) was correlated positively with mineral soil C stocks (R ²  = 0.72, P < 0.001), such that a nearly two-fold increase in acidity_tot was associated with a more than two-fold increase of organic C. We attribute this correlation to effects of tree species on soil acidification and subsequent mineral weathering reactions, which make hydrolyzing cations available for stabilization of soil organic matter. The effects of tree species on soil acidity were better understood by measuring multiple components of soil acidity, including pH, the abundance of hydrolyzing cations in soil solutions and on cation exchange sites, and acidity_tot. Soil pH and acidity_tot were correlated with proton-producing components of the soil N cycle (e.g. nitrification), which were positively correlated with species-level variability in fine root N concentrations. Soluble components of soil acidity, such as aluminum in saturated paste extracts, were more strongly related to plant traits associated with calcium cycling, including leaf and root calcium concentrations. Our results suggest conceptual models of plant impacts on soil biogeochemistry should be revised to account for underappreciated plant traits and biogeochemical processes.     

Nitrogen balance sheet of (NH4)2SO4-gypsum pellets and mixtures and ureaformaldehyde applied to corn in pots of sand

Summary A comparison of ammonium sulphate added to sand pots in different ways and ureaformaldehyde as sources of N to corn plants was carried out. The results showed that nitrogen utilization by plants from ammonium sulphategypsum pellets was greater than its utilization when ammonium sulphate was mixed with gypsum or when the pellets were ground or from ureaformaldehyde. The leached nitrogen from the pellets, ammonium sulphate applied in 3 portions and ureaformaldehyde was not significantly different and was lower than other ammonium sulphate treatments. The nitrogen remaining in pots fertilized by ureaformaldehyde was much greater than the corresponding amount in the case of all ammonium sulphate treatments. Gaseous loss of nitrogen took place in all nitrogen treatments with the loss from ammonium sulphate-gypsum pellets being the lowest.Incubation in sand of ureaformaldehyde, urea, and ammonium sulphate was carried out to understand better the growth conditions of corn fertilized by ureaformaldehyde. In the case of ureaformaldehyde- and urea-sand systems, the pH increased, NO₂ ^– accumulated and considerable loss of nitrogen took place. The pH, the NO₂ ^– accumulation and the loss of N tended to decrease with gypsum increments. re]19720801     

Allantoinase from nodules of pigeonpea (Cajanus cajan)

Allantoinase was purified about 10-fold from nitrogen fixing root nodules of pigeonpea (Cajanus cajan) using (NH₄)₂S0₄ fractionation and chromatography on Sephadex G-100. The purified preparation showed a specific activity of 1.73 nkat/mg protein, M_r of 125 000, pH optimum between 7.5 and 7.7 and K_m of 13.3 mM. The enzyme was heat stable up to 70dg and metal ions, except Hg²⁺, had no effect on the enzyme activity. The enzyme was inhibited significantly by reducing agents. Amino acids, ammonium, nitrate, potential precursors of allantoin and a number of other intermediate metabolites of ureide biosynthetic pathway had no effect on enzyme activity. It is suggested that allantoinase is unlikely to regulate the production of ureides in the nodule tissue.     

Phytohormones, Rhizobium Mutants, and Nodulation in Legumes : III. Auxin Metabolism in Effective and Ineffective Pea Root Nodules

High specific activity [³H]indole-3-acetic acid (IAA) was applied to the apical bud of intact pea (Pisum sativum L. cv Greenfeast) plants. Radioactivity was detected in all tissues after 24 hours. More radioactivity accumulated in the nodules than in the parent root on a fresh weight basis and more in effective (nitrogen-fixing) nodules than in ineffective nodules (which do not fix nitrogen).

For most samples, thin layer chromatography revealed major peaks of radioactivity at the R_F values of IAA and indole-3-acetylaspartic acid (IAAsp) and further evidence of the identity of these compounds was obtained by chromatography in other systems. Disintegrations per minute due to IAA per unit fresh weight were significantly greater for root than for nodule tissue, but were not significantly different for effective and ineffective nodules. Radioactivity due to IAAsp, expressed both on a percentage basis and per unit fresh weight, was significantly greater for nodule than for root tissue and significantly greater for the effective nodules than for the ineffective nodules. When [³H]IAA was applied to effective nodules, IAAsp was the dominant metabolite in the nodule. The data suggest that metabolism of auxins may be important for the persistence of a functional root nodule.

     

Nitrogen use efficiency (NUE) in rice links to NH4 + toxicity and futile NH4 + cycling in roots

Aims

Rice is known as an ammonium (NH₄ ⁺)-tolerant species. Nevertheless, rice can suffer NH₄ ⁺ toxicity, and excessive use of nitrogen (N) fertilizer has raised NH₄ ⁺ in many paddy soils to levels that reduce vegetative biomass and yield. Examining whether thresholds of NH₄ ⁺ toxicity in rice are related to nitrogen-use efficiency (NUE) is the aim of this study.

Methods

A high-NUE (Wuyunjing 23, W23) and a low-NUE (Guidan 4, GD) rice cultivar were cultivated hydroponically, and growth, root morphology, total N and NH₄ ⁺ concentration, root oxygen consumption, and transmembrane NH₄ ⁺ fluxes in the root meristem and elongation zones were determined.

Results

We show that W23 possesses greater capacity to resist NH₄ ⁺ toxicity, while GD is more susceptible. We furthermore show that tissue NH₄ ⁺ accumulation and futile NH₄ ⁺ cycling across the root-cell plasma membrane, previously linked to inhibited plant development under elevated NH₄ ⁺, are more pronounced in GD. NH₄ ⁺ efflux in the root elongation zone, measured by SIET, was nearly sevenfold greater in GD than in W23, and this was coupled to strongly stimulated root respiration. In both cultivars, root growth was affected more severely by high NH₄ ⁺ than shoot growth. High NH₄ ⁺ mainly inhibited the development of total root length and root area, while the formation of lateral roots was unaffected.

Conclusions

It is concluded that the larger degree of seedling growth inhibition in low- vs. high-NUE rice genotypes is associated with significantly enhanced NH₄ ⁺ cycling and tissue accumulation in the elongation zone of the root.     

A nitrogen-dependent switch in the high affinity ammonium transport in Medicago truncatula

Ammonium transporters (AMTs) are crucial for the high affinity primary uptake and translocation of ammonium in plants. In the model legume Medicago truncatula, the genomic set of AMT-type ammonium transporters comprises eight members. Only four genes were abundantly expressed in young seedlings, both in roots and shoots. While the expression of all AMTs in the shoot was not affected by the nitrogen availability, the dominating MtAMT1;1 gene was repressed by nitrogen in roots, despite that cellular nitrogen concentrations were far above deficiency levels. A contrasting de-repression by nitrogen was observed for MtAMT1;4 and MtAMT2;1, which were both expressed at intermediate level. Weak expression was found for MtAMT1;2 and MtAMT2;3, while the other AMTs were not detected in young seedlings. When expressed from their endogenous promoters, translational fusion proteins of MtAMT1;1 and MtAMT2;1 with green fluorescent protein were co-localized in the plasma membrane of rhizodermal cells, but also detected in cortical root layers. Both transporter proteins similarly functionally complemented a yeast strain that is deficient in high affinity ammonium transport, both at acidic and neutral pH. The uptake into yeast mediated by these transporters saturated with K_{m AMT1;1} = 89 µM and K_{m AMT2;1} = 123 µM, respectively. When expressed in oocytes, MtAMT1;1 mediated much larger ¹⁵N-ammonium uptake than MtAMT2;1, but NH₄ ⁺ currents were only recorded for MtAMT1;1. These currents saturated with a voltage-dependent K_m = 90 µM at ?80 mV. The cellular localization and regulation of the AMTs suggests that MtAMT1;1 encodes the major high affinity ammonium transporter gene in low nitrogen grown young M. truncatula roots and despite the similar localization and substrate affinity, MtAMT2;1 appears functionally distinct and more important at higher nitrogen supply.     

Rhizosphere pH along different root zones of Douglas-fir (Pseudotsuga menziesii), as affected by source of nitrogen

Douglas-fir, grown on strongly acid soil (pH-H₂O 3.87), was fertilized with ammonium, nitrate or ammonium nitrate as N-source. Rhizosphere pH along the root axis was measured with microelectrodes. Pure ammonium supply resulted in acidification of the rhizosphere, almost along the entire root axis; only the extreme root tip was slightly alkaline compared with the bulk soil. With nitrate supply, the alkalization of the rhizosphere at the root tip was considerably stronger than with ammonium supply and the length of the alkalization zone greater, extending over the entire growth zone of the root. Acidification of the rhizosphere along the older parts of the root was less pronounced in the case of nitrate. It is concluded that nitrate nutrition enables the plant to protect its most essential root zone from the adverse effects of strong acidity by locally raising the rhizosphere pH.     

Combined Effect and Mechanism of Acidity and Lead Ion on Soybean Biomass

Heavy metal pollution and soil acidification are serious global environmental issues. The combined pollution from acidification and heavy metal has become a new environmental issue in regions where the two issues simultaneously occur. However, studies on combined pollution are still limited. In the current study, we investigated the combined effect and mechanism of acidity and heavy metal [lead ion (Pb²⁺)] on soybean biomass as well as on growth, nitrogen nutrition, and antioxidant system in soybean roots. Results showed that the combined treatment with acidity and Pb²⁺ decreased the soybean biomass. At pH 4.5, the soybean biomass in the combined treatment with acidity and 0.9 mmol L^?1 Pb²⁺ was lower than that in the combined treatment with acidity and Pb²⁺ at 0.3 or 1.5 mmol L^?1. This result was also observed at pH 3.5 and 3.0. The combined treatment with acidity and Pb²⁺ also resulted in the following consequences: root growth inhibition; decrease in nitrate, ammonium, and malondialdehyde contents; increase in nitrite reductase activity; and decrease in peroxidase activity. The extent at which the test indexes decreased/increased in the combined treatment was higher than that in the single acidity treatment. The correlation analysis results indicated that the decrease in the soybean biomass in the combined treatment with acidity and Pb²⁺ resulted from the decrease in the root growth, nitrate–nitrogen assimilation, and peroxidase activity.     

Ammonium formation in cape timiris (Mauritania) upwelling

Ammonium formation was studied in two consecutive years by following a newly upwelled ammonium-free ‘parcel’ of water with a drogue. High concentrations (> 2 μg-at. NH₄-N l^?1) were found at the end of each study. The relative importance of WP-2 net mesozooplankton excretion on the ammonium values is greater before the phytoplanktonic bloom (35–48%) than during it (10–16%). From carbon grazing estimates, it is shown that other herbivore excretion does not take the place of WP-2 mesozooplankton and because of synchronous variations of chlorophyll with dissolved organic nitrogen, and bacterial activity with ammonium, it is suggested that much of the ammonium is produced from phytoplankton decay or nitrogen excretion during the bloom.     

Artemisinin production in Artemisia annua hairy root cultures with improved growth by altering the nitrogen source in the medium

Murashige & Skoog medium was modified for enhancing artemisinin production in Artemisia annua hairy root cultures by altering the ratio of NO ₃ ^– /NH ₄ ⁺ and the total amount of initial nitrogen. Increasing ammonium to 60 mM decreased both growth and artemisinin accumulation in hairy root cultures. With NO ₃ ^– /NH ₄ ⁺ at 5:1 (w/w), the optimum concentration of total initial nitrogen for artemisinin production was 20 mM. After 24 days of cultivation with 16.7 mM nitrate and 3.3 mM ammonium, the maximum artemisinin production of hairy roots was about 14 mg l^–1, a 57% increase over that in the standard MS medium.     

Potassium influx into maize root systems : influence of root potassium concentration and ambient ammonium

Potassium influx into roots of dark-grown decapitated maize seedling (Zea mays L., cv Pioneer 3369A) was examined in presence and absence of ambient ammonium and at various root potassium concentrations. Six-day old seedlings which were dependent on the endosperm reserves for their energy source were exposed to KCl (labeled with ⁸⁶Rb) ranging from 5 to 200 micromolar. At both low (13 micromoles per gram fresh weight) and high (100 micromoles per gram fresh weight) root potassium concentration, isotherms indicated two potassium influx systems, one approaching saturation at 50 to 100 micromolar potassium and an additional one tentatively considered to be linear. A mixed-type inhibition by ammonium for the low-concentration saturable system was indicated by a concomitant decrease in V_max and increase in K_m. High root potassium concentration decreased V_max but had little effect on K_m of this system. The rate constant for the second quasilinear system was decreased by ambient ammonium and by high root potassium status. Transfer of high potassium roots to potassium-free solutions resulted in an increase in influx within 2 hours; by 24 hours influx significantly exceeded that of roots not previously exposed to potassium. In roots of both low and high root potassium concentrations, potassium influx was restricted progressively as ambient ammonium increased to about 100 micromolar, but there was little further inhibition as ammonium concentrations increased beyond that to 500 micromolar. The data imply that potassium influx has two components, one subject to inhibition by ambient ammonium and one relatively resistant.     

Anatomy and photosynthetic parameters of roots and leaves of two shade-adapted orchids, Dichaea cogniauxiana Shltr. and Epidendrum secundum Jacq.

This study compares photosynthetic and structural features of Dichaea cogniauxiana and Epidendrum secundum leaves and roots. The diurnal titratable acidity fluctuations indicated crassulacean acid metabolism (CAM) in E. secundum leaves, associated with anatomical features like thick cuticle, large and vacuolated cells, and reduced stomata size and frequency. Roots of both species had chloroplasts in their cortical parenchyma. However, neither the roots nor D. cogniauxiana leaves did show tissue sap acidity fluctuations. This indicates C₃ metabolism in these organs. This lack of oscillation of organic acids in Epidendrum roots was at odds with a CAM-like ¹³C ratio, suggesting that in spite of active CO₂ fixation in roots during the day, the bulk of carbon is imported from the leaves. Roots of both species showed Fv/Fm, ΔF/Fm′, ETR values similar to reports from other non-foliar photosynthetic organs. Besides reducing root carbon cost, root photosynthesis may also be important by alleviating potential hypoxia, since water-saturated velamen severely impedes the gas exchange between radicular cortex.     

Transport of nitrogen and zinc to rhodes grass by arbuscular mycorrhiza and roots as affected by different nitrogen sources (NH<Subscript>4</Subscript><Superscript>+</Superscript>-N and NO<Subscript>3</Subscript><Superscript>−</Superscript>-N)

We investigated the effect of mineral nitrogen forms on transfer of nitrogen (N) and zinc (Zn) from attached compartments to rhodes grass (Chloris gayana) colonised with arbuscular mycorrhizal fungi (AMF). After being pre-cultivated in substrates with adequate nutrient supply and either AMF inoculated (+AM) or left non-inoculated (?AM), rhodes grass was positioned adjacent to an outer compartment holding a similar substrate but applied with labelled nitrogen (¹⁵N) either as ammonium (NH₄ ⁺) or nitrate (NO₃ ^?), and a high supply of Zn (150 mg kg^?1 DS). Plant roots together with fungal mycelium were either allowed to explore the outer compartment (with root access) or only mycorrhizal hyphae were allowed (without root access). Within each access treatment, biomasses of rhodes grass were not significantly affected by AMF inoculation or N form. AMF contribution to plant ¹⁵N uptake was about double in NH₄ ⁺ compared with NO₃ ^?-supplied treatments while the mycorrhizal influence on plant Zn uptake was insignificant. Without root access, the shoot ¹⁵N/Zn concentration ratio was up to ten-fold higher in +AM than –AM treatments and this ratio increase was clearly more pronounced in NH₄ ⁺ than NO₃ ^?-supplied treatments. In conclusion, rhodes grass in symbiosis with the tested AMF acquired more N when supplied with ammonium. Moreover, there is clear indication that although the AMF have transported both nutrients (N and Zn), N was preferentially transferred as compared to Zn. We confirmed that, while rhodes grass is not able to prevent excessive Zn uptake via roots under conditions of high Zn, mycorrhiza is able to avoid excessive Zn supply to the host plant when the fungus alone has access to contaminated patches.     

Elevated CO2 and aboveground–belowground herbivory by the clover root weevil

Predicted increases in atmospheric carbon dioxide (CO₂) concentrations are expected to increase primary productivity in many terrestrial ecosystems, which could lead to plants becoming N limited. Studies suggest that legumes may partially overcome this by increasing biological nitrogen fixation. However, these studies have not yet considered how these changes may be affected by the altered dynamics of insect herbivores feeding on the plant. This study investigated how elevated CO₂ (700 μl l^?1) affected the clover root weevil (Sitona lepidus), a significant pest of white clover (Trifolium repens). Adults feed on leaves aboveground where they lay eggs; soil-dwelling larvae initially feed on root nodules that house N₂-fixing bacteria. Foliar C:N ratios rose by 9% at elevated CO₂, but the biggest responses were observed belowground, with increases in root mass (85% greater) and nodule abundance (220% more abundant). Root C:N ratios increased significantly from 10.95 to 11.60 under elevated CO₂, which increased even further to 13.13 when nodules were attacked by larval S. lepidus. Adult S. lepidus consumed significantly more leaf tissue at elevated CO₂ (0.47 cm² day^?1) compared with ambient CO₂ (0.35 cm² day^?1), suggesting compensatory feeding, but laid 23% fewer eggs at elevated CO₂. Even though fewer eggs were laid at elevated CO₂, 38% more larvae were recovered suggesting that larval survival was much better under elevated CO₂. Increased larval abundance and performance at elevated CO₂ were positively correlated with the number of nodules available. In conclusion, reduced foliar quality at elevated CO₂ was generally disadvantageous for adult S. lepidus living aboveground, but extremely beneficial for S. lepidus larvae living belowground, due to the enhanced nodulation. Climate change may, therefore, enhance biological nitrogen fixation by T. repens, but potential benefits (e.g. provision of N without chemical fertilizers) may be undermined by larger populations of S. lepidus larvae belowground.     

Root respiration associated with ammonium and nitrate absorption and assimilation by barley

We examined nitrate assimilation and root gas fluxes in a wild-type barley (Hordeum vulgare L. cv Steptoe), a mutant (nar1a) deficient in NADH nitrate reductase, and a mutant (nar1a;nar7w) deficient in both NADH and NAD(P)H nitrate reductases. Estimates of in vivo nitrate assimilation from excised roots and whole plants indicated that the nar1a mutation influences assimilation only in the shoot and that exposure to NO₃⁻ induced shoot nitrate reduction more slowly than root nitrate reduction in all three genotypes. When plants that had been deprived of nitrogen for several days were exposed to ammonium, root carbon dioxide evolution and oxygen consumption increased markedly, but respiratory quotient—the ratio of carbon dioxide evolved to oxygen consumed—did not change. A shift from ammonium to nitrate nutrition stimulated root carbon dioxide evolution slightly and inhibited oxygen consumption in the wild type and nar1a mutant, but had negligible effects on root gas fluxes in the nar1a;nar7w mutant. These results indicate that, under NH₄⁺ nutrition, 14% of root carbon catabolism is coupled to NH₄⁺ absorption and assimilation and that, under NO₃⁻ nutrition, 5% of root carbon catabolism is coupled to NO₃⁻ absorption, 15% to NO₃⁻ assimilation, and 3% to NH₄⁺ assimilation. The additional energy requirements of NO₃⁻ assimilation appear to diminish root mitochondrial electron transport. Thus, the energy requirements of NH₄⁺ and NO₃⁻ absorption and assimilation constitute a significant portion of root respiration.