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

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

Molecular identification and functional characterization of GhAMT1.3 in ammonium transport with a high affinity from cotton (Gossypium hirsutum L.)

Ammonium (NH₄⁺) represents a primary nitrogen source for many plants, its effective transport into and between tissues and further assimilation in cells determine greatly plant nitrogen use efficiency. However, biological components involved in NH₄⁺ movement in woody plants are unclear. Here, we report kinetic evidence for cotton NH₄⁺ uptake and molecular identification of certain NH₄⁺ transporters (AMTs) from cotton (Gossypium hirustum). A substrate‐influx assay using ¹⁵N‐isotope revealed that cotton possessed a high‐affinity transport system with a K_m of 58 μM for NH₄⁺. Sequence analysis showed that GhAMT1.1–1.3 encoded respectively a membrane protein containing 485, 509 or 499 amino acids. Heterologous functionality test demonstrated that GhAMT1.1–1.3 expression mediated NH₄⁺ permeation across the plasma membrane (PM) of yeast and/or Arabidopsis qko‐mutant cells, allowing a growth restoration of both mutants on NH₄⁺. Quantitative PCR measurement showed that GhAMT1.3 was expressed in roots and leaves and markedly up‐regulated by N‐starvation, repressed by NH₄⁺ resupply and regulated diurnally and age‐dependently, suggesting that GhAMT1.3 should be a N‐responsive gene. Importantly, GhAMT1.3 expression in Arabidopsis improved plant growth on NH₄⁺ and enhanced total nitrogen accumulation (～50% more), conforming with the observation of 2‐fold more NH₄⁺ absorption by GhAMT1.3‐transformed qko plant roots during a 1‐h root influx period. Together with its targeting to the PM and saturated transport kinetics with a K_m of 72 μM for NH₄⁺, GhAMT1.3 is suggested to be a high‐affinity NH₄⁺ permease that may play a significant role in cotton NH₄⁺ acquisition and utilization, adding a new member in the plant AMT family.     

Elevated CO2 plus chronic warming reduce nitrogen uptake and levels or activities of nitrogen‐uptake and ‐assimilatory proteins in tomato roots

Atmospheric CO₂ enrichment is expected to often benefit plant growth, despite causing global warming and nitrogen (N) dilution in plants. Most plants primarily procure N as inorganic nitrate (NO₃^?) or ammonium (NH₄⁺), using membrane‐localized transport proteins in roots, which are key targets for improving N use. Although interactive effects of elevated CO₂, chronic warming and N form on N relations are expected, these have not been studied. In this study, tomato (Solanum lycopersicum) plants were grown at two levels of CO₂ (400 or 700 ppm) and two temperature regimes (30 or 37°C), with NO₃^? or NH₄⁺ as the N source. Elevated CO₂ plus chronic warming severely inhibited plant growth, regardless of N form, while individually they had smaller effects on growth. Although %N in roots was similar among all treatments, elevated CO₂ plus warming decreased (1) N‐uptake rate by roots, (2) total protein concentration in roots, indicating an inhibition of N assimilation and (3) shoot %N, indicating a potential inhibition of N translocation from roots to shoots. Under elevated CO₂ plus warming, reduced NO₃^?‐uptake rate per g root was correlated with a decrease in the concentration of NO₃^?‐uptake proteins per g root, reduced NH₄⁺ uptake was correlated with decreased activity of NH₄⁺‐uptake proteins and reduced N assimilation was correlated with decreased concentration of N‐assimilatory proteins. These results indicate that elevated CO₂ and chronic warming can act synergistically to decrease plant N uptake and assimilation; hence, future global warming may decrease both plant growth and food quality (%N).     

Uptake, metabolism and distribution of nitrogen in crop plants traced by enriched and natural 15N: Progress over the last 30 years

The major findings of many years of research into plant N cycling are summarised in this review, firstly as revealed by ¹⁵N-enriched methods and secondly, in relation to natural ¹⁵N abundance (δ¹⁵N) in plants and their metabolites. This work has mainly been done in an agricultural context. As many groups especially attempt to relate δ¹⁵N to N cycling, atmospheric N deposition and the interactions of N with carbon budgets, we deem it useful to synthesize these major findings. Primary assimilation and distribution of N within plants were investigated from the ¹⁵N enrichment in individual plant organs and in individual amino acids after feeding them ¹⁵N-labelled compounds. In both roots and leaves, NH₄ ⁺ and NO₃ ^? were assimilated into amino acids, largely by a combination of glutamine synthetase (GS) and glutamate synthase (GOGAT). In the leaves, the transfer of glutamine (amide) N to glutamic acid was accelerated in the light, and amino N in some amino acids was deaminated to ammonia in the dark, followed by its incorporation into glutamine. The N in the growing parts such as growing leaves, filling grains and growing root parts were from two sources: re-allocation (phloem supply) of reserved N (amino acids), and currently-absorbed N. The metabolites from the mature parts may perform the roles of substrates for plant growth and signals for gene expression. δ¹⁵N values, measured for plants/soils and plant metabolites (inorganic N, amino acids, polyamines) were related with the acquisition, metabolism and distribution of N in plants. Small ¹⁵N/¹⁴N fractionation in the acquisition of N₂ and NO₃ ^? and large ¹⁵N/¹⁴N fractionation in NH₄ ⁺ uptake were found. The δ¹⁵N values of whole shoots or grains from field-grown crops were largely reflected major sources of N. In some legumes, ¹⁵N was enriched in their nodules and an extremely ¹⁵N-enriched compound was homospermidine. Nitrate reduction to ammonia (NR) and ammonia assimilation to glutamine (GS) showed large ¹⁵N/¹⁴N fractionations. Specific attention was paid to the δ¹⁵N values in xylem and phloem exudates compared to those of plant organs.     

Belowground competition and the response of developing forest communities to atmospheric CO2 and O3

As human activity continues to increase CO₂ and O₃, broad expanses of north temperate forests will be simultaneously exposed to elevated concentrations of these trace gases. Although both CO₂ and O₃ are potent modifiers of plant growth, we do not understand the extent to which they alter competition for limiting soil nutrients, like nitrogen (N). We quantified the acquisition of soil N in two 8‐year‐old communities composed of trembling aspen genotypes (n= 5) and trembling aspen–paper birch which were exposed to factorial combinations of CO₂ (ambient and 560 μL L⁻¹) and O₃ (ambient = 30–40 vs. 50–60 nL L⁻¹). Tracer amount of ¹⁵NH₄⁺ were applied to soil to determine how these trace gases altered the competitive ability of genotypes and species to acquire soil N. One year after isotope addition, we assessed N acquisition by measuring the amount of ¹⁵N tracer contained in the plant canopy (i.e. recent N acquisition), as well as the total amount of canopy N (i.e. cumulative N acquisition). Exposure to elevated CO₂ differentially altered recent and cumulative N acquisition among aspen genotypes, changing the rank order in which they obtained soil N. Elevated O₃ also altered the rank order in which aspen genotypes obtained soil N by eliciting increases, decreases and no response among genotypes. If aspen genotypes respond similarly under field conditions, then rising concentrations of CO₂ and O₃ could alter the structure of aspen populations. In the aspen–birch community, elevated CO₂ increased recent N (i.e. ¹⁵N) acquisition in birch (68%) to a greater extent than aspen (19%), suggesting that, over the course of this experiment, birch had gained a competitive advantage over aspen. The response of genotypes and species to rising CO₂ and O₃ concentrations, and how these responses are modified by competitive interactions, has the potential to change the future composition and productivity of northern temperate forests.     

Is nitrogen transfer among plants enhanced by contrasting nutrient‐acquisition strategies?

Nitrogen (N) transfer among plants has been found where at least one plant can fix N₂. In nutrient‐poor soils, where plants with contrasting nutrient‐acquisition strategies (without N₂ fixation) co‐occur, it is unclear if N transfer exists and what promotes it. A novel multi‐species microcosm pot experiment was conducted to quantify N transfer between arbuscular mycorrhizal (AM), ectomycorrhizal (EM), dual AM/EM, and non‐mycorrhizal cluster‐rooted plants in nutrient‐poor soils with mycorrhizal mesh barriers. We foliar‐fed plants with a K¹⁵NO₃ solution to quantify one‐way N transfer from ‘donor’ to ‘receiver’ plants. We also quantified mycorrhizal colonization and root intermingling. Transfer of N between plants with contrasting nutrient‐acquisition strategies occurred at both low and high soil nutrient levels with or without root intermingling. The magnitude of N transfer was relatively high (representing 4% of donor plant N) given the lack of N₂ fixation. Receiver plants forming ectomycorrhizas or cluster roots were more enriched compared with AM‐only plants. We demonstrate N transfer between plants of contrasting nutrient‐acquisition strategies, and a preferential enrichment of cluster‐rooted and EM plants compared with AM plants. Nutrient exchanges among plants are potentially important in promoting plant coexistence in nutrient‐poor soils.     

Changes in the C/N balance caused by increasing external ammonium concentrations are driven by carbon and energy availabilities during ammonium nutrition in pea plants: the key roles of asparagine synthetase and anaplerotic enzymes

An understanding of the mechanisms underlying ammonium (NH₄⁺) toxicity in plants requires prior knowledge of the metabolic uses for nitrogen (N) and carbon (C). We have recently shown that pea plants grown at high NH₄⁺ concentrations suffer an energy deficiency associated with a disruption of ionic homeostasis. Furthermore, these plants are unable to adequately regulate internal NH₄⁺ levels and the cell‐charge balance associated with cation uptake. Herein we show a role for an extra‐C application in the regulation of C–N metabolism in NH₄⁺‐fed plants. Thus, pea plants (Pisum sativum) were grown at a range of NH₄⁺ concentrations as sole N source, and two light intensities were applied to vary the C supply to the plants. Control plants grown at high NH₄⁺ concentration triggered a toxicity response with the characteristic pattern of C‐starvation conditions. This toxicity response resulted in the redistribution of N from amino acids, mostly asparagine, and lower C/N ratios. The C/N imbalance at high NH₄⁺ concentration under control conditions induced a strong activation of root C metabolism and the upregulation of anaplerotic enzymes to provide C intermediates for the tricarboxylic acid cycle. A high light intensity partially reverted these C‐starvation symptoms by providing higher C availability to the plants. The extra‐C contributed to a lower C4/C5 amino acid ratio while maintaining the relative contents of some minor amino acids involved in key pathways regulating the C/N status of the plants unchanged. C availability can therefore be considered to be a determinant factor in the tolerance/sensitivity mechanisms to NH₄⁺ nutrition in plants.     

A critical experimental evaluation of methods for determination of NH4+ in plant tissue, xylem sap and apoplastic fluid

Ammonium (NH₄⁺) is a central intermediate in the N metabolism of plants, but the quantitative importance of NH₄⁺ in transporting N from root to shoot and the capability of plants to store NH₄⁺ in leaves are still matters of substantial controversy. This paper shows that some of these controversies have to be related to the use of inadequate analytical procedures used for extraction and quantification of NH₄⁺ in plants. The most frequently used methods for determination of NH₄⁺, viz. colorimetric methods based on the classical Berthelot reaction, suffered severely from interference caused by amino acids, amines, amides and proteins. For some of these metabolites the interference was positive, while for others it was negative, making correction impossible. Consequently, colorimetric analysis is inapplicable for determination of NH₄⁺ in plants. Results obtained by ion chromatography may overestimate the NH₄⁺ concentration due to co‐elution of NH₄⁺ with amines like methylamine, ethylamine, ethanolamine and the non‐protein amino acid Γ‐aminobutyric acid. Derivatization of NH₄⁺ with o‐phthalaldehyde at alkaline pH and subsequent quantification of NH₄⁺ by fluorescence spectroscopy was also associated with interference. However, when pH was lowered to 6.8 during derivatization and 2‐mercaptoethanol was used as reductant, NH₄⁺ could be determined with a high selectivity and sensitivity down to a detection limit of 3.3 μM in a 10‐μl sample volume. Derivatization was performed on‐line using a column‐less HPLC system, enabling rapid quantification of NH₄⁺ in a few minutes. Flow injection analysis with on‐line gas dialysis was, likewise, free from interference, except when applied on highly senescent plant material containing volatile amines. Labile N metabolites in leaf tissue extract, xylem sap and apoplastic fluid were degraded to NH₄⁺ during extraction and subsequent instrumental analysis if the samples were not stabilised. A simple and efficient stabilisation could be obtained by addition of 10 mM ice‐cold HCOOH to the plant extraction medium or to the samples of apoplastic fluid or xylem sap. We conclude that significant concentrations of NH₄⁺, exceeding 1 mM, may occur in xylem sap, leaf apoplastic fluid and leaf tissue water of nitrate‐grown tomato and oilseed rape plants. The measured NH₄⁺ concentrations were not a result of excessive N supplies, as even plants grown under mildly N‐deficient conditions contained NH₄⁺.     

Optimization of ammonium acquisition and metabolism by potassium in rice (Oryza sativa L. cv. IR-72)

We present the first characterization of K⁺ optimization of N uptake and metabolism in an NH₄⁺‐tolerant species, tropical lowland rice (cv. IR‐72). ¹³N radiotracing showed that increased K⁺ supply reduces futile NH₄⁺ cycling at the plasma membrane, diminishing the excessive rates of both unidirectional influx and efflux. Pharmacological testing showed that low‐affinity NH₄⁺ influx may be mediated by both K⁺ and non‐selective cation channels. Suppression of NH₄⁺ influx by K⁺ occurred within minutes of increasing K⁺ supply. Increased K⁺ reduced free [NH₄⁺] in roots and shoots by 50–75%. Plant biomass was maximized on 10 mm NH₄⁺ and 5 mm K⁺, with growth 160% higher than 10 mm NO₃^‐‐grown plants, and 220% higher than plants grown at 10 mm NH₄⁺ and 0.1 mm K⁺. Unlike in NH₄⁺‐sensitive barley, growth optimization was not attributed to a reduced energy cost of futile NH₄⁺ cycling at the plasma membrane. Activities of the key enzymes glutamine synthetase and phosphoenolpyruvate carboxylase (PEPC) were strongly stimulated by elevated K⁺, mirroring plant growth and protein content. Improved plant performance through optimization of K⁺ and NH₄⁺ is likely to be of substantial agronomic significance in the world's foremost crop species.     

Plant and microbial N acquisition under elevated atmospheric CO2 in two mesocosm experiments with annual grasses

The impact of elevated CO₂ on terrestrial ecosystem C balance, both in sign or magnitude, is not clear because the resulting alterations in C input, plant nutrient demand and water use efficiency often have contrasting impacts on microbial decomposition processes. One major source of uncertainty stems from the impact of elevated CO₂ on N availability to plants and microbes. We examined the effects of atmospheric CO₂ enrichment (ambient+370 μmol mol^?1) on plant and microbial N acquisition in two different mesocosm experiments, using model plant species of annual grasses of Avena barbata and A. fatua, respectively. The A. barbata experiment was conducted in a N‐poor sandy loam and the A. fatua experiment was on a N‐rich clayey loam. Plant–microbial N partitioning was examined through determining the distribution of a ¹⁵N tracer. In the A. barbata experiment, ¹⁵N tracer was introduced to a field labeling experiment in the previous year so that ¹⁵N predominantly existed in nonextractable soil pools. In the A. fatua experiment, ¹⁵N was introduced in a mineral solution [(¹⁵NH₄)₂SO₄ solution] during the growing season of A. fatua. Results of both N budget and ¹⁵N tracer analyses indicated that elevated CO₂ increased plant N acquisition from the soil. In the A. barbata experiment, elevated CO₂ increased plant biomass N by ca. 10% but there was no corresponding decrease in soil extractable N, suggesting that plants might have obtained N from the nonextractable organic N pool because of enhanced microbial activity. In the A. fatua experiment, however, the CO₂‐led increase in plant biomass N was statistically equal to the reduction in soil extractable N. Although atmospheric CO₂ enrichment enhanced microbial biomass C under A. barbata or microbial activity (respiration) under A. fatua, it had no significant effect on microbial biomass N in either experiment. Elevated CO₂ increased the colonization of A. fatua roots by arbuscular mycorrhizal fungi, which coincided with the enhancement of plant competitiveness for soluble soil N. Together, these results suggest that elevated CO₂ may tighten N cycling through facilitating plant N acquisition. However, it is unknown to what degree results from these short‐term microcosm experiments can be extrapolated to field conditions. Long‐term studies in less‐disturbed soils are needed to determine whether CO₂‐enhancement of plant N acquisition can significantly relieve N limitation over plant growth in an elevated CO₂ environment.     

Ingestion and excretion of nitrogen by larvae of a cabbage armyworm: the effects of fertilizer application

• 1 Insect frass has significant impacts on decomposition and soil nitrogen dynamics. Although the frass contains various forms of nitrogen that may differently influence nitrogen dynamics in the decomposition process, how the nitrogen form in the insect frass is influenced by host plant quality remains poorly understood.
• 2 The present study examined the effects of application of fertilizer on leaf quality of Brassica rapa L. var. perviridis Bailey (Brassicaceae), and on the consumption, frass excretion and frass quality of its insect pest Mamestra brassicae (L.) (Lepidoptera: Noctuidae), with a particular focus on the dynamics of inorganic nitrogen.
• 3 Brassica rapa increased total nitrogen concentration, and accumulated inorganic nitrogen [i.e. leaf nitrate‐nitrogen (NO₃^?‐N) and ammonium‐nitrogen (NH₄⁺‐N)] in the leaves in response to the application of fertilizer.
• 4 Although leaf consumption and frass excreted by M. brassicae was not affected by fertilizer treatment, frass quality was influenced by host plant quality as altered by fertilizer applications. Frass contained high concentrations of total nitrogen, NO₃^?‐N, and NH₄⁺‐N under high fertilizer treatment. In particular, the larvae excreted much more NH₄⁺‐N than ingested. The relationship between host plant quality and insect frass quality, as well as the potential implications for decomposition and nutrient dynamics, are discussed.

     

H+‐pyrophosphatase from Salicornia europaea confers tolerance to simultaneously occurring salt stress and nitrogen deficiency in Arabidopsis and wheat

High salinity and nitrogen (N) deficiency in soil are two key factors limiting crop productivity, and they usually occur simultaneously. Here we firstly found that H⁺‐PPase is involved in salt‐stimulated NO₃^? uptake in the euhalophyte Salicornia europaea. Then, two genes (named SeVP1 and SeVP2) encoding H⁺‐PPase from S. europaea were characterized. The expression of SeVP1 and SeVP2 was induced by salt stress and N starvation. Both SeVP1 or SeVP2 transgenic Arabidopsis and wheat plants outperformed the wild types (WTs) when high salt and low N occur simultaneously. The transgenic Arabidopsis plants maintained higher K⁺/Na⁺ ratio in leaves and exhibited increased NO₃^? uptake, inorganic pyrophosphate‐dependent vacuolar nitrate efflux and assimilation capacity under this double stresses. Furthermore, they had more soluble sugars in shoots and roots and less starch accumulation in shoots than WT. These performances can be explained by the up‐regulated expression of ion, nitrate and sugar transporter genes in transgenic plants. Taken together, our results suggest that up‐regulation of H⁺‐PPase favours the transport of photosynthates to root, which could promote root growth and integrate N and carbon metabolism in plant. This work provides potential strategies for improving crop yields challenged by increasing soil salinization and shrinking farmland.     

Constraints to nitrogen acquisition of terrestrial plants under elevated CO2

A key part of the uncertainty in terrestrial feedbacks on climate change is related to how and to what extent nitrogen (N) availability constrains the stimulation of terrestrial productivity by elevated CO₂ (eCO₂), and whether or not this constraint will become stronger over time. We explored the ecosystem‐scale relationship between responses of plant productivity and N acquisition to eCO₂ in free‐air CO₂ enrichment (FACE) experiments in grassland, cropland and forest ecosystems and found that: (i) in all three ecosystem types, this relationship was positive, linear and strong (r² = 0.68), but exhibited a negative intercept such that plant N acquisition was decreased by 10% when eCO₂ caused neutral or modest changes in productivity. As the ecosystems were markedly N limited, plants with minimal productivity responses to eCO₂ likely acquired less N than ambient CO₂‐grown counterparts because access was decreased, and not because demand was lower. (ii) Plant N concentration was lower under eCO₂, and this decrease was independent of the presence or magnitude of eCO₂‐induced productivity enhancement, refuting the long‐held hypothesis that this effect results from growth dilution. (iii) Effects of eCO₂ on productivity and N acquisition did not diminish over time, while the typical eCO₂‐induced decrease in plant N concentration did. Our results suggest that, at the decennial timescale covered by FACE studies, N limitation of eCO₂‐induced terrestrial productivity enhancement is associated with negative effects of eCO₂ on plant N acquisition rather than with growth dilution of plant N or processes leading to progressive N limitation.     

Responses to K deficiency and waterlogging interact via respiratory and nitrogen metabolism

K deficiency and waterlogging are common stresses that can occur simultaneously and impact on crop development and yield. They are both known to affect catabolism, with rather opposite effects: inhibition of glycolysis and higher glycolytic fermentative flux, respectively. But surprisingly, the effect of their combination on plant metabolism has never been examined precisely. Here, we applied a combined treatment (K availability and waterlogging) to sunflower (Helianthus annuus L.) plants under controlled greenhouse conditions and performed elemental quantitation, metabolomics, and isotope analyses at different sampling times. Whereas separate K deficiency and waterlogging caused well‐known effects such as polyamines production and sugar accumulation, respectively, waterlogging altered K‐induced respiration enhancement (via the C₅‐branched acid pathway) and polyamine production, and K deficiency tended to suppress waterlogging‐induced accumulation of Krebs cycle intermediates in leaves. Furthermore, the natural ¹⁵N/¹⁴N isotope composition (δ¹⁵N) in leaf compounds shows that there was a change in nitrate circulation, with less nitrate influx to leaves under low K availablity combined with waterlogging and more isotopic dilution of lamina nitrates under high K. Our results show that K deficiency and waterlogging effects are not simply additive, reshape respiration as well as nitrogen metabolism and partitioning, and are associated with metabolomic and isotopic biomarkers of potential interest for crop monitoring.     

Plant nitrogen acquisition and interactions under elevated carbon dioxide: impact of endophytes and mycorrhizae

Both endophytic and mycorrhizal fungi interact with plants to form symbiosis in which the fungal partners rely on, and sometimes compete for, carbon (C) sources from their hosts. Changes in photosynthesis in host plants caused by atmospheric carbon dioxide (CO₂) enrichment may, therefore, influence those mutualistic interactions, potentially modifying plant nutrient acquisition and interactions with other coexisting plant species. However, few studies have so far examined the interactive controls of endophytes and mycorrhizae over plant responses to atmospheric CO₂ enrichment. Using Festuca arundinacea Schreb and Plantago lanceolata L. as model plants, we examined the effects of elevated CO₂ on mycorrhizae and endophyte (Neotyphodium coenophialum) and plant nitrogen (N) acquisition in two microcosm experiments, and determined whether and how mycorrhizae and endophytes mediate interactions between their host plant species. Endophyte‐free and endophyte‐infected F. arundinacea varieties, P. lanceolata L., and their combination with or without mycorrhizal inocula were grown under ambient (400 μmol mol⁻¹) and elevated CO₂ (ambient + 330 μmol mol⁻¹). A ¹⁵N isotope tracer was used to quantify the mycorrhiza‐mediated plant acquisition of N from soil. Elevated CO₂ stimulated the growth of P. lanceolata greater than F. arundinacea, increasing the shoot biomass ratio of P. lanceolata to F. arundinacea in all the mixtures. Elevated CO₂ also increased mycorrhizal root colonization of P. lanceolata, but had no impact on that of F. arundinacea. Mycorrhizae increased the shoot biomass ratio of P. lanceolata to F. arundinacea under elevated CO₂. In the absence of endophytes, both elevated CO₂ and mycorrhizae enhanced ¹⁵N and total N uptake of P. lanceolata but had either no or even negative effects on N acquisition of F. arundinacea, altering N distribution between these two species in the mixture. The presence of endophytes in F. arundinacea, however, reduced the CO₂ effect on N acquisition in P. lanceolata, although it did not affect growth responses of their host plants to elevated CO₂. These results suggest that mycorrhizal fungi and endophytes might interactively affect the responses of their host plants and their coexisting species to elevated CO₂.     

Soil nitrogen transformations under Populus tremuloides, Betula papyrifera and Acer saccharum following 3 years exposure to elevated CO2 and O3

Increases in atmospheric CO₂ and tropospheric O₃ may affect forest N cycling by altering plant litter production and the availability of substrates for microbial metabolism. Three years following the establishment of our free‐air CO₂–O₃ enrichment experiment, plant growth has been stimulated by elevated CO₂ resulting in greater substrate input to soil; elevated O₃ has counteracted this effect. We hypothesized that rates of soil N cycling would be enhanced by greater plant productivity under elevated CO₂, and that CO₂ effects would be dampened by O₃. We found that elevated CO₂ did not alter gross N transformation rates. Elevated O₃ significantly reduced gross N mineralization and microbial biomass N, and effects were consistent among species. We also observed significant interactions between CO₂ and O₃: (i) gross N mineralization was greater under elevated CO₂ (1.0 mg N kg^?1 day^?1) than in the presence of both CO₂ and O₃ (0.5 mg N kg^?1 day^?1) and (ii) gross NH₄⁺ immobilization was also greater under elevated CO₂ (0.8 mg N kg^?1 day^?1) than under CO₂ plus O₃ (0.4 mg N kg^?1 day^?1). We used a laboratory ¹⁵N tracer method to quantify transfer of inorganic N to organic pools. Elevated CO₂ led to greater recovery of NH₄⁺‐¹⁵N in microbial biomass and corresponding lower recovery in the extractable NO₃^? pool. Elevated CO₂ resulted in a substantial increase in NO₃^?‐¹⁵N recovery in soil organic matter. We observed no O₃ main effect and no CO₂ by O₃ interaction effect on ¹⁵N recovery in any soil pool. All of the above responses were most pronounced beneath Betula papyrifera and Populus tremuloides, which have grown more rapidly than Acer saccharum. Although elevated CO₂ has increased plant productivity, the resulting increase in plant litter production has yet to overcome the influence of the pre‐existing pool of soil organic matter on soil microbial activity and rates of N cycling. Ozone reduces plant litter inputs and also appears to affect the composition of plant litter in a way that reduces microbial biomass and activity.     

Interactions between K+ and NH4+: effects on ion uptake by rice roots

Interactive effects of K⁺ and N (principally NH₄⁺) on plant growth and ion uptake were investigated using hydroponically grown rice (Oryza sativa L. cv. M202) seedlings by varying the availability of NH₄⁺ or NO₃^? and K⁺ during an 18d growth period, a 3d pretreatment period and during flux measurements. Plants grew best in media containing 100 mmol m^?3 NH₄⁺ and 200mmolm^?3 K⁺ (N100/K200), followed by N2/K200 < N100/K2 < N2/K2. ⁸⁶Rb⁺(K⁺) fluxes were increased by exposure to N during the 18 d growth period and the 3 d of pretreatment, but decreased by the presence of NH₄⁺ during flux measurements. This inhibition was a function of prior N/K provision and the [NH₄⁺]₀ present during flux determinations. NH₄⁺ was least inhibitory to 86Rb⁺(K⁺) influx in high-N/low-K plants. Pretreatments with K⁺ failed to stimulate NH₄⁺ uptake, and the presence of K⁺ in the uptake solutions reduced NH₄⁺ fluxes only in high-N/low-K plants.     

Nutrient uptake in mycorrhizal symbiosis

The role of mycorrhizal fungi in acquisition of mineral nutrients by host plants is examined for three groups of mycorrhizas. These are; the ectomycorrhizas (ECM), the ericoid mycorrhizas (EM), and the vesicular-arbuscular mycorrhizas (VAM). Mycorrhizal infection may affect the mineral nutrition of the host plant directly by enhancing plant growth through nutrient acquisition by the fungus, or indirectly by modifying transpiration rates and the composition of rhizosphere microflora. A capacity for the external hyphae to take up and deliver nutrients to the plant has been demonstrated for the following nutrients and mycorrhizas; P (VAM, EM, ECM), NH₄ ⁺ (VAM, EM, ECM), NO₃ ^- (ECM), K (VAM, ECM), Ca (VAM, EM), SO₄ ^2- (VAM), Cu (VAM), Zn (VAM) and Fe (EM). In experimental chambers, the external hyphae of VAM can deliver up to 80% of plant P, 25% of plant N, 10% of plant K, 25% of plant Zn and 60% of plant Cu. Knowledge of the role of mycorrhiza in the uptake of nutrients other than P and N is limited because definitive studies are few, especially for the ECM. Although further quantification is required, it is feasible that the external hyphae may provide a significant delivery system for N, K, Cu and Zn in addition to P in many soils. Proposals that ECM and VAM fungi contribute substantially to the Mg, B and Fe nutrition of the host plant have not been substantiated. ECM and EM fungi produce ectoenzymes which provide host plants with the potential to access organic N and P forms that are normally unavailable to VAM fungi or to non mycorrhizal roots. The relative contribution of these nutrient sources requires quantification in the field. Further basic research, including the quantification of nutrient uptake and transport by fungal hyphae in soil and regulation at the fungal-plant interface, is essential to support the selection and utilization of mycorrhizal fungi on a commercial scale.