The ability of several high arctic plant species to utilize nitrate nitrogen under field conditions

The ability to utilize NO _inf3 ^sup– in seven high arctic plant species from Truelove Lowland, Devon Island, Canada was investigated, using an in vivo assay of maximum potential nitrate reductase (NR) activity and applications of ¹⁵N. Plant species were selected on the basis of being characteristic of nutrient-poor and nutrient-rich habitats. In all species leaves were the dominant site of NR activity. Root NR activity was negligible in all species except Saxifraga cernua. NO _inf3 ^sup– availability per se did not appear to limit NR activity of the species typically found on nutrient-poor sites (Dryas integrifolia, Saxifraga oppositifolia, and Salix arctica), or in Cerastium alpinum, as leaf NR activities remained low, even after NO _inf3 ^sup– addition. ¹⁵NO _inf3 ^sup– uptake was limited in D. integrifolia and Salix arctica. However, the lack of field induction of NR activity in C. alpinum and Saxifraga oppositifolia was not due to restricted nitrate uptake, as ¹⁵NO _inf3 ^sup– labelled NO _inf3 ^sup– entered the roots and shoots of both species. Leaf NR activity rates were low in three of the species typical of nutrient-rich habitats (O. digyna, P. radicatum and Saxifraga cernua), sampled from a site containing low soil NO _inf3 ^sup– . Additions of NO _inf3 ^sup– significantly increased leaf NR activity in these latter species, suggesting that potential NR activity was limited by the availability of NO _inf3 ^sup– . ¹⁵N labelled NO _inf3 ^sup– was taken up by O. digyna. P. radicatum and Saxifraga cernua. Although two species (D. integrifolia and Salix arctica) showed little utilization of NO _inf3 ^sup– , we concluded that five of the seven selected high arctic plant species (C. alpinum, O. digyna, P. radicatum, Saxifraga cernua and Saxifraga oppositifolia) do have the potential to utilize NO _inf3 ^sup– as a nitrogen source under field conditions, with the highest potential to utilize NO _inf3 ^sup– occurring in three of the species typically found on fertile habitats.     

Effect of ammonium and nitrate on ferric chelate reductase and nitrate reductase in Vaccinium species

BACKGROUND AND AIMS: Most Vaccinium species have strict soil requirements for optimal growth, requiring low pH, high iron availability and nitrogen primarily in the ammonium form. These soils are limited and are often located near wetlands. Vaccinium arboreum is a wild species adapted to a wide range of soils, including high pH, low iron, and nitrate-containing soils. This broader soil adaptation in V. arboreum may be related to increased efficiency of iron or nitrate uptake compared with the cultivated Vaccinium species. METHODS: Nitrate, ammonium and iron uptake, and nitrate reductase (NR) and ferric chelate reductase (FCR) activities were compared in two Vaccinium species grown hydroponically in either nitrate or ammonia, with or without iron. The species studied were the wild V. arboreum and the cultivated V. corymbosum interspecific hybrid, which exhibits the strict soil requirements of most Vaccinium species. RESULTS: Ammonium uptake was significantly greater than nitrate uptake in both species, while nitrate uptake was greater in the wild species, V. arboreum, compared with the cultivated species, V. corymbosum. The increased nitrate uptake in V. arboreum was correlated with increased root NR activity compared with V. corymbosum. The lower nitrate uptake in V. corymbosum was reflected in decreased plant dry weight in this species compared with V. arboreum. Root FCR activity increased significantly in V. corymbosum grown under iron-deficient conditions, compared with the same species grown under iron-sufficient conditions or with V. arboreum grown under either iron condition. CONCLUSIONS: V. arboreum appears to be more efficient in acquiring nitrate compared with V. corymbosum, possibly due to increased NR activity and this may partially explain the wider soil adaptation of V. arboreum.     

Differences in uptake kinetics of ammonium and nitrate in legumes and cereals

Influx isotherms were obtained for nitrate and ammonium from three legumes, Cajanus cajan (L.) Millsp., Cicer arietinum L. and Arachis hypogaea L. and three cereals, Sorghum bicolor (L.) Moench., Pennisetum glaucum L. and Zea mays L. The transition in influx isotherms for both nitrogen sources was found to be within the concentration range (0.05–2.5 mM) tested. There were significant differences in Km and Vmax for ammonium between legumes and cereals. The difference in the kinetic properties for nitrate uptake between the two groups of plants only became apparent at the higher concentration tested. Legumes translocated absorbed nitrate and ammonium to shoots more rapidly than cereals. Results show that there are significant differences in uptake and translocation of ammonium and nitrate between legumes and cereals.     

Ammonium and nitrate uptake by Eucalyptus nitens: effects of pH and temperature

Ammonium and nitrate uptake by roots of Eucalyptus nitens was characterised with respect to pH and temperature. Uptake of ammonium and nitrate was measured as depletion from solutions by roots of intact 11 week old solution-cultured seedlings. Uptake rates of ammonium were consistently higher than those of nitrate in all experiments. Uptake rates for ammonium were 200% higher at pH 4 than at pH 6, but for nitrate were unchanged. Uptake rates of ammonium and nitrate were both reduced to a similar extent (70%) with a decrease in temperature from 20 °C to 10 °C. For ammonium uptake, there was rapid (<24 hr) adaptation to a reduction in root temperature. The apparent preference shown here for ammonium over nitrate could be indicative of E. nitens growing in cold, acidic forest soils where ammonium is commonly more available than nitrate. These results suggest that N uptake rates of E. nitens may be maximised under a wide variety of conditions if N is supplied predominantly in the ammonium form. This revised version was published online in June 2006 with corrections to the Cover Date.     

The effect of endogenous and externally supplied nitrate on nitrate uptake and reduction in sugarbeet seedlings

The pericarp of the dormant sugarbeet fruit acts as a storage reservoir for nitrate, ammonium and -amino-N. These N-reserves enable an autonomous development of the seedling for 8–10 d after imbibition. The nitrate content of the seed (1% of the whole fruit) probably induces nitrate-reductase activity in the embryo enclosed in the pericarp. Nitrate that leaks out of the pericarp is reabsorbed by the emerging radicle. Seedlings germinated from seeds (pericarp was removed) without external N-supply are able to take up nitrate immediately upon exposure via a low-capacity uptake system (v_max = 0.8 mol NO ₃ ^- ·(g root FW)^–1·h^–1; K_s = 0.12 mM). We assume that this uptake system is induced by the seed nitrate (10 nmol/seed) during germination. Induction of a high-capacity nitrate-uptake system (v_max = 3.4 mol NO ₃ ^- ·(g root FW)^–1·h^–1; K_s = 0.08 mM) by externally supplied nitrate occurs after a 20-min lag and requires protein synthesis. Seedlings germinated from whole fruits absorb nitrate via a highcapacity uptake mechanism induced by the pericarp nitrate (748 nmol/pericarp) during germination. The uptake rates of the high-capacity system depend only on the actual nitrate concentration of the uptake medium and not on prior nitrate pretreatments. Nitrate deprivation results in a decline of the nitrate-uptake capacity (t_1/2 of v_max = 5 d) probably caused by the decay of carrier molecules. Small differences in K_s but significant differences in v_max indicate that the low- and high-capacity nitrate-uptake systems differ only in the number of identical carrier molecules.Abbreviations NR nitrate reductase - pFPA para-fluorophenylalanine This work was supported by a grant from Bundesministerium für Forschung und Technologie and by Kleinwanzlebener Saatzucht AG, Einbeck.     

Physiological dissection revealed that both uptake and assimilation are the major components regulating different growth responses of two tobacco cultivars to nitrogen nutrition

• K326 and HD represent major tobacco cultivars in China, which required large N fertiliser input but at different application rates. To understand primary components affecting tobacco N use physiology, we adopted these two varieties as valuable genetic material to assess their growth response to N nutrition.
• We established a hydroponic culture system to grow plants supplied with different N regimes. Plant biomass, N, ammonium, nitrate, arginine, GS and NR activity, N transfer and use efficiency as well as root uptake were examined.
• Our data revealed the preference of K326 and HD to utilise nitrate or ammonium nitrate but not ammonium alone, with 2 mm N supply probably sufficient and economical to achieve good biomass production at the vegetative stage. Moreover, both varieties were very sensitive to ammonium, perhaps due to lack of or abnormal signalling related to nitrate and/or arginine rather than impairment of N acquisition and initial assimilation; this was supported by measurements of the plant content of N, ammonium and activities of GS and NR. Notably, short‐term ¹⁵N root influx studies identified differential uptake kinetics of K326 and HD, with distinct affinities and transport rates for ammonium and nitrate.
• The data suggest that the growth adaptation of K326 or HD to higher or lower N may be ascribed to different competences for effective N uptake/translocation and assimilation. Thus, our work provides valuable information to prompt deeper investigation of the molecular basis controlling plant N use efficiency.

     

Nitrate regulation of nitrate uptake and nitrate reductase expression in barley grown at different nitrate:ammonium ratios at constant relative nitrogen addition rate

Barley (Hordeum vulgare L. cv. Golf) was cultured using the relative addition rate technique, where nitrogen is added in a fixed relation to the nitrogen already bound in biomass. The relative rate of total nitrogen addition was 0.09 day^?1 (growth limiting by 35%), while the nitrate addition was varied by means of different nitrate: ammonium ratios. In 3- to 4-week-old plants, these ratios of nitrate to ammonium supported nitrate fluxes ranging from 0 to 22 μmol g^?1 root dry weight h^?1, whereas the total N flux was 21.8 ± 0.25 μmol g^?1 root dry weight h^?1 for all treatments. The external nitrate concentrations varied between 0.18 and 1.5 μM. The relative growth rate, root to total biomass dry weight ratios, as well as Kjeldahl nitrogen in roots and shoots were unaffected by the nitrate:ammonium ratio. Tissue nitrate concentration in roots were comparable in all treatments. Shoot nitrate concentration increased with increasing nitrate supply, indicating increased translocation of nitrate to the shoot. The apparent V_max for net nitrate uptake increased with increased nitrate fluxes. Uptake activity was recorded also after growth at zero nitrate addition. This activity may have been induced by the small, but detectable, nitrate concentration in the medium under these conditions. In contrast, nitrate reductase (NR) activity in roots was unaffected by different nitrate fluxes, whereas NR activity in the shoot increased with increased nitrate supply. NR-mRNA was detected in roots from all cultures and showed no significant response to the nitrate flux, corroborating the data for NR activity. The data show that an extremely low amount of nitrate is required to elicit expression of NR and uptake activity. However, the uptake system and root NR respond differentially to increased nitrate flux at constant total N nutrition. It appears that root NR expression under these conditions is additionally controlled by factors related to the total N flux or the internal N status of the root and/or plant. The method used in this study may facilitate separation of nitrate-specific responses from the nutritional effect of nitrate.     

Ammonium and nitrate uptake by barley genotypes in diurnally fluctuating root temperatures simulating till and no-till conditions

The morphological development and N uptake patterns of spring barley (Hordeum vulgare L.) genotypes of Northern European (Nordic) and Pacific Northwest US (PNW) origin were compared under two diurnally fluctuating root temperature regimes in solution culture. The two regimes, 15/5°C and 9/5°C day maximum/night minimum temperatures, simulated soil temperature differences between tilled vs. heavy-residue, no-till conditions, respectively, observed during early spring in eastern Washington. Previous field experiments indicated that some of the Nordic genotypes accumulated more N and dry matter than the PNW cultivars during early spring under no-till conditions. The objective of this experiment was to determined whether these differences 1) are dependent on the temperature of the rooting environment, and 2) are correlated with genotypic differences in NH₄ ⁺ and NO₃ ^– uptake. Overall, shoot N and dry matter accumulation was reduced by 40% due to lower root temperatures during illumination. Leaf emergence was slowed by 14 to 22%, and tiller production was also inhibited. All genotypes absorbed more ammonium than nitrate from equimolar solutions, and the proportion of total N absorbed as NH₄ ⁺ was slightly higher in the 9/5°C than the 15/5°C regime. A Finnish genotype, HJA80201, accumulated significantly more shoot N than the PNW cultivars, Clark and Steptoe, and also more than a Swedish cultivar, Pernilla, in the 9/5°C regime. In the 15/5°C regime Steptoe did not differ in shoot N from the Nordic genotypes, while Clark remained significantly lower. These differences were not correlated to relative propensity for N form. Root lengths of the Nordic genotypes were significantly greater than the PNW genotypes grown under the 9/5°C regime, while the root lengths in the warmer root temperture regime were not significantly different among genotypes. Higher root elongation rates under low soil temperature conditions may be an inherent adaptive mechanism of the Nordic genotypes. Overall, the data indicate that lower maximum daytime temperatures of the soil surface layer likely account for a significant portion of the growth reductions and lower N uptake observed in no-till systems.     

Organic nitrogen content and nitrate and nitrite reductase activities in tritordeum and wheat grown under nitrate or ammonium

Tritordeum is a fertile amphiploid derived from durum wheat (Triticum turgidum L. conv. durum) × a wild barley (Hordeum chilense Roem. et Schultz.). The organic nitrogen content of tritordeum grain (34 mg g^-1 DW) was significantly higher than that of its wheat parent (25 mg g^-1 DW). Leaf and root nitrogen content became higher in tritordeum than in wheat after four weeks of growth, independently of the nitrogen source (either NO₃ ^- or NH₄ ⁺). Under NO₃ ^- nutrition, tritordeum generally exhibited higher levels of nitrate reductase (NR) activity than wheat. Nitrite reductase (NiR) levels were however lower in tritordeum than in its wheat parent. In NH₄ ⁺-grown plants, both NR and NiR activities progressively decreased in the two species, becoming imperceptible after 3 to 5 weeks of growth. Results indicate that, in addition to a higher rate of NO₃ ^- reduction, other physiological factors must be responsible for the greater accumulation of organic nitrogen in tritordeum grain.     

The course of 15N-ammonium nitrate in a spring barley cropping system

¹⁵N-labelled ammonium nitrate was applied to spring barley growing on a Cambisol soil in western Switzerland. Immobilization, plant uptake and disappearance of inorganic nitrogen were followed at frequent intervals. Fertilizer nitrogen disappeared shortly after its application, mainly through immobilization by soil microorganisms and absorption by the crop. Some of the added nitrogen was probably denitrified as a result of humid conditions during the first days after fertilizer application. At the end of the growing season, 31% of the added nitrogen was recovered from the aerial barley plants, and 56% was immobilized by microorganisms. Most of the fertilizer nitrogen not used by the crop was immobilized in the upper 0–30 cm soil layer. This prevented downward movement of nitrate and limited nitrogen losses. Fertilizer efficiency was mainly determined by the competition between crop uptake and microbial immobilization. Careful consideration of the time of fertilization, taking into account plant growth and weather conditions, can result in an increase in fertilizer efficiency and minimal pollution.     

Off-season uptake of nitrogen in temperate heath vegetation

In this field study we show that temperate coastal heath vegetation has a significant off-season uptake potential for nitrogen, both in the form of ammonium and as glycine, throughout winter. We injected ¹⁵N-ammonium and ¹⁵N 2×(¹³C)-glycine into the soil twice during winter and once at spring. The winter temperatures were similar to those of an average winter in the northern temperate region of Europe, with only few days of soil temperatures below zero or above 5°C. The vegetation, consisting of the evergreen dwarf shrub Calluna vulgaris, the deciduous dwarf shrub Salix arenaria, and the graminoids Carex arenaria and Deschampsia flexuosa, showed high root uptake of both forms of nitrogen, both 1 day after labelling and after a month, in species specific temporal patterns. Plant uptake of ¹³C was not significant, providing no further evidence of intact uptake of glycine. Translocation of the labelled nitrogen to shoots was generally evident after 1 month and increased as spring approached, with different translocation strategies in the three plant functional types. Furthermore, only the graminoids showed shoot growth during winter. Increasing plant nitrogen concentration from fall to spring at temperate heaths may, hence, be due to nitrogen uptake. Our results suggest that the potential for nitrogen uptake in plants at winter is of the same order of magnitude as at summer. Hence, winter nitrogen uptake in ecosystems in the temperate/boreal region should be considered when making annual nitrogen budgets of heath ecosystems, and the view of plant nutrient uptake as low in this climatic region during winter should be revised.     

Relationships between external nitrate availability,nitrate uptake and expression of nitrate reductase in roots of barley grown in N-limited split-root cultures

Despite the large number of studies of nitrate metabolism in plants, it remains undetermined to what extent this key plant system is controlled by overall plant N nutrition on the one hand, and by the nitrate ion itself on the other hand. To investigate these questions, V _max for nitrate uptake (high-affinity range), and nitrate reductase (NR) mRNA and activity, were measured in roots of N-limited barley (Hordeum vulgare L. cv. Golf) grown under conditions of constant relative addition of nitrate, with the seminal roots split between two culture compartments. The total amount of nitrate added per unit time (0.09·d^-1) was distributed between the two root parts (subroots) in ratios of 1000, 982, 955, 9010, 8020, and 5050. These nitrate-addition ratios resulted in nitrate fluxes ranging from 0 to 23 mol nitrate·g^-1 DW root·h^-1, while the external nitrate concentrations varied between 0 and 1.2 M. The apparent V _max for net nitrate uptake showed saturation-type responses to nitrate flux maintained during preceding growth. The flux resulting in half-maximal induction of nitrate uptake was approximately 4 mol nitrate·g^-1 DW root·h^-1, corresponding to an external nitrate concentration of 0.7 M. The activity of NR and levels of NR mRNA did not saturate within the range of nitrate fluxes studied. None of the parameters studied saturated with respect to the steady-state external nitrate concentration. At the zero nitrate addition — the 0%-root — initial uptake activity as determined in short-term ¹⁵N-labelling experiments was insignificant, and NR activity and NR mRNA were not detectable. However, nitrate uptake was rapidly induced, showing that the 0%-root had retained the capacity to respond to nitrate. These results suggest that local nitrate availability has a significant impact on the nitrate uptake and reducing systems of a split-root part when the total plant nitrate nutrition is held constant and limiting.Abbreviation NR nitrate reductase This work was supported by the Lars Hierta Memory Foundation, the Royal Swedish Academy of Sciences, and by the Swedish Natural Science Research Council via project grants (to C.-M.L. and B.I.) and visiting scientist grant (to W.H.C.). We thank Mrs. Ellen Campbell for technical advice, and Mrs. Judith V. Purves, Long Ashton Research Station, Long Ashton, UK, for analyses of ¹⁵N-labelling in tissue samples.     

Relatively large nitrate efflux can account for the high specific respiratory costs for nitrate transport in slow-growing grass species

In this paper we address the question why slow-growing grass species appear to take up nitrate with greater respiratory costs than do fast-growing grasses when all plants are grown with free access to nutrients. Specific costs for nitrate transport, expressed as moles of ATP per net mole of nitrate taken up, were 1.5 to 4 times higher in slow-growing grasses than in fast-growing ones (Scheurwater et al., 1998, Plant, Cell & Environ. 21, 995–1005). The net rate of nitrate uptake is determined by two opposing nitrate fluxes across the plasma membrane: influx and efflux. To test whether differences in specific costs for nitrate transport are due to differences in the ratio of nitrate influx to net rate of nitrate uptake, nitrate influx and the net rate of nitrate uptake were measured in the roots of two fast-growing ( Dactylis glomerata L. and Holcus lanatus L.) and two slow-growing (Deschampsia flexuosa L. and Festuca ovina L.) grass species at four points during the diurnal cycle, using ¹⁵NO₃ ^-. Efflux was calculated by subtraction of net uptake from influx; it was assumed that efflux of nitrogen represents the flux of nitrate. Transfer of the plants to the solution containing the labelled nitrate did not significantly affect nitrate uptake in the present grass species. The net rate of nitrate uptake was highest during the middle of the light period in all species. Diurnal variation in the net rate of nitrate uptake was mostly due to variation in nitrate influx. Variation in nitrate efflux did not occur in all species, but efflux per net mole of nitrate taken up was higher during darkness than in the light in the slow-growing grasses. The two fast-growing species, however, did not show diurnal variation in the ratio of efflux to net nitrate uptake. Integrated over 24 hours, the slow-growing grasses clearly exhibited higher ratios of influx to net uptake than the fast-growing grass species. Our results indicate that the higher ratio of nitrate influx to net nitrate uptake can account for higher specific costs for nitrate transport in slow-growing grass species compared with those in their fast-growing counterparts, possibly in combination with greater activity of the non-phosphorylating alternative respiratory path. Therefore, under our experimental conditions with plants grown at a non-limiting nitrate supply, nitrate uptake is less efficient (from the point of ATP consumption) in slow-growing grasses than in fast-growing grass species. This revised version was published online in June 2006 with corrections to the Cover Date.     

Reassessing the nitrogen relations of Arctic plants: a mini-review

The Arctic is often assumed to be an NH₄⁺-dominated ecosystem. This review assesses the validity of this assumption. It also addresses the question of whether Arctic plant growth is limited by the ability to use the forms of nitrogen that are available. The review demonstrates that several sources of soil nitrogen are available to Arctic plants, including soluble organic nitrogen (e.g. glycine, aspartic acid and glutamic acid), NH₄⁺ and NO^?₃. In mesic Arctic soils, soluble organic nitrogen is potentially more important than either NH⁺₄ or NO^?₃. Many Arctic species are capable of taking up soluble organic nitrogen (either directly and/or in association with ectomycorrhizae), with the greatest potential for soluble organic nitrogen uptake being exhibited by deciduous species. The ability to take up soluble organic nitrogen may enable some Arctic plants to avoid nitrogen limitations imposed by the slow rate of organic matter decomposition. NO^?₃ is also present in many Arctic soils, especially calcareous soils and soils near flowing water, animal burrows and bird cliffs. Arctic species characteristic of mesic and xeric habitats are capable of taking up and assimilating NO^?₃. Even when present in lower concentrations in soils than NH⁺₄, NO^?₃ is still an important source of nitrogen for some Arctic plants. Arctic-plants therefore have a variety of nitrogen sources available to them, and are capable of using those nitrogen sources. Taken together, these findings demonstrate that the Arctic is not an NH⁺₄dominated ecosystem. Symbiotic fixation of atmospheric N₂ does not appear to be an important source of nitrogen for Arctic plants. The reliance of Arctic plants on internal recycling of nitrogen substantially reduces their dependence on soil nitrogen uptake (this is particularly the case for slow-growing evergreens). Despite the high level of internal nitrogen recycling, Arctic plant growth remains limited by the low levels of available soil nitrogen. However, Arctic plant growth is not limited by an inability to utilize any of the available forms of nitrogen. The potential effects of climatic warming on nitrogen availability and use are discussed. The question of whether the Arctic ecosystem is uniquely different from temperate nitrogen-deficient ecosystems is also assessed.     

Mineralization-immobilization and plant uptake of nitrogen as influenced by the spatial distribution of cattle slurry in soils of different texture

The effect of incorporating cattle slurry in soil, either by mixing or by simulated injection into a hollow in soil, on the ryegrass uptake of total N and ¹⁵NH₄ ⁺-N was determined in three soils of different texture. The N accumulation in Italian ryegrass (Lolium multiflorum L.) from slurry N and from an equivalent amount of NH₄ ⁺-N in (¹⁵NH₄) SO₄ (control) was measured during 6 months of growth in pots. After this period the total recovery of labelled N in the top soil plus herbage was similar in the slurry and the control treatments. This indicated that gaseous losses from slurry NH₄ ⁺-N were insignificant. Consequently, the availability of slurry N to plants was mainly influenced by the mineralization-immobilization processes. The apparent utilization of slurry NH₄ ⁺-N mixed into soil was 7%, 14% and 24% lower than the utilization of (NH₄)₂SO₄-N in a sand soil, a sandy loam soil and a loam soil, respectively. Thus, the net immobilization of N due to slurry application increased with increasing soil clay content, whereas the recovery in plants of ¹⁵N-labelled NH₄ ⁺-N from slurry was similar on the three soils. A parallel incubation experiment showed that the immobilization of slurry N occurred within the first week after slurry application. The incorporation of slurry N by simulated injection increased the plant uptake of both total and labelled N compared to mixing the slurry into the soil. The apparent utilization of injected slurry NH₄ ⁺-N was 7% higher, 8% lower and 4% higher than the utilization of (NH₄)₂SO₄-N in the sand, the sandy loam and the loam soil, respectively. It is concluded that the spatial distribution of slurry in soil influenced the net mineralization of N to the same degree as did the soil type.     

植物氮素吸收与转运的研究进展

氮素是植物生长发育所必须的基本营养元素,在植物生长发育和形态建成中起着重要作用.土壤中植物所利用的主要氮素形式是铵态氮和硝态氮,在进化过程中植物形成不同的吸收和转运铵态氮和硝态氮的分子机制.该文对植物吸收与转运氮素的生理学特征、分子机制及涉及的相关基因等研究进行概括性综述,为研究水稻中氮素吸收、转运相关基因提供理论基础...     

The vertical distribution of N and K uptake in relation to root distribution and root uptake capacity in mature Quercus robur, Fagus sylvatica and Picea abies stands

We have measured the uptake capacity of nitrogen (N) and potassium (K) from different soil depths by injecting ¹⁵N and caesium (Cs; as an analogue to K) at 5 and 50 cm soil depth and analysing the recovery of these markers in foliage and buds. The study was performed in monocultures of 40-year-old pedunculate oak (Quercus robur), European beech (Fagus sylvatica) and Norway spruce (Picea abies (L.) Karst.) located at an experimental site in Palsgård, Denmark. The markers were injected as a solution through plastic tubes around 20 trees of each species at either 5 or 50 cm soil depth in June 2003. After 65 days foliage and buds were harvested and the concentrations of ¹⁵N and Cs analysed. The recovery of ¹⁵N in the foliage and buds tended to be higher from 5 than 50 cm soil depth in oak whereas they where similar in spruce and beech after compensation for differences in immobilization of ¹⁵N in the soil. In oak more Cs was recovered from 5 than from 50 cm soil depth whereas in beech and spruce no difference could be detected. Out of the three investigated tree species, oak was found to have the lowest capacity to take up Cs at 50 cm soil depth compared to 5 cm soil depth also after compensating for differences in discrimination against Cs by the roots. The uptake capacity from 50 cm soil depth compared with 5 cm was higher than expected from the root distribution except for K in oak, which can probably be explained by a considerable overlap of the uptake zones around the roots and mycorrhizal hyphae in the topsoil. The study also shows that fine roots at different soil depths with different physiological properties can influence the nutrient uptake of trees. Estimates of fine root distribution alone may thus not reflect the nutrient uptake capacity of trees with sufficient accuracy. Our study shows that deep-rooted trees such as oak may have lower nutrient uptake capacity at deeper soil layers than more shallow-rooted trees such as spruce, as we found no evidence that deep-rooted trees obtained proportionally more nutrients from deeper soil layers. This has implications for models of nutrient cycling in forest ecosystems that use the distribution of roots as the sole criterion for predicting uptake of nutrients from different soil depths.     

Ammonium can stimulate nitrate and nitrite reductase in the absence of nitrate in Clematis vitalba

Nitrogen assimilation was studied in the deciduous, perennial climber Clematis vitalba. When solely supplied with NO₃^– in a hydroponic system, growth and N-assimilation characteristics were similar to those reported for a range of other species. When solely supplied with NH₄⁺, however, nitrate reductase (NR) activity dramatically increased in shoot tissue, and particularly leaf tissue, to up to three times the maximum level achieved in NO₃^– supplied plants. NO₃^– was not detected in plant material that had been solely supplied with NH₄⁺, there was no NO₃^– contamination of the hydroponic system, and the NH₄⁺-induced activity did not occur in tobacco or barley grown under similar conditions. Western Blot analysis revealed that the induction of NR activity, either by NO₃^– or NH₄⁺, was matched by NR and nitrite reductase protein synthesis, but this was not the case for the ammonium assimilation enzyme glutamine synthetase. Exposure of leaf disks to N revealed that NO₃^– assimilation was induced in leaves directly by NO₃^– and NH₄⁺ but not glutamine. Our results suggest that the NH₄⁺-induced potential for NO₃^– assimilation occurs when externally sourced NH₄⁺ is assimilated in the absence of any NO₃^– assimilation. These data show that the potential for nitrate assimilation in C. vitalba is induced by a nitrogenous compound in the absence of its substrate and suggest that NO₃^– assimilation in C. vitalba may have a significant role beyond the supply of reduced N for growth.