Ion currents and the nitrogen status of roots of Hordeum vulgare and non-nodulated Trifolium repens

Abstract. Profiles of self-generated ion currents associated with the growing primary root tips of intact Hordeum vulgare L. and Trifolium repens L. (nonnodulated) seedlings were measured using a highly sensitive vibrating electrode in media containing NH⁺₄ or NO^-₃, and compared to control roots growing in nitrogen free media. Under these three nutrient regimes, positive current entered the root at regions corresponding to the meristematic tissues and main elongation zones of root tips and left from the mature root tissues. Mapping the surface of the roots with a pH-sensitive microelectrode revealed regions of external alkalinity where positive electrical current entered the root, and external acidity where positive current exited. The correlation between pH-profile and the pattern of ion current generation in these experiments suggests that H⁺ ions were responsible for carrying the bulk of the root-generated current. Assimilation of NHJ results in net H⁺ extrusion while assimilation of NO^-₃, results in net OH^-₃ efflux. Growth on NH⁺₄, as compared to growth on NO^-₃, stimulated the magnitude of the electrical current but did not affect significantly the growth rate of the roots. However, despite the differing stresses on internal pH regulation that arise due to growth on the two exogenous forms of combined nitrogen, the current profiles were qualitatively similar under the different conditions that were examined. The role of the circulating proton current is not yet known; however, the constancy of the current profile under different nutrient regimes sustains the hypothesis that the current may have a role in the regulation of root polarity.     

Using porewater profiles to assess nutrient availability in seagrass-vegetated carbonate sediments

We measured porewater profiles of inorganic (NH₄ ⁺, NO₃ ^–(+NO₂ ^–), PO₄ ^3– (hereafter referred to as DIP)) and organic (DON, DOP) nutrients in seagrass-vegetated sediments at two sites in a shallow bay in Bermuda within close proximity (200 m) but subject to different nutrient loading. At both sites, total dissolved and inorganic nutrient concentrations were usually 1–2 orders of magnitude higher in the sediments than in the water column, with the exception of NO₃ ^–. Organic N and P were significant components of the total dissolved nutrient pools both in the sediment porewater and in the overlying water column (up to 75% for DON and 40% for DOP), and may be important in meeting plant nutrient demands. We used two approaches to examine how well porewater nutrient concentrations reflected the relative availabilities of N and P for seagrasses: (1) a simple stoichiometric nutrient regeneration model based on the N:P ratio of decomposing organic matter and porewater NH₄ ⁺ concentrations to predict porewater DIP, and (2) fitting of the porewater profiles to estimate rates of net nutrient production (or consumption), which reflects the balance between nutrient sources and sinks in the rhizosphere. The stoichiometric model indicated that sediment porewaters were depleted in P relative to N in the low-nutrient outer bay site, and enriched in P relative to N in the higher-nutrient inner bay site. These results are consistent with the mechanism of carbonate sediments in oligotrophic tropical environments being a strong sink for dissolved inorganic P and our previous work suggesting that nutrient enrichment causes P to become disproportionately more available than N. Net nutrient production rates of porewater P at both sites and N at the inner bay site were low (typically < 2%) relative to the nutrient demands of the seagrasses. The implications of the profile interpretation are two-fold: (1) the low rates of net nutrient production indicate diffusive losses from the root zone were insignificant and that nutrient turnover rates were high, except in the P-limited outer bay where N accumulated in sediment porewaters; and (2) because standing stock nutrient concentrations often represent a small fraction of the total nutrients cycled in the sediments, they are in many cases a poor indicator of nutrient availability. Based on our estimates of losses from the root zone, decomposition, and plant uptake we have constructed a rough budget for the cycling of P and N at our two sites.     

Ammonium, nitrate, and proton fluxes along the maize root

Ion-selective microelectrodes were used to measure NH₄⁺, NO₃^– and H⁺ fluxes along the primary root of maize seedlings. Plants were exposed to nutrient solutions containing NH₄⁺, NO₃^– or both ions. Nitrogen fluxes along the root varied substantially among the different treatments. Net NH₄⁺ and NO₃^– uptake and H⁺ extrusion were low at the very apex of the root and generally increased in the more basal regions. In the absence of nitrogen or in the presence of NO₃^– alone, net H⁺ uptake (and root surface alkalinization) occurred at the root tip (0–1 mm), whereas net H⁺ extrusion occurred in all other regions. In the presence of NH₄⁺ alone, a dramatic increase in net H⁺ extrusion was detected in all regions except for the region 6–11 mm from the apex. In contrast, when NO₃^– alone was supplied, net H⁺ extrusion was depressed at all locations except for the tip (0–1 mm). When both NH₄⁺ and NO₃^– were supplied, NO₃^– uptake was suppressed at all locations while net H⁺ extrusion was increased relative to NO₃^– alone. The capacities to absorb NH₄⁺ and NO₃^– at the tip were similar, as indicated by flux rates when NH₄⁺ or NO₃^– were supplied as sole sources, but when supplied together, net NO₃^– uptake was half that of net NH₄⁺ uptake, indicating that NH₄⁺ may satisfy the nitrogen requirements of the poorly vascularized apical tissue in the most energy-efficient way. The high spatial resolution of the measurements enabled us to establish that acidification in the root expansion zone is maintained regardless of nitrogen source.     

Combined effects of water column nitrate enrichment, sediment type and irradiance on growth and foliar nutrient concentrations of Potamogeton alpinus

1. High water column NO₃^? concentrations, low light availability and anoxic, muddy sediments are hypothesised to be key factors hampering growth of rooted submerged plants in shallow, eutrophic fresh water systems. In this study, the relative roles and interacting effects of these potential stressors on survival, growth, allocation of biomass and foliar nutrient concentrations of Potamogeton alpinus were determined in a mesocosm experiment using contrasting values of each factor (500 versus 0 μmol L^?1 NO₃^?; low irradiance, corresponding to the eutrophic environment, versus ambient irradiance; and muddy versus sandy sediment). 2. Low irradiance, high NO₃^? and sandy sediment led to reduced growth. In a muddy sediment, plants had lower root : shoot ratios than in a sandy sediment. 3. Growth at high NO₃^? and on the sandy sediment resulted in lower foliar N and C concentrations than in the contrasting treatments. The C : N ratio was higher at high NO₃^? and on the sandy sediment. Foliar P was higher on the muddy than on the sandy sediment but was not affected by irradiance or NO₃^?. The N : P ratio was lowest at high NO₃^? on the sandy sediment. 4. Total foliar free amino acid concentration was lowest on sand, low irradiance and high NO₃^?. Total free amino acid concentration and growth were not correlated. 5. Turbidity and ortho‐PO₄^3? concentration of the water layer were lower at high water column NO₃^? indicating that the growth reduction was not associated with increased algal growth but that physiological mechanisms were involved. 6. We conclude that high water column NO₃^? concentrations can significantly reduce the growth of ammonium preferring rooted submerged species such as P. alpinus, particularly on sediments with a relatively low nutrient availability. Further experiments are needed to assess potential negative effects on other species and to further elucidate the underlying physiological mechanisms.     

Water supply and not nitrate concentration determines primary root growth in Arabidopsis

Understanding how root system architecture (RSA) adapts to changing nitrogen and water availability is important for improving acquisition. A sand rhizotron system was developed to study RSA in a porous substrate under tightly regulated nutrient supply. The RSA of Arabidopsis seedlings under differing nitrate (NO₃^‐) and water supplies in agar and sand was described. The hydraulic conductivity of the root environment was manipulated by using altered sand particle size and matric potentials. Ion‐selective microelectrodes were used to quantify NO₃^‐ at the surface of growing primary roots in sands of different particle sizes. Differences in RSA were observed between seedlings grown on agar and sand, and the influence of NO₃^‐ (0.1–10.0 mm ) and water on RSA was determined. Primary root length (PRL) was a function of water flux and independent of NO₃^‐. The percentage of roots with laterals correlated with water flux, whereas NO₃^‐ supply was important for basal root (BR) growth. In agar and sand, the NO₃^‐ activities at the root surface were higher than those supplied in the nutrient solution. The sand rhizotron system is a useful tool for the study of RSA, providing a porous growth environment that can be used to simulate the effects of hydraulic conductivity on growth.     

Variation in nutrient availability and plant species diversity across forb and graminoid zones of a Northern New England high salt marsh

Plant zonation patterns across New England salt marshes have been investigated for years, but how nutrient availability differs between zones has received little attention. We investigated how N availability, P availability, and plant N status varied across Juncus gerardii, Spartina patens, and mixed forb zones of a Northern New England high salt marsh. We also investigated relationships between several edaphic factors and community production and diversity across the high marsh. P availability, soil salinity, and soil moisture were higher in the mixed forb zone than in the two graminoid zones. NH⁺ ₄-N availability was highest in the J. gerardii zone, but NO^– ₃-N availability and mid season net N mineralization rates did not vary among zones. Plant tissue N concentrations were highest in the mixed forb zone and lowest in the S. patens zone, reflecting plant physiologies more so than soil N availability. Community production was highest in the J. gerardii zone and was positively correlated with N availability and negatively correlated with soil moisture. Plant species diversity was highest in the mixed forb zone and was positively correlated with P availability and soil salinity. Thus, nutrient availability, plant N status, and plant species diversity varied across zones of this high marsh. Further investigation is needed to ascertain if soil nutrient availability influences or is a result of the production and diversity differences that exist between vegetation zones of New England high salt marshes.     

MACROALGAL RESPONSES TO NITROGEN SOURCE AND AVAILABILITY: AMINO ACID METABOLIC PROFILING AS A BIOINDICATOR USING GRACILARIA EDULIS (RHODOPHYTA)1

The use of macroalgae as biological indicators of dissolved nutrient source and availability in the water column was investigated. Total tissue nitrogen (N) content, pigments, and amino acids of the red alga Gracilaria edulis (Gmelin) Silva were compared to N source and availability in laboratory and field incubations to identify responses that would serve as bioindicators of N. Field-collected algae were preincubated (6–8 wk) in low-nutrient seawater to deplete their luxury reserves ofN. Incubations were then conducted for periods of 3 d in laboratory aquaria (N-spiked seawater) and in the field using macroalgal incubation chambers. After incubation in different N sources (NH₄⁺, NO₃^?, and urea) in laboratory aquaria, photosynthetic pigments (phycoerythrin and chlorophyll a) and total tissue N increased, in response to increasing [NH₄⁺] but not to [NO₃^?] or [urea]. Incubation in two ranges of [NH₄⁺], one from 0 to 80 μM and the other from 0 to 800 μM, in laboratory aquaria increased the total amino acid pool. Citrulline concentrations were the most responsive to [NH₄⁺] (r²= 0. 84). NH₄⁺ source treatments produced increases in citrulline, phenylalanine, serine, and free NH₄⁺ and decreases in alanine; NO₃^? treatments produced increases in glutamic acid, citrulline, and alanine; and urea treatments produced increases in free NH₄⁺ and decreases in phenylalanine and serine. The observed variations in amino acid content facilitated the development of an index for each N source based on relative concentrations of various amino acids (i. e. metabolic profiling). Gracilaria edulis was incubated along a field N gradient in the Brisbane River (three sites) and Moreton Bay (four sites), Queensland, Australia. Both phycoerythrin and tissue N appeared to respond equally to NH₄⁺ and NO₃^? availability in the field. N source indices, based on amino acid concentration, were effective predictors of both [NH₄⁺] and [NO₃^?] over a wide range of concentrations along the field gradient. Macroalgal physiological responses, particularly amino acid content, to changes in source and availability of N appear to be useful as sensitive bioindicators of N.     

翠菊根系养分捕获形态塑性及其生理机制

为验证以下3个假设: 1) NO₃ ^-和NH₄ ⁺及其不同供给方式显著影响根系生长; 2) NO₃ ^-和NH₄ ⁺以及不同供给方式对根内激素含量影响显著; 3)根构型(1级根长、单位2级根上1级侧根密度(分枝强度)和1级根在2级根上的根间距)与根内激素(生长素(IAA)、脱落酸(ABA)和细胞分裂素(玉米素核苷+玉米素) (CK (ZR + Z))含量显著相关, 采用营养液培养方法, 使实验植物翠菊(Callistephus chinensis)在两种氮肥(NO₃ ^-和NH₄ ⁺)、不同施氮浓度(NO₃ ^-: 0.2、1.0和18.0 mmol·L ^-1; NH₄ ⁺: 0.2、4.0和20.0 mmol·L ^-1), 以及脉冲和稳定两种施用方式处理下生长。在处理35天后收获植物, 测定根系生物量、根系构型指标(根系1级根长、单位2级根上1级侧根数和1级根在2级根上的根间距)和根系中激素含量(IAA、ABA和CK (ZR + Z))。结果显示: 1)实验处理对根生物量和根系中IAA、ABA和CK (ZR + Z)含量均有不同程度的显著影响: 施用NH₄ ⁺使根生物量和根内IAA含量显著低于施用NO₃ ^-; 高浓度NO₃ ^-和NH₄ ⁺处理亦使根生物量和IAA降低; 相对于稳定处理, 脉冲施氮显著降低根生物量和根内IAA含量; NO₃ ^-使根内CK (ZR + Z)含量显著高于施用NH₄ ⁺, 且与施氮浓度及施氮方式无关; NO₃ ^-处理下, 高浓度使根内ABA含量提高, 且脉冲处理使ABA含量升高。NH₄ ⁺处理下, 高浓度使根内ABA含量降低, 而施氮方式对其没有显著影响。2)根构型因素与根内激素关系各异: 各激素与1级根间距无显著关系; IAA和CK (ZR + Z)与1级根长和侧根密度有显著回归关系。3)根构型因素与根生物量的关系是根生物量与1级根长和侧根密度有显著正回归关系, 与1级根间距无显著回归关系。实验结果表明翠菊根生长的 “反常”可能是由于其对脉冲高浓度NH₄ ⁺耐受阈值低所致。该研究通过实验建立了氮养分种类/供应方式通过改变激素、影响根构型而影响根生长的联系, 进一步探究了植物根养分捕获塑性机制。     

凋落物和根系输入对南亚热带季风常绿阔叶林土壤养分的影响

植被凋落物和根系输入在调节森林土壤元素生物地球化学循环中扮演着关键作用。目前仍然不清楚凋落物和根系输入对热带原始林土壤主要元素含量的调控作用。针对该研究现状,以中国南亚热带季风常绿阔叶林为研究对象,通过开展凋落物与根系输入改变的控制试验（6个处理,每处理4次重复：对照、凋落物加倍、凋落物去除、断根、断根+凋落物加倍、断根+去除凋落物）,探讨了凋落物和断根处理对土壤可溶性离子、土壤酸中和能力（ANC）和阳离子交换量（CEC）的短期影响。凋落物与根系处理半年后的结果显示：（1）凋落物去除与加倍处理都显著增加了0-40 cm土壤NO₃^-含量,并且凋落物去除效应大于添加效应;去除凋落物增加了表层土壤（0-20 cm） Ca²⁺、Mg²⁺、Na⁺的含量。（2）断根处理显著增加0-40 cm土壤NO₃^-和表层土壤Ca²⁺、Mg²⁺含量。（3）断根和去除凋落物交互处理显著增加了0-40 cm土壤NO₃^-和表层土壤Ca²⁺、Mg²⁺、K⁺含量,产生了叠加效应。（4）凋落物和断根处理并没有改变土壤pH,但降低了土壤酸中和能力（除凋落物加倍外）,其降低的原因主要与阳离子交换量的降低和NO₃^-含量的增加有关。这些结果表明,土壤养分离子的可利用性（尤其是NO₃^-和Ca²⁺、Mg²⁺）和酸缓冲能力对凋落物和根系输入改变响应敏感,森林植物及其凋落物对土壤养分保留和缓冲性能具有重要调节作用。在人为干扰和气候变化加剧背景下,该研究可为森林生态系统可持续管理提供重要的理论参考。此外,植被凋落物和根系输入改变引起的长期生态学效应仍值得进一步关注。     

Localized and internal effect of nitrate on symbiotic dinitrogen fixation

Alfalfa (Medicago sativa L.) is a deeply rooted perennial legume which, under field conditions, may be exposed to varying NO₃^? concentrations with depth. Our objective was to characterize the effect of localized (deep vs shallow) exposure of alfalfa root systems to NO₃^? on symbiotic N₂ fixation and NO₃^?-N uptake. Cuttings of a single alfalfa plant were grown in vertical split root systems in a controlled environment chamber. The split root system was a rigid acrylic tube (5 cm diam. by 60 cm long) filled with silica sand and divided into upper and lower sections at the 30-cm depth by a 5-mm-thick wax layer. Roots penetrated the wax layer, but mixing of nutrient solutions between the sections was prevented. Nodulation was restricted to the upper section. The plants were subjected for 10 days to the following treatments: both sections of the split root system received nutrient solution containing either 0.5, 5.25, or 10 mM NO₃^?; the upper section received 0.5 mM NO₃^? while the lower section received 10 mM NO₃^?; or the upper section received 10 mM NO₃^? while the lower section received 0.5 mM NO₃^?. Increasing supply of NO₃^? in the nutrient solution to both sections resulted in higher NO₃^?-N uptake, lower nodule mass and lower specific nitrogenase activity. Although NO₃^?-N uptake did not differ, plants exposed to 10 mM NO₃^? for 10 days in the upper, nodulated section of the root system had a 20% lower nodule mass than plants exposed to the same NO₃^? concentration in the lower, non-nodulated section of the root system. Specific nitrogenase activity was not different between these two treatments. Therefore, we conclude that: (1) nodule mass was dependent on two factors, the amount of NO₃^?-N taken up and the concentration of NO₃^? within the nodulated root zone; and (2) specific nitrogenase activity was little affected by the concentration of NO₃^? surrounding the nodules, but was strongly inhibited by NO₃^?-N taken up.     

A dynamic whole-plant model of integrated metabolism of nitrogen and carbon. 2. Balanced growth driven by C fluxes and regulated by signals from C and N substrate

A whole-plant model of C and N metabolism is presented for the juvenile stage. It is aimed at comparing the growth performance of (wild) plant species in a range of environments with respect to irradiance and availability of nitrate (NO₃ ^-) and ammonium (NH₄ ⁺). State variables are the structural masses of leaves, stem and root, NO₃ ^- concentrations in root and shoot, non-structural carbohydrate (C) densities in leaves, stem and root and non-structural organic N concentration in the whole plant. Explicit expressions for NO₃ ^- influx, efflux, translocation and assimilation, and for NH₄ ⁺ uptake and assimilation have been formulated in an accompanying paper. Photosynthetic rate is derived from electron-transport rate which depends on irradiance and chlorophyll concentration on a leaf-area basis. The latter is proportional to non-structural organic N concentration. Photosynthetic N is considered non-structural. Unique features of the model are the use of metabolite signals and the treatment of C allocation and balanced growth. Metabolite signals are dimensionless functions of non-structural compounds (NO₃ ^-, C, organic N) and modify rate variables involved in N uptake and assimilation, C allocation and growth. Carbon allocation is driven by concentration differences of the cytosolic C pools in stem and root and is modified by the N status of the plant such that a high N status increases the apparent size of the shoot. Photosynthate is unloaded into C buffers which degrade at a constant specific rate. The sugar fluxes which arise from these buffers drive the growth rate of stem and root. No parameters are included for maximum specific growth or for activity or strength of sinks. Primary stem growth is proportional to growth of the leaf compartment: leaves arise from stems in a modular fashion. Leaves are autonomous with respect to their C balance. The model is presented as a system of differential equations which is integrated numerically. Parameter values, e.g., for uptake and assimilation capacities and costs of uptake, assimilation, maintenance and growth, are estimated for a grass species, Dactylis glomerata. Juvenile growth is simulated under optimal conditions with respect to irradiance and NO₃ ^- availability and compared with literature data. Diurnal and daily patterns of C utilisation and respiration, expressed as percentages of gross photosynthetic rate, are discussed. The model satisfactorily simulates typical responses to nutrient and light limitation and pruning, such as redirected C allocation, adjusted root and leaf weight ratios and compensatory growth. A sensitivity analysis is included for selected parameters. This revised version was published online in June 2006 with corrections to the Cover Date.     

Cotton root and shoot response to localized supply of nitrate,phosphate and potassium: Split-pot studies with nutrient solution and vermiculitic soil

Vertical stratification of plant-available K in vermiculitic soil profiles contributes to a late-season K deficiency that limits cotton (Gossypium hirsutum L.) yields on affected soils. Split-root solution culture and split-pot soil experiments were conducted to determine whether root distribution and cultivar differences in root extension in these stratified profiles result from a compensatory response to localized enrichment with NO₃-N, PO₄-P, and/or K in the root zone. Compensatory root growth was greatest in response to localized NO₃-N enrichment. For two cultivars examined in solution culture, 74% of new root development occurred in the half-pot providing 90% of the total NO₃-N supply. Only 60% of cultivar root development occurred in the half-pot providing 90% of the PO₄-P. No compensatory root growth was observed in response to localized K enrichment. In the split-pot system, the proportion of total root surface area developing in a half-pot was highly correlated with localized soil NO₃-N levels (r²=0.81), while increased K availability in one half of the root zone did not affect root distribution. Mean soil NO₃-N supply to the whole root system determined shoot N accumulation (r²=0.97). Shoot K accumulation was not related to soil K availability but was strongly correlated with mean root surface area density (r²=0.86). Cultivar Acala GC510, known to be less sensitive to K deficiency than Acala SJ-2, had significantly larger root diameter in all nutrient-supply environments. Under conditions of K stress, Acala GC510 had increased root branching and allocated greater dry matter to roots relative to shoots than Acala SJ-2. The results demonstrate that K acquisition by cotton is strongly influenced by the quantity and distribution of NO₃-N in the root zone through its effects on root proliferation, and that distinct cultivar differences associated with crop performance on low K soils can be detected in short-term, solution culture growth systems.     

Effects of long-term alleviation of nutrient limitation on shoot growth and foliar phenolics of Empetrum hermaphroditum

Alpine tundra ecosystems are characterised by low productivity, due in part to low nutrient availability. These ecosystems are often dominated by ‘stress tolerant’ species such as Empetrum hermaphroditum, which contribute to stress by producing and releasing biologically active phenolic compounds into the environment. In a nine‐year field experiment in alpine tundra, we investigated changes in growth and the levels (concentrations and contents) of foliar redox‐active phenolics of current‐year shoots of E. hermaphroditum in response to nine long‐term environmental manipulation treatments. The treatments were aimed at reducing ecological stresses commonly present in high‐latitude ecosystems, primarily stresses associated with low availability of N and other nutrients. Treatments included additions of various forms of N (single and combined applications of NH₄⁺ and NO₃^?, inorganic N as a component of a full nutrient treatment, and protein as a source of organic N), and additions of glucose, activated carbon, and lime. Shoot growth and levels of foliar phenolics varied greatly between years, but the variation was not clearly explained by the inter‐annual variation in macroclimate. Addition of inorganic N generally stimulated growth (especially stem biomass) and increased levels of leaf phenolics. The responses were, however, slow, and varied both between years and between individual inorganic N treatments. Compared to the other treatments, application of inorganic N as a component of a full nutrient treatment had the most consistent positive effect on shoot growth and phenolic content, but it did not affect the concentration of phenolics, suggesting that the treatment did not affect the net rate of phenolic production per unit shoot biomass. During some years of the experiment, the combined application of NH₄⁺ and NO₃^? resulted in increased production of phenolics per unit biomass accumulation. In contrast to inorganic N fertilisation, application of organic N generally reduced both shoot biomass and phenolic content. Non‐N treatments had no substantial effects on either the growth or levels of foliar phenolics of E. hermaphroditum. The observed long‐term responses of E. hermaphroditum to environmental manipulation treatments may be important for evaluating potential effects of variation in phenolics production and interference potential of this species under conditions of environmental change and for predicting long‐term responses of nutrient‐poor communities to environmental changes.     

Effects of different nitrogen forms on the growth and cytokinin content in xylem sap of tomato (Lycopersicon esculentum Mill.) seedlings

In order to investigate the effects of homogeneous and localized supply of different nitrogen forms (nitrate, NO₃ ^? vs ammonium, NH₄ ⁺) on the growth of tomato seedlings, root morphology and six cytokinin (CTK) fractions in xylem sap were analyzed. Whole roots were supplied with different ratios of NO₃ ^? to NH₄ ⁺ (100:0, as 100-0NA; 75:25, as 75-25NA; 50:50, as 50-50NA) under homogeneous supply. In split-root experiments, three treatments were compared: a sole NO₃ ^? supply (N|N), a spatially separated supply of NO₃ ^? and NH₄ ⁺ (N|A), and a spatially separated supply of NO₃ ^? and a mixture of NO₃ ^? and NH₄ ⁺ nutrition at a ratio of 75:25 (N|AN). All concentrations of total N were set at 5 mM. The results showed that (1) homogeneous 75% NO₃ ^? plus 25% NH₄ ⁺ supply to the whole root zone led to maximum shoot and root dry matter (DM), root surface area (RS) and root volume (RV). The spatially separated supply of NO₃ ^? and NH₄ ⁺ (N|A) resulted in a contrasting effect on root morphology: in comparison to N|N, root DM in the NO₃ ^?-containing pot was increased by 50% whereas it was depressed by 50% in the NH₄ ⁺-containing pot. The 75% NO₃ ^? plus 25% NH₄ ⁺ supply in the split-root experiment led to no significant effects either on shoot DM and root DM, or on RS and RV when compared to N|N. (2) The presence of NH₄ ⁺ in the external medium led to a significantly reduced total xylem-CTK concentration, and a close negative correlation was found between xylem NH₄ ⁺ and total CTK concentration irrespective of culture mode. A relatively high level of zeatin riboside (ZR) was maintained both in 75-25NA and N|A treatments. It was concluded that, in addition to the percentage of NH₄ ⁺ to NO₃ ^? in the nutrient solution, whether NH₄ ⁺ was supplied to the whole root system or to only part of the root system was also an important factor affecting plant growth. The fact that the 75-25NA and N|A treatments resulted in optimal growth of tomato seedlings might be attributed to the higher ZR concentration in xylem.     

Effect of N concentration and N source on root colonization by Pseudomonas fluorescens 2-79RLI

Wheat (Triticum aestivum L.) was grown in nutrient solution with low or high N supply (NH₄NO₃ as N source). To further evaluate the influence of N form and its interaction with the nutrient solution pH, wheat plants were grown with NH ₄ ⁺ or NO ₃ ^- either in an conventional nutrient solution or in a nutrient solution in which the pH was maintained at pH 6.5 using a pH-stat system. The nutrient solution was inoculated with Pseudomonas fluorescens 2-79RLI, a genetically modified bacterium that contains lux genes activated by a ribosomal promoter. Cell numbers and physiological status of P. fluorescens 2-79RLI (length of the lag phase of bioluminescence) in the rhizosphere were determined at the root tip and in the lateral root zone. Nitrogen deficiency decreased both plant growth and root colonization by P. fluorescens 2-79RLI at the root tip while it had no effect on root colonization in the lateral root zone. The physiological status of P. fluorescens 2-79RLI was not affected by nitrogen deficiency. Ammonium nutrition increased root colonization by P. fluorescens 2-79RLI at the root tip and in the lateral root zone when the pH of the nutrient solution was allowed to change according to the N form provided. Under these conditions, the physiological status of P. fluorescens 2-79RLI was higher in the lateral root zone than at the root tip. In contrast, N source had no effect on root colonization or physiological status of P. fluorescens 2-79RLI in the nutrient solution maintained at pH 6.5. It is concluded that the stimulation of root colonization by NH ₄ ⁺ in the nutrient solution, not maintained at a constant pH, may be due to increased leakage of solutes into the rhizosphere as a result of impaired exudate retention by high H⁺ concentration in the rhizosphere or the apoplast. This revised version was published online in June 2006 with corrections to the Cover Date.     

Interactive Effects of Vegetation Type and Topographic Position on Nitrogen Availability and Loss in a Temperate Montane Ecosystem

Determining the fate of deposited nitrogen (N) in natural ecosystems remains a challenge. Heterogeneity of vegetation types and resulting plant–soil feedbacks interact with topo-hydrologic gradients to mediate spatial patterns of N availability and loss, yet net effects of variation in these two factors together across complex terrain remain unclear. Here we measured a suite of N-cycle pools and fluxes in sites that differed factorially in vegetation type (mixed forest vs. herbaceous) and topographic position (upslope vs. downslope) in a protected montane watershed near Salt Lake City, UT. Vegetation type was associated with large variation in N availability—herbaceous sites had larger NO₃ ^? pools, higher NO₃ ^?:NH₄ ⁺ ratios, higher nitrification potentials, lower soil C:N values, enriched δ¹⁵N values, and lower microbial biomass compared to forests, especially those upslope. Downslope sites tended to exhibit higher N availability and indicators of N-cycle openness, but patterns were moderated by vegetation type. In downslope forest, soil NO₃ ^? depth profiles and higher foliar N content suggested trees were accessing deep soil N and transferring it to the surface via litterfall, while more deep soil NO₃ ^? but no change in surface or foliar N suggested herbaceous cover was not N limited or deeper N pools were not accessible. Soil NO₃ ^? leaching from below the rooting zone closely tracked N availability, revealing a link between N status and hydrologic loss as well as an important role for roots in N retention. NO₃ ^? isotopes did not reveal a similar link for gaseous losses (that is, denitrification), instead reflecting nitrification and/or transport dynamics. Together, these results suggest a coupled ecological, topo-hydrologic perspective can help assess the fate of N in complex landscapes.     

Organic and inorganic nitrogen nutrition of western red cedar,western hemlock and salal in mineral N-limited cedar–hemlock forests

Western red cedar (Thuja plicata Donn.), western hemlock (Tsuga heterophylla Raf. Sarge) and salal (Gaultheria shallon Pursh) are the main species growing in cedar–hemlock forests on Vancouver Island, Canada. Based on the dominance of organic N in these systems, we tested the hypotheses that: (1) organic N can be utilized by the three plant species; and (2) salal, which is ericoid mycorrhizal and has high tannin concentration in its tissues, would absorb more N from the complex organic N compounds than the other two species. The abilities of cedar, hemlock and salal to take up ¹⁵N,¹³C-labelled glutamic acid were measured and the capacities of the three species to use nitrate (NO₃^–), ammonium (NH₄⁺), glutamic acid, protein and protein–tannin N were compared over a 20-day period. Based on ¹³C enrichment, all three species absorbed at least a portion of glutamic acid intact. Cedar, hemlock and salal also showed similar patterns of N uptake from the NO₃^–, NH₄⁺, glutamic acid, protein and protein–tannin treatments. The largest proportions of applied N were taken up from the NO₃^– and NH₄⁺ treatments while smaller amounts of N were absorbed from the organic N compounds. Thus organic N was accessed to a modest degree by all three species, and salal did not have a greater capacity to utilize protein and protein–tannin–N.     

Effects of inorganic nitrogen application on the dynamics of the soil solution composition in the root zone of maize

The effect of inorganic nitrogen (N) fertilizer on the ionic composition of the soil solution under maize (Zea mays L.) was studied. A pot experiment was carried out with two treatments combined factorially, with or without N application (Ca(NO₃)₂; +N and –N treatments, respectively), and with or without plants. Three looped hollow fiber samplers were installed in each pot to sample soil solutions nondestructively from the root zone, seven times during the 50-day growth period. Plants were harvested on the 50th day, and their nutrient contents determined.Effects of N fertilizer on the soil solutions were observed by the first sampling, 2 days after sowing. The concentrations of Ca and NO₃ ^– and electrical conductivity (EC) increased significantly in the +N treatments as direct effects of fertilizer application. In addition, the concentrations of Mg, K, Na and H⁺ also increased and that of P decreased significantly as indirect effects caused by the re-establishment of chemical equilibria. This suggested the greater supply as well as the greater possibility of leaching loss not only of NO₃ ^– but also of Ca, Mg and K. In the treatments with plants, the concentrations of NO₃ ^–, Ca, Mg and K decreased with time and pH increased significantly compared with the unplanted soil. The depletion of N in the soil solution roughly agreed with the amount of N taken up by the plant. The depletions of K from the soil solution amounted to less than 10% of the amount of the K taken up, suggesting intensive replenishment of K from exchange sites in the soil. Depletions of Ca and Mg were several times higher than the amounts taken up, indicating that the depletions resulted from the adsorption of the divalent cations by the soil rather than uptake by plants. Because NO₃ ^– is hardly absorbed by exchange sites in soil and was the dominant anion in solution, it was concluded that NO₃ ^– had a major role in controlling cation concentrations in the soil solution and, consequently, on their availability for uptake by plants as well as their possible leaching loss. ei]H Marschner     

Effects of a localized supply of nitrate on NO3 - uptake rate and growth of roots in Lolium multiflorum Lam.

Roots of higher plants are usually exposed to varying spatial and temporal changes in concentrations of soil mineral nitrogen. A split root system was used to see how Lolium multiflorum Lam. roots adapt to such variations to cope with their N requirements. Plants were grown in hydroponic culture with their root system split in two spatially separated compartments allowing them to be fed with or without KNO₃. Net NO₃ ^- uptake, ¹⁵NO₃ ^- influx and root growth were studied in relation to time. Within less than 24 h following deprivation of KNO₃ to half the roots, the influx in NO₃ ^- fed roots was observed to increase (about 200% of the influx measured in plant uniformly NO₃ ^- supplied control plant) thereby compensating the whole plant for the lack of uptake by the N deprived roots. Due to the large NO₃ ^- concentrations in the roots, the NO₃ ^- efflux was also increased so that the net uptake rate increased only slightly (35% maximum) compared with the values obtained for control plants uniformly supplied with NO₃ ^-. This increase in net NO₃ ^- uptake rate was not sufficient to compensate the deficit in N uptake rate of the NO₃ ^- deprived split root in the short term. Over a longer period (>1 wk), root growth of the part of the root system locally supplied with NO₃ ^- was stimulated. An increase in root growth was mainly responsable for the greater uptake of nitrate in Lolium multiflorum so that it was able to fully compensate the deficit in N uptake rate of the NO₃ ^- deprived split root.