Plant and microbial nitrogen use and turnover: Rapid conversion of nitrate to ammonium in soil with roots

Immobilization of ammonium (NH ₄ ⁺ ) by plants and microbes, a controlling factor of ecosystem nitrogen (N) retention, has usually been measured based on uptake of¹⁵NH ₄ ⁺ solutions injected into soil. To study the influence of roots on N dynamics without stimulating consumption of NH ₄ ⁺ , we estimated gross nitrification in the presence or absence of live roots in an agricultural soil. Tomato (Lycopersicon esculentum var. Peto76) plants were grown in microcosms containing root exclosures. When the plants were 7 weeks old,¹⁵N enriched nitrate (NO ₃ ⁻ ) was applied in the 0–150 mm soil layer. After 24 h, > 30 times more¹⁵NH ₄ ⁺ was found in the soil with roots than in the soil of the root exclosures. At least 18% of the NH ₄ ⁺ -N present at this time in the soil with roots had been converted from NO ₃ ⁻ . We estimated rates of conversion of NO ₃ ⁻ to NH ₄ ⁺ , and rates ofNH ₄ ⁺ immobilization by plants and microbes, by simulating N-flow of¹⁴⁺¹⁵N and¹⁵N in three models representing mechanisms that may be underlying the experimental data: Dissimilatory NO ₃ ⁻ reduction to NH ₄ ⁺ (DNRA), plant N efflux, and microbial biomass nitrogen (MBN) turnover. Compared to NO ₃ ⁻ uptake, plant NH ₄ ⁺ uptake was modest. Ammonium immobilization by plants and microbes was equal to at least 35% of nitrification rates. The rapid recycling of NO ₃ ⁻ to NH ₄ ⁺ via plants and/or microbes contributes to ecosystem N retention and may enable plants growing in agricultural soils to capture more NH ₄ ⁺ than generally assumed.     

Soil solute concentration and water uptake by single lupin and radish plant roots

Application of computer assisted tomography to gamma and X-ray attenuation measurements and Na⁺-LIX microelectrodes were used to determine the spatial distributions of soil water content and Na⁺ concentrations respectively near single roots of eighteen day old lupin and radish plants. These quantities were monitored at root depths of 3, 6 and 9 cm and at zero, 2, 4, 6, and 8 hour intervals from the diurnal commencement of transpiration. The plants were subjected to two levels of transpirational demand and five Na⁺ soil solution concentration levels. Water extraction rates for the lupin and radish roots increased continuously with time but were substantially reduced with increasing Na⁺ concentration in the treatment. Water uptake was uniform along the length of the essentially constant diameter lupin roots but decreased along the tapering radish roots as the diameter and hence the surface area per unit length of the roots decreased. The accumulation of Na⁺ at the root surfaces of both plants increased gradually with time in a near linear fashion and was slightly higher under the higher transpiration demand. These increases were not exponential as would be expected with non-absorption by the roots and this is considered to be due to back diffusion at the relatively high water contents used. At these water contents matric potentials had a much smaller influence on transpiration than osmotic potentials. The relationships between leaf water potentials (Ψ₁) and osmotic potentials at the root surfaces were linear with the decreases in Ψ₁ almost exactly reflecting the decreases in Ψ_π indicating rapid plant adjustment. Leaf water potentials decreased progressively with time and the relationships between leaf water potential and the transpiration rate were also linear supporting the suggestion of constant plant resistances at any given concentration.     

The importance of initial exchangeable ammonium in the nitrogen nutrition of lowland rice soils

Summary The importance of initial exchangeable soil NH ₄ ⁺ in nitrogen nutrition and grain yield of rice was studied in a number of representative lowland rice soils in the Philippines. The initial exchangeable soil NH ₄ ⁺ +fertilizer N plotted against nitrogen uptake by the crop resulted in a highly significant linear relationship (R²=0.91), suggesting that the presence of exchangeable NH ₄ ⁺ in the soil at transplanting behaved like fertilizer nitrogen. The correlation between N fertilizer rate and N uptake by the rice crop was relatively poor (R²=0.73). On the other hand, relative grain yield was more closely correlated with the initial exchangeable soil NH ₄ ⁺ +fertilizer N than with fertilizer nitrogen applied alone. These results indicate that the initial exchangeable NH ₄ ⁺ in the soil contributed substantially to the nitrogen uptake of the crop.Critical nitrogen levels in the soil defined as the initial exchangeable soil NH ₄ ⁺ +fertilizer N at which the optimum grain yield (95% of the maximum yield) is obtained, varied from 60 to 100 kg N/ha in the wet season and from 100 to 120 kg N/ha in the dry season for the different fertilizer treatments. The results further suggest that the initial exchangeable soil NH ₄ ⁺ should serve as a guide in selecting an optimum nitrogen fertilizer rate for high grain yields.     

Effects of litter incorporation and nitrogen fertilization on the contents of extractable aluminium in the rhizosphere soil of tea plant (Camallia sinensis (L.) O. Kuntze)

The effects of litter incorporation and nitrogen application on the properties of rhizosphere and bulk soils of tea plants (Camellia sinensis (L.) O. Kuntze) were examined in a pot experiment. Total of 8 treatments included four levels of tea litter additions at 0, 4.9, 9.8, and 24.5 g kg^–1 in combination with two N levels (154.6 mg kg^–1 and without). After 18 months of growth the rhizosphere soil was collected by removing the soil adhering to plant roots and other soil was referred to as bulk soil. The dry matter productions of tea plants were significantly increased by N fertilization and litter incorporation. The effect of litter was time-depending and significantly decreased the content of exchangeable Al (Al_ex, by 1 mol L^–1 KCl) and Al saturation at 9 months after litter incorporation whereas soil pH was not affected, although the litter contained high Al content. After 18 months, the contents of extractable Al by dilute CaCl₂, CuCl₂ + KCl, NH₄OAC, ammonium oxalate and sodium citrate (Al_CaCl2, Al_Cu/KCl, Al_NH4OAC, Al_Oxal, and Al_Cit respectively) and Al_ex, were not affected by litter application, except that of Al_CaCl2 in the rhizosphere soil which was decreased following litter additions. Nitrogen fertilization with NH₄ ⁺ (urea and (NH₄)₂SO₄) significantly reduced soil pH, the contents of exchangeable Ca, K, Mg and base saturation while raised extractable Al levels (Al_CaCl2, Al_Cu/KCl, Al_NH4OAC, and Al_ex). In the rhizosphere soils exchangeable K accumulated in all treatments while exchangeable Ca and Mg depleted in treatments without litter application. The depletions of Ca and Mg were no longer observed following litter incorporation. This change of distribution gradients in rhizosphere was possibly due to the increase of nutrient supplies from litter decomposition and/or preferable root growth in soil microsites rich in organic matter. Lower pH and higher extractable Al (Al_CaCl2, Al_ex, and Al_NH4OAC) in the rhizosphere soils, regardless of N and litter treatments, were distinct and consistent in all treatments. Such enrichments of extractable Al in the rhizosphere soil might be of importance for tea plants capable of taking up large amounts of Al.     

Depletion of soil mineral N by roots of Cucumis sativus L. colonized or not by arbuscular mycorrhizal fungi

Two experiments were conducted where Cucumis sativus were grown in uncompartmented pots either alone or in symbiosis with Glomus intraradices Schenck and Smith (Experiment 1) or Glomus sp. (Experiment 2) in order to investigate if root colonization by arbuscular mycorrhizal (AM) fungi has an effect on depletion of the soil mineral N pool. All pots were gradually supplied with 31 mg NH₄NO₃-N kg^-1 dry soil from 12–19 days after planting and an additional 50 mg (NH₄)₂SO₄-N kg^-1 dry soil (¹⁵N-labelled in Experiment 1) was supplied at 21 or 22 days after planting in Experiments 1 and 2, respectively. Dry weight of plant parts, total root length, mycorrhizal colonization rate and soil concentration of NH ₄ ⁺ and NO ₃ ^- were recorded at five sequential harvest events: 21, 24, 30, 35 and 42 days (Experiment 1) and 22, 25, 28, 31 and 35 days (Experiment 2) after planting. In Experiment 1, plants were also analysed for total content of N and ¹⁵N. The mycorrhizal colonization rate increased during time: from 25 to 40% in Experiment 1 and from 50 to 60% in Experiment 2. Plant dry matter accumulation was unaffected by mycorrhizal colonization, except in Experiment 1 where shoot dry weights were slightly increased and in Experiment 2 where root dry weights were slightly decreased compared to non-mycorrhizal control plants. The total root length was similar in the control and mycorrhizal treatments in Experiment 1, while it was decreased (20–30%) by mycorrhizal colonization in the last two harvest in Experiment 2. Mycorrhizal colonization affected the rate of depletion of soil mineral N in Experiment 1, where both NH ₄ ⁺ and NO ₃ ^- concentrations were markedly lower in the first two harvests, when plants were mycorrhizal. As the root length was similar in mycorrhizal and control treatments, this may indicate that the external AM hyphae contributed to the depletion of the soil mineral N pool. A similar pattern was observed in Experiment 2, although the effect was less pronounced. The ¹⁵N enrichment in mycorrhizal plants (Experiment 1) also indicated a faster NH ₄ ⁺ uptake than in the non-mycorrhizal controls in the first two harvests after application of the ¹⁵N-labelled N source. However, the external hyphae and roots seemed to have access to the same N sources as the ¹⁵N enrichment and total N content were similar in mycorrhizal and control plants at the end of the experiment. This revised version was published online in June 2006 with corrections to the Cover Date.     

Ammonium and nitrate uptake rates and rhizosphere pH in non-mycorrhizal roots of Norway spruce [Picea abies (L.) Karst.]

Summary Relationships between root zone temperature, concentrations and uptake rates of NH ₄ ⁺ and NO ₃ ^– were studied in non-mycorrhizal roots of 4-year-old Norway spruce under controlled environmental conditions. Additionally, in a forest stand NH ₄ ⁺ and NO ₃ ^– uptake rates along the root axis and changes in the rhizosphere pH were measured. In the concentration (C_min) range of 100–150 M uptake rates of NH ₄ ⁺ were 3–4 times higher than those of NO ₃ ^– The preference for NH ₄ ⁺ uptake was also reflected in the minimum concentration (C_min) values. Supplying NH₄NO₃, the rate of NO ₃ ^– uptake was very low until the NH ₄ ⁺ concentrations had fallen below about 100 M. The shift from NH ₄ ⁺ to NO ₃ ^– uptake was correlated with a corresponding shift from net H⁺ production to net H⁺ consumption in the external solution. The uptake rates of NH ₄ ⁺ were correlated with equimolar net production of H⁺. With NO ₃ ^– nutrition net consumption of H⁺ was approximately twice as high as uptake rates of NO ₃ ^– In the forest stand the NO ₃ ^– concentration in the soil solution was more than 10 times higher than the NH ₄ ⁺ concentration (<100 M), and the rhizosphere pH of non-mycorrhizal roots considerably higher than the bulk soil pH. The rhizosphere pH increase was particularly evident in apical root zones where the rates of water and NO ₃ ^– uptake and nitrate reductase activity were also higher. The results are summarized in a model of water and nutrient transport to, and uptake by, non-mycorrhizal roots of Norway spruce in a forest stand. Model calculations indicate that delivery to the roots by mass flow may meet most of the plant demand of nitrogen and calcium, and that non-mycorrhizal root tips have the potential to take up most of the delivered nitrate and calcium.     

The chemistry of the lowland rice rhizosphere

Models and experimental studies of the rhizosphere of rice plants growing in anaerobic soil show that two major processes lead to considerable acidification (1–2 pH units) of the rhizosphere over a wide range of root and soil conditions. One is generation of H⁺ in the oxidation of ferrous iron by O₂ released from the roots. The other is release of H⁺ from roots to balance excess intake of cations over anions, N being taken up chiefly as NH₄ ⁺. CO₂ exchange between the roots and soil has a much smaller effect. The zone of root-influence extends a few mm from the root surface. There are substantial differences along the root length and with time. The acidification and oxidation cause increased sorption of NH₄ ⁺ ions on soil solids, thereby impeding the movement of N to absorbing root surfaces. But they also cause solubilization and enhanced uptake of soil phosphate.     

Ammonium versus Nitrate Nutrition of Plants Stimulates Microbial Activity in the Rhizosphere

Using an alkaline calcareous soil, pot experiments were conducted to elucidate the effects of NH ₄ ⁺ vs. NO ₃ ⁻ nutrition (50 or 100 mg kg⁻¹ soil) of wheat and maize on microbial activity in the rhizosphere and bulk soils. Dicyandiamide was used as nitrification inhibitor to maintain NH ₄ ⁺ as the predominant N source for plants grown in NH ₄ ⁺ -treated soil. While maize grew equally well on both N sources, root and shoot growth of wheat was higher under NH ₄ ⁺ than under NO ₃ ⁻ nutrition. Bacterial population density on roots, but not in the rhizosphere soil, was higher under NH ₄ ⁺ than under NO ₃ ^– supplied at 150 mg N kg⁻¹ soil; whereas at both N levels applied, NH ₄ ⁺ compared to NO ₃ ⁻ nutrition of wheat and maize significantly increased microbial biomass in the rhizosphere soil. Under both plant species, NH ₄ ⁺ vs. NO ₃ ⁻ nutrition also increased aerobic and anaerobic respiration, and dehydrogenase activity in the rhizosphere. As microbial activity in the planted bulk and unplanted soils was hardly affected by the N-source, we hypothesize that the stimulation by NH ₄ ⁺ of the rhizosphere microbial activity was probably due to higher availability of root exudates under NH ₄ ⁺ than under NO ₃ ⁻ nutrition.     

Effect of Sesbania,Azolla and rice straw incorporation on the kinetics of NH4, K,Fe, Mn,Zn and P in some flooded rice soils

In a greenhouse study, with and without rice plants, of five flooded Philippine rice soils whose organic C (OC) content varied from 0.5 to 3.6%, incorporation ofSesbania rostrata, Azolla microphylla and rice straw affected the kinetics of soil solution NH ₄ ⁺ −N, K⁺, Fe²⁺, Mn²⁺, Zn²⁺, and P. Sesbania and Azolla increased NH ₄ ⁺ −N concentration above the control treatment, whereas rice straw depressed it. In all soils Azolla released less NH ₄ ⁺ −N than Sesbania. The apparent net N release depended on the soil and ranged from 44–81% for Sesbania and 27–52% for Azolla. These effects persisted throughout the growth of IR36. Soil solution and exchangeable NH ₄ ⁺ −N increased initially but levelled off between 30 to 80 days and between 20 to 40 days after flooding (DF), respectively. With rice, soil solution NH ₄ ⁺ −N concentration, reached a peak at 15–40 DF and declined to very low levels (<4mg L⁻¹). In the 3 soils of low OC content nitrogen derived from green manure ranged from 34–53% and the apparent revovery of added green manure N varied from 29–67%. Almost all N released from both Azolla and Sesbania were recovered in the rice plant in all soils except Concepcion with only 77%. The concentration of K⁺, Fe²⁺, Mn²⁺ and P in the soil solution were higher with rice straw than Sesbania and Azolla in all soils except Hanggan which showed no change in Fe²⁺ and Mn²⁺ but increased K⁺ and P. In general, rice straw, Sesbania and Azolla decreased Zn²⁺ concentration in all soils.     

Low concentrations of ammonium inhibit specific nodulation (nodule number g-1 root DW) in soybean (Glycine max [L.] Merr.)

Experiments on peas (Gulden and Vessey, 1997) have indicated that NH ₄ ⁺ stimulates both whole plant (nodules plant^-1) and specific nodulation (nodules g^-1 root DW). The effect of low concentrations of NH ₄ ⁺ on the soybean/Bradyrhizobium symbiosis is unknown. The objectives of the current study were to determine the immediate and residual effects of NH ₄ ⁺ on nodulation and N₂ fixation in soybean (Glycine max [L.] Merr.) in sand culture. Soybean (cv. Maple Ridge) were exposed to 0.0, 0.5, 1.0 and 2.0 mM of ¹⁵N-labelled (NH₄)₂SO₄ for 28 days after inoculation (DAI). From 29 to 56 DAI the plants were grown on NH ₄ ⁺ -free nutrient solution. Plants were harvested at 7, 14, 21, 28 and 56 DAI for root, shoot and nodule dry weight (DW), total N content, nodule counts and ¹⁵N enrichment of plant composites. Nitrogenase activity was measured by gas exchange at 28 DAI. The plants in the control (0.0 mM NH ₄ ⁺ ) treatment had consistently lower relative growth rates than the plants in the NH ₄ ⁺ treatments during the first 28 DAI. Plant growth was also less at 2.0 mM NH ₄ ⁺ compared to growth at 0.5 and 1.0 mM NH ₄ ⁺ . At 28 DAI, plants exposed to 0.5 and 1.0 mM NH ₄ ⁺ had significantly more nodules per plant and larger individual nodules than either the NH ₄ ⁺ -free controls or the 2.0 mM NH ₄ ⁺ -supplied plants. However, specific nodulation (nodule number g^-1 root DW) and specific nitrogenase activity (nitrogenase activity g^-1 nodule DW) were on average approximately 286% and 60% higher in the control plants, respectively, than for plants in the NH ₄ ⁺ treatments at 28 DAI. Also at 28 DAI, specific nodule DW (nodule DW g^-1 root DW) were 17, 44 and 53% higher in control plants than plants that had been exposed to 0.5, 1.0 and 2.0 mM NH ₄ ⁺ . At 56 DAI, after an additional 4 weeks of NH ₄ ⁺ -free nutrition, the plants which had previously received 0.5 and 1.0 mM NH ₄ ⁺ still maintained the highest plant DW and N contents, however, specific nodule DW had become similar at 600 mg nodule DW g^-1 root DW among all treatments. It is concluded that NH ₄ ⁺ has a negative effect on the nodulation process in the soybean/Bradyrhizobium symbiosis (as best indicated by the negative effect of NH ₄ ⁺ on specific nodulation). Despite this negative effect on specific nodulation, 0.5 and 1.0 mM NH ₄ ⁺ resulted in higher whole plant nodulation and N₂ fixation due to a compensating, positive effect on overall plant growth (i.e. fewer nodules g^-1 root DW, but much larger roots). Once NH ₄ ⁺ was removed from all treatments, the soybean plants appeared to move toward a consistent level of nodule DW relative to root DW.     

The dynamics of protons,aluminium, and calcium in the rhizosphere of maize cultivated in tropical acid soils: experimental study and modelling

The goals of this work were to understand the dynamics of H⁺, Al and Ca in the rhizosphere of maize cultivated in tropical acid soils, and to evaluate the contribution of the dissolution kinetics of the Al-hydroxides to Al dynamics. The study of the dissolution kinetics was based on a comparison between experimental and simulated data, using a model of the chemical processes in the rhizosphere. Two Oxisols, pH 5.1 and 4.6, and one Ultisol, pH 5.2, were studied. An Al-tolerant maize variety (Zea mays L.) was grown for 14 days on a 3-mm thick soil layer. The composition of the soil and the soil solution, together with the concentration of Al in the roots, were determined throughout the experiment. The results showed that root growth (i) decreased the soil solution pH, up to one pH unit, (ii) increased Al concentration in the soil solution, (iii) increased exchangeable Al, and (iv) decreased exchangeable Ca. Soil solution pH, exchangeable Al, and exchangeable Ca were closely linked. Exchangeable Al increased 1.5 – 3.0 times, due to the dissolution of easily mobilised Al components. In addition, Al accumulation in roots depended mainly on Al in the soil solution. Modelling the interactions between H⁺, Al, and Ca proved that the main factor determining Al in the soil solution was the kinetic reactivity of the easily mobilised Al components. These components, probably poorly crystallised Al-hydroxides, are key players in the functioning of the rhizosphere in tropical acid soils.     

Localization and quantification of net fluxes of H+ along maize roots by combined use of pH-indicator dye videodensitometry and H+-selective microelectrodes

Two methods for measuring proton fluxes along intact maize roots grown with NH ₄ ⁺ or NO ₃ ⁻ at pH 6.5 were compared. Videodensitometric measurement of changes in a pH-indicator dye by video camera was used to map pH around roots and determine the amounts of protons released by various root regions. This method was compared with potentiometric determination of the concentration of H⁺ in the unstirred layer at the root surface using ion-selective microelectrodes. With NH ₄ ⁺ the roots released large amounts of H⁺ in preferential regions where the rate of flux can reach 1.4 or even 2.5 nmol m⁻¹ s⁻¹. Videodensitometry indicated a first region of root acidification in the subapical zone, but this was more difficult to localize with microelectrodes. With NO₃ ⁻ both methods showed that the roots released small amounts of H⁺ and that the apical region took up H⁺ in the first 10 mm then sometimes released H⁺ over the following 10 mm of root. The H⁺ flux profiles obtained by both methods were in good agreement in terms of both order of magnitude of the fluxes and spatial differences along the root. These results suggest that videodensitometry, which is easier to use than potentiometry, can be used to screen different plant species or cultivars under various experimental conditions. The microelectrode technique is indispensable, however, for studying the underlying mechanisms of net H⁺ fluxes. This revised version was published online in June 2006 with corrections to the Cover Date.     

Regulation of ammonium and potassium uptake by glutamine and asparagine in Pisum arvense plants

Pisum arvense plants were subjected to 5 days of nitrogen deprivation. Then, in the conditions that increased or decreased the root glutamine and asparagine pools, the uptake rates of 0.5 mM NH₄ ⁺ and 0.5 mM K⁺ were examined. The plants supplied with 1 mM glutamine or asparagine took up ammonium and potassium at rates lower than those for the control plants. The uptake rates of NH₄ ⁺ and K⁺ were not affected by 1 mM glutamate. When the plants were pre-treated with 100 μM methionine sulphoximine, an inhibitor of glutamine synthesis, the efflux of NH₄ ⁺ from roots to ambient solution was enhanced. On the other hand, exposure of plants to methionine sulphoximine led to an increase in potassium uptake rate. The addition of asparagine, glutamine or glutamate into the incubation medium caused a decline in the rate of NH₄ ⁺ uptake by plasma membrane vesicles isolated from roots of Pisum arvense, whereas on addition of methionine sulphoximine increased ammonium uptake. The results indicate that both NH₄ ⁺ and K⁺ uptake appear to be similarly affected by glutamine and asparagine status in root cells. The research was supported by grant of KBN No. 6PO4C 068 08     

Combination Effect of NaCl Salinity and Nitrogen Form on Mineral Composition of Sunflower Plants

The effect of two N-forms (NH₄ ⁺ and NO₃ ^–) and NaCl on pattern of accumulation of some essential inorganic nutrients was examined in sunflower (Helianthus annuus L.) cv. Hisun 33. Eight-day-old plants of were subjected for 21 d to Hoagland's nutrient solution containing 8 mM N as NH₄ ⁺ or NO₃ ^·, and salinized with and addition of NaCl to the growth medium had no significant effect on total leaf N. However, root N of NH₄-supplied plants decreased significantly with increase in NaCl concentration, whereas that of NO₃-supplied plants remained unaffected. There was no significant effect of NaCl on leaf or root P, but the NO₃-supplied plants had significa concentration of leaf P than that of NH₄-supplied plants at varying salt treatments. Salinity of the rooting med did not show any significant effect on Na⁺ concentrations of leaves or roots of plants subjected to two differen N. NH₄-treated plants generally had greater concentrations of Cl^– in leaves and roots and lower K⁺ content in leaves than NO₃-supplied plants. Ca²⁺ concentrations of leaves and roots and Mg²⁺ concentrations of leaves decreased in NH₄-supplied plants due to NaCl, but they remained unaffected in NO₃-treated plants.     

Effect of ammonium ion on morphogenesis from cultured cotyledon explants ofPanax ginseng

Cotyledon expiants ofPanax ginseng were cultured on modified Murashige and Skoog medium with various concentrations of NH₄C1 and KNO,. Morphogenesis such as somatic embryo, embryogenic callus, or adventitious root formation from cotyledon expiants differently occurred according to the concentrations of NH/ and NO₃. Somatic embryos were actively formed in a moderate concentration of NH₄ ⁺ (20 mM) in combination of NO₃, but in a high concentration of NH₄ ⁺ (60 mM), only embryogenie calli were formed. In little or no NH₄ ⁺, adventitious roots were formed at a high rate. The influence of NO₃ on those morphogenesis was slight but combination of NO₃ with NH₄ ⁺ was indispensable since the cotyledon expiants were necrotized on medium containing only NH₄ ⁺ as a nitrogen source. Histological observation revealed that somatic embryo and embryogénie callus formation occurred from the same origin (cotyledon epidermis), whereas, adventitious roots were originated from the cells near vascular strands.     

Root growth as a function of ammonium and nitrate in the root zone

We examined the effect of soil NH₄⁺ and NO₃^? content upon the root systems of field-grown tomatoes, and the influence of constant, low concentrations of NH₄⁺ or NO₃^? upon root growth in solution culture. In two field experiments, few roots were present in soil zones with low extractable NH₄⁺ or NO₃^?; they increased to a maximum in zones having 2μg-N NO₃^? g^?1 soil and 6 μg-N NO₃⁼ g^?1 soil, but decreased in zones having higher NH₄⁺ or NO₃^? levels. Root branching was relatively insensitive to available mineral nitrogen. Plants maintained in solution culture at constant levels of NH₄⁺ or NO₃^?, had similar shoot biomass, but all root parameters – biomass, length, branching and area – were greater under NH4 nutrition than under NO₃^?. These results suggest that the size of root system depends on a functional equilibrium between roots and shoots (Brouwer 1967) and on the balance between soil NH₄⁺ and NO₃^?.     

Rates of rhizodeposition and ammonium depletion in the rhizosphere of axenic oat roots

Summary A multiple split root chamber and artificial soil were developed which allowed for maintenance of axenic conditions and for the isolation of soil from specific regions of single roots. A sterile minirhizotron was used to measure patterns and rates of root extension under sterile conditions. Carbon and nitrogen distributions in the rhizosphere of sterile oat roots were measured in combination with rates of root elongation to calculate specific rates of rhizodeposition and ammonium nitrogen uptake. The highest rates of rhizodeposition C production and N depletion occurred at the root tip (first day segment). Rhizodeposited soluble and insoluble C compounds represented up to 50% of the standing root biomass C. Within 48 hours after root entry, levels of rhizosphere ammonium-N decreased by 40–50%. The results were summarized in a simple model of root growth, rhizodeposition, and NH ₄ ⁺ −H uptake. From a dissertation by the senior author submitted to the Academic Faculty of Colorado State University in partial fulfillment of the requirements for the Ph.D. degree.     

Effects of gypsum on the uptake,assimilation and cycling of 15N-labelled ammonium and nitrate-N by perennial ryegrass

Solution culture studies have shown that plant uptake of NH₄ ⁺ and NO₃ ^- can be improved by increasing the concentration of Ca²⁺ in the root environment: the same may be true for grass grown in soil culture. An experiment was set up to see whether gypsum (CaSO₄ 2H₂O) increased the rate at which perennial ryegrass absorbed ¹⁵NH₄ ⁺ and ¹⁵NO₃ ^- from soil.The results demonstrated that gypsum increases the rates of uptake of both NH₄ ⁺ and NO₃ ^- by perennial ryegrass. However because there was little potential for mineral-N loss from the experimental system, either by gaseous emission or by N immobilization, long term improvements in fertilizer efficiency were not observed. Nitrogen cycling from shoots to roots commenced once net uptake of N into plants had ceased. Labelled N transferred thus to roots underwent isotopic exchange with unlabelled soil N. It was suggested that this exchange of N might constitute an energy drain from the plant, if plant organic N was exchanged for soil inorganic N. The fact that the exchange occurred at all cast doubt on the suitability of the ¹⁵N-isotope dilution technique for assessing fertilizer efficiency in medium to long term experiments. There was evidence that the extra NO₃ ^--N taken up by plants on the all-nitrate treatments as a result of gypsum application, was reduced in root tissue rather than in shoots, but to the detriment of subsequent root growth and N uptake.     

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).     

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

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