Seasonal distribution of topsoil ammonium and nitrate under legume-grass and grass swards

Losses of soil N through leaching and N₂ fixation by legumes often are related to soil nitrate concentration. The seasonal distribution of soil ammonium and nitrate concentrations under ungrazed legume-grass and grass swards were evaluated on two experiments that were established in 1983 (Exp. 1) and in 1984 (Exp. 2). Treatments were white clover (Trifolium repens L.) (WC), red clover (Trifolium pratense L.) (RC), and birdsfoot trefoil (Lotus corniculatus L.) (BT), each grown with tall fescue (Festuca arundicacea Schreb.) (TF) at two legume proportions, and a pure stand of TF. The concentrations of both forms of N were measured in the top 20-cm layer during 2 years in Exp. 1 and for 1 year in Exp. 2. The concentrations of nitrate and ammonium were least in winter and spring, and greatest in summer. The concentration of nitrate for the mixtures decreased in the order WC-TF, RC-TF, and BT-TF in both summers of Exp. 1 but there were no mixture differences in Exp. 2. The concentration of soil ammonium was not affected by the treatments applied. We conclude that the concentration of soil nitrate usually was small for these swards but became greater and often dependent on species and legume proportion during summer. The concentration of soil ammonium also was greater in summer but was not affected by species or legume proportion. Journal of Paper no. J.-13359 of the Iowa Agric. and Home Econ. Exp Stn., Ames. Project 2281. Supported in part by the Facultad de Agronomía, Montevideo, Uruguay. Journal of Paper no. J.-13359 of the Iowa Agric. and Home Econ. Exp Stn., Ames. Project 2281. Supported in part by the Facultad de Agronomía, Montevideo, Uruguay.     

Nitrogen acquisition in four co-existing species from an upland acidic grassland

The concentration of both nitrate and ammonium nitrogen was measured in soil taken from an upland acidic (pH 4.5) grassland habitat, containing four co-existing species, Deschampsia flexuosa (L.) Trin., Festuca ovina L., Juncus squarrosus L. and Nardus stricta L. Both nitrate and ammonium nitrogen were found to be present in the soil, in similarly small quantities. The effect of both sources of nitrogen on relative growth rate was studied, and an attempt was made to determine whether nitrate or ammonium nitrogen is the immediate source of nitrogen for these plants using assays of nitrate reductase (EC 1.6.6.2) and ammonium uptake. All four species showed larger growth rates on the same concentration of ammonium nitrogen compared to nitrate nitrogen. All species showed low activities of leaf nitrate reductase, even in plants grown on 18 mol nitrate m⁻³. Ammonium uptake activity appeared to be higher in species which showed the lowest nitrate reductase activity and least response to increasing nitrate concentration in the growth medium.     

Response of 12 weedy species to elevated CO2 in low-phosphorus-availability soil

We conducted an experiment on responses of weedy species from an orchard ecosystem to elevated CO₂ (700–800 μmol mol⁻¹) under low phosphorus (P) soil in an environment-controlled growth chamber. Twelve local weedy species, Poa annua L., Lolium perenne L., Avena fatua L., Vicia cracca L., Medicago lupulina L., Kummerowia striata (Thunb.) Schindl., Veronica didyma Ten., Plantago virginica L., Gnaphalium affine D.Don., Echinochloa crusgalli var. mitis (L.) Beauv., Eleusine indica (L.) Gaertn. and Setaria glauca (L.) P. Beauv., grouped into four functional groups (C₃ grass, C₃ forb, legume and C₄ grass), were used in the experiment. The total plant biomass, P uptake, and mycorrhizal colonization were measured. The results showed that the total biomass of the 12 weedy species tended to increase under elevated CO₂. But changes in the total biomass under elevated CO₂ significantly differed among functional groups: legumes showed the greatest increase in the total biomass of all functional groups, following the order C₃ forbs > C₄ grasses > C₃ grasses. Elevated CO₂ significantly increased mycorrhizal colonization and P uptake of legumes, C₃ forbs and C₄ grasses but did not change C₃ grasses. Positive correlations between mycorrhizal colonization and shoot P concentration, and between total P uptake and total biomass were found under elevated CO₂. The results suggested that the interspecific difference in CO₂ response at low P availability was caused by the difference in CO₂ response in mycorrhizae and P uptake. These differences among species imply that plant interaction in orchard ecosystems may change under future CO₂ enrichment.     

Nitrate and ammonium uptake and solution pH changes for Al-tolerant and Al-sensitive sorghum (Sorghum bicolor) genotypes grown with and without aluminium

Plant tolerance to Al toxicity has been associated with differential nitrate and ammonium uptake and solution pH changes. Sorghum [Sorghum bicolor (L.) Moench] genotypes with tolerance (SC283) and sensitivity (ICA-Nataima) to Al toxicity were grown with different nitrate/ammonium ratios (39:1, 9:1, and 3:1) at 0 and 300 μM Al to determine genotypic differences in nitrate and ammonium uptake, changes in nutrient solution pH, and relationships of these traits to Al toxicity tolerance in the genotypes. ICA-Nataima had greater reductions in nitrate and ammonium uptake than SC283 when plants were grown with Al, but SC283 had higher nitrate and ICA-Nataima had higher ammonium uptake when plants were grown without Al. Differences in nitrate and ammonium uptake were associated with changes in solution pH; pH decreased as long as ammonium was in solution and increased when ammonium was depleted from solution. Greater changes in solution pH occurred when plants were grown with 39:1 compared to 9:1 and 3:1 nitrate/ammonium ratios. Solution pH values were lower when plants were grown with than without Al. The genotypes maintained their relative differences in Al toxicity tolerance when plants were grown separately or together in the same container with Al and with different nitrate/ammonium ratios.     

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.     

Differences in nitrate and ammonium uptake between Scots pine and European larch

In forest soils, ammonium is usually the predominant form of inorganic nitrogen. However, the capacity of trees to utilize both NO₃ ^- and NH₃ ⁺ may provide greater flexibility in responding to changes of nitrogen supply from the environment. Such capacity has been studied in seedlings of Scots pine (Pinus sylvestris L.) and European larch (Larix decidua Mill.) grown in the presence or absence of either nitrate or ammonium. Nitrate-induced plants showed a higher nitrate uptake rate than non-induced plants; this difference was almost negligible after 24 h of exposure to NO₃ ^-. Ammonium uptake in both species was consistently higher than that of nitrate, regardless of prior nitrogen provision. In both nutrient conditions, larch showed a more efficient transport system in comparison with Scots pine, with higher ammonium and nitrate uptake rates in both induced and non-induced plants. This was consistent also with the activity of nitrate reductase, measured in vivo in roots and leaves. This revised version was published online in June 2006 with corrections to the Cover Date.     

Nitrate and ammonium absorption by plants growing at a sufficient or insufficient level of phosphorus in nutrient solutions

Summary Absorption of nitrate and ammonium was studied in water culture experiments with 4 to 6 weeks old plants of barley (Hordeum vulgare L.), buckwheat (Fagopyrum esculentum L. Moench) and rape (Brassica napus L.). The plants were grown in a complete nutrient solution with nitrate (5.7±0.2 mM) or nitrate (5.6±0.2 mM) + ammonium (0.04±0.02 mM). The pH of the nutrient solution was kept at 5.0 using a pH-stat. It was found that phosphorus deficiency reduced the rate of nitrate uptake by 58±3% when nitrate was the sole N source and by 83±1% when both nitrate and ammonium were present. The reduction occurred even before growth was significantly impeded by P deficiency. The inhibition of the uptake of ammonium was less,i.e. ammonium constituted 10±1% of the total N uptake in the P sufficient plants and 30±5% in the P deficient plants. The reduction of nitrate absorption greatly decreased the difference between the uptake of anions and cations. It is suggested that P deficiency reduced the assimilation of NO ₃ ^– into the proteins, which might cause a negative feedback on NO ₃ ^– influx and/or stimulate NO ₃ ^– efflux.     

Nitrogen uptake in relation to excess supply and its effects on the lichens<Emphasis Type="Italic"> Evernia prunastri</Emphasis> (L.) Ach and<Emphasis Type="Italic"> Xanthoria parietina</Emphasis> (L.) Th. Fr.

The aim of this study was to compare the physiological responses to increased nitrogen (N) supply between the nitrophytic lichen Xanthoria parietina (L.) Th. Fr. and the acidophytic lichen Evernia prunastri (L.) Ach. The two lichens were exposed to a weekly dosage of 0.05, 0.1, 0.2, 0.6 or 2.4 g N m^–2 for 2 months, administered as NH₄NO₃ dissolved in artificial rainwater (1 l m^–2). After the treatments, in vivo chlorophyll a fluorescence was determined to assess vitality; concentrations of total N, ammonium, nitrate and dominant amino acids, including glutamate, glutamine and arginine, were quantified in order to follow changes in N status; and the polyols ribitol, arabitol and mannitol were quantified to follow changes in the lichens carbon (C) status. The uptake of N was quantified by labelling the fertiliser with ¹⁵N in the ammonium position; chlorophyll a was used as an indirect marker for algal activity, and ergosterol as an indirect marker of fungal activity. Nitrogen uptake was higher in E. prunastri than in X. parietina, although the latter species may have used the mannitol reserves to obtain C skeletons and energy for N assimilation. Chlorophyll a and ergosterol concentrations remained unaltered in X. parietina irrespective of N dosage while ergosterol decreased with increasing N uptake in E. prunastri. The latter species had accumulated a large pool of ammonium at the highest N dosage, whilst in X. parietina a significant nitrate pool was instead observed. Taken together, these short-term responses to high N supply observed in the two lichens, and the differences between them, can partly explain the higher tolerance of X. parietina towards increased atmospheric N levels.     

Influx,efflux and net uptake of nitrate in Quercus suber seedlings

Nitrate influx, efflux and net nitrate uptake were measured for the slow-growing Quercus suber L. (cork-oak) to estimate the N-uptake efficiency of its seedlings when grown with free access to nitrate. We hypothesise that nitrate influx, an energetically costly process, is not very efficiently controlled so as to avoid losses through efflux, because Q. suber has relatively high respiratory costs for ion uptake. Q. suber seedlings were grown in a growth room in hydroponics with 1 mM NO₃ ^-. Seedlings were labelled with ¹⁵NO₃ ^- in nutrient solution for 5 min to measure influx and for 2 h for net uptake. Efflux was calculated as the difference between influx and net uptake. Measurements were made in the morning, afternoon and night. The site of nitrate reduction was estimated from the ratio of NO₃ ^- to amino acids in the xylem sap; the observed ratio indicated that nitrate reduction occurred predominantly in the roots. Nitrate influx was always much higher than net acquisition and both tended to be lower at night. High efflux occurred both during the day and at night, although the proportion of ¹⁵NO₃ ^- taken up that was loss through efflux was proportionally higher during the night. Efflux was a significant fraction of influx. We concluded that the acquisition system is energetically inefficient under the conditions tested. This revised version was published online in June 2006 with corrections to the Cover Date.     

Differences in nitrate uptake of species from habitats rich or poor in nitrogen when grown at low nitrate concentrations,using a new growth technique

Summary A method is described for culturing plants at extremely low nutrient concentrations. Using a Braun infusion pump, a fixed amount of nitrate or ammonium was supplied continuously to plants growing in a culture vessel at a rate limiting the uptake of the plants. At a very low nitrogen concentration an equilibrium was established where uptake rate of the plants is equal to the rate of supply by the infusion pump. The nitrogen concentrations reached appeared to be in the order of 1 μM. The method compared the nitrate uptake byHypochaeris radicata L.ssp.radicata, H. radicata ssp.ericetorum Van Soest andUrtica dioica L. and ammonium uptake byH. radicata ssp.radicata andH. radicata ssp.ericetorum. Plants were cultivated in monocultures or in mixed cultures (two species per culture vessel). For the mixed cultures competition for nitrate (or ammonium) between the species was maintained for long periods. The capacities of the uptake systems of two subspecies ofH. radicata from places different in nitrogen supply and pH were adapted equally well to both low nitrate and low ammonium concentrations. Apparently factors other than nitrogen uptake play a part in the distribution of the subspecies. The capacity of the uptake system ofU. dioica, a nitrophilous species, was lower than that ofH. radicata ssp.radicata, a species from places poorer in nitrogen. This difference is related to the different distribution of the two species in the field. The present results are compared with those of previous experiments where K_m and V_max were measured and the significance of both parameters is discussed.     

Kinetics of nitrate uptake by New Zealand marine macroalgae and evidence for two nitrate transporters in <Emphasis Type="Italic">Ulva intestinalis</Emphasis> L.

Kinetic constants were determined for nitrate uptake in three species, Pterocladiella capillacea (S.G. Gmelin) Santelices et Hommersand (Rhodophyceae, Gelidiales), Ulva intestinalis L. (Chlorophyceae, Ulvales) and Xiphophora chondrophylla (Turner) Montagne ex Harvey (Phaeophyceae, Fucales), of New Zealand macroalgae, with K _m values ranging from 10 to 17 μM and V _max values from 3 to 65 μmole g⁻¹ dry weight h⁻¹. There was no effect of ammonium on nitrate uptake by Pterocladiella capillacea or Xiphophora chondrophylla. Ammonium inhibited nitrate uptake by 40% in Ulva intestinalis from a site with relatively low seawater ammonium concentrations. In contrast, U. intestinalis from an ammonium-enriched site had lower rates of nitrate uptake that were insensitive to inhibition by ammonium. It is suggested that there are (at least) two transport systems for nitrate in U. intestinalis; a constitutive transporter, which is insensitive to ammonium, and a transporter that is sensitive to ammonium inhibition and down-regulation by ammonium; the implications of this for our understanding of macroalgal blooms is discussed. Handling editor: K. Martens     

Measuring Nitrogen and Phosphorus Uptake by Intact Roots of Mature Acer saccharum Marsh., Pinus resinosa Ait., and Picea abies (L.) Karst

A common method for measuring uptake by intact roots in situ is the depletion method, wherein intact fine roots are separated from soil and placed in nutrient solution. The difference between initial and final amounts of nutrient in solution is attributed to root uptake. Variations on this method include applying pretreatment solutions, training roots to grow into bags or trays, and varying concentrations of nutrient solution. We tested whether variations in methods affected measured net uptake rates of NH ₄ ⁺ , NO ₃ ⁻ , and PO ₄ ³⁻ . Intact roots of 60 year-old sugar maple (Acer saccharum Marsh.), red pine (Pinus resinosa Ait.), and Norway spruce (Picea abies (L.) Karst.) were given one of four treatments prior to measuring net uptake. “Trained” roots were grown in a sand-soil mixture. “Recovered” roots were excavated and allowed to recover in nutrient solution for two or four days (“two-day recovery” and “four-day recovery”, respectively). “No recovery” roots were excavated and used immediately in experiments. We exposed roots to three concentrations of nutrient solutions to observe the effects of initial nutrient solution concentration. Initial nutrient solution concentration was an important source of variation in measured uptake rates, and N uptake was stimulated by low antecedent concentrations. We found no significant differences in net uptake rates between pretreatments for any of the species studied, indicating that our pretreatments were not effective in improving measurement of uptake. Such pretreatments may not be necessary for measuring net uptake and may not hinder the comparison of rates measured using variations of the depletion method.     

Fate and efficiency of N fertilizers applied to pearl millet in Niger

Field studies were conducted in Niger using ¹⁵N-labeled fertilizers to assess the fate and efficiency of fertilizer N in pearl millet (Pennisetum glaucum [L.] R.Br.) production. Total plant uptake of fertilizer N was low in all cases (20%–37%), and losses were severe (25%–53%). The majority of N remaining in the soil was found in the 0- to 15-cm layer though some enrichment at lower depths was found when the N fertilizer was calcium ammonium nitrate (CAN). In a comparison of urea placement methods (band, broadcast, or point placement), no significant differences in ¹⁵N uptake or yield were noted though point placement did exacerbate ¹⁵N loss. The mechanism of N loss is believed to have been ammonia volatilization. Yields were similar whether urea or CAN was used, but ¹⁵N uptake from CAN was higher. A statistical model was developed relating millet yield and N response to midseason rainfall. In drought years, no N response was found, whereas in years of good rainfall a response was found of 15 kg grain for each kilogram of N applied (at 30 kg N ha^-1 rate).     

Influence of ammonium,nitrate and nitrite on the performance of the pure culture of <Emphasis Type="Italic">Acinetobacter junii</Emphasis>

The influence of different concentration ranges (0–500 mg/L) of ammonium, nitrate and nitrite presence in the wastewater, on the performance of the pure culture of phosphate-accumulating bacterium Acinetobacter junii in the anaerobic and aerobic conditions, was investigated. A. junii was able to use ammonium and nitrate salts as the source of nitrogen, unlike in the case of nitrite salt. Comparing to the control reactors with the peptone and yeast extract as the sources of nitrogen, at the lowest tested concentration of ammonium and nitrate the performance of the system was inhibited due to the nitrogen deficit in the wastewater, while at the highest concentration it was positively influenced. Nitrite in all concentrations detrimentally affected the phosphate release and uptake rates, chemical oxygen demand uptake rates, nitrogen uptake rates, as well as multiplication of A. junii. The higher the nitrite concentration, the more pronounced was the effect. At the highest nitrite concentration tested a complete failure of the system was observed.     

Nitrate reductase activity of free-living and symbiotic uptake hydrogenase-positive and uptake hydrogenase-negative strains of Bradyrhizobium japonicum

The nitrate reductase activity (NR) of selected uptake hydrogenase-positive (hup ⁺) and uptake hydrogenase-negative (hup ^-) strains of Bradyrhizobium japonicum were examined both in free-living cells and in symbioses with Glycine max L. (Marr.) cv. Williams. Bacteria were cultured in a defined medium containing either 10 mM glutamate or nitrate as the sole nitrogen source. Nodules and bacteriods were isolated from plants that were only N₂-dependent or grown in the presence of 2 mM KNO₃. Rates of activity in nodules were determined by an in vivo assay, and those of cultured cells and bacteriods were assayed after permeabilization of the cells with alkyltrimethyl ammonium bromide. All seven strains examined expressed NR activity as free-living cells and as symbiotic forms, regardless of the hup genotype of the strain used for inoculation. Although the presence of nitrate increased nitrate reduction by cultures cells and nodules, no differences in NR activity were observed between bacteroids isolated from nodules of plants fed with nitrate or grown on N₂-fixation exclusively. Cultured cells, nodules and bacteriods of strains with hup ^- genotype (USDA 138, L-236, 3. 15B3 and PJ17) had higher rates of NR activity than those with hup ⁺ genotype (USDA 110, USDA 122 DES and CB1003). These results suggest that NR activity is reduced in the presence of a genetic determinant associated with the hup region of B. japonicum.Abbreviations EDTA ethylene-diamine tetraacetic acid - Hup hydrogen uptake - MOPS 3-(N-morpholino)-propane sulfonic acid - NR nitrate reductase - PVP polyvinyl-polypyrrolidone - Tris Tris(hydroxymethyl)-aminomethane     

Rhizospheric NO affects N uptake and metabolism in Scots pine (Pinus sylvestris L.) seedlings depending on soil N availability and N source

We investigated the interaction of rhizospheric nitric oxide (NO) concentration (i.e. low, ambient or high) and soil nitrogen (N) availability (i.e. low or high) with organic and inorganic N uptake by fine roots of Pinus sylvestris L. seedlings by ¹⁵N feeding experiments under controlled conditions. N metabolites in fine roots were analysed to link N uptake to N nutrition. NO affected N uptake depending on N source and soil N availability. The suppression of nitrate uptake in the presence of ammonium and glutamine was overruled by high NO. The effects of NO on N uptake with increasing N availability showed different patterns: (1) increasing N uptake regardless of NO concentration (i.e. ammonium); (2) increasing N uptake only with high NO concentration (i.e. nitrate and arginine); and (3) decreasing N uptake (i.e. glutamine). At low N availability and high NO nitrate accumulated in the roots indicating insufficient substrates for nitrate reduction or its storage in root vacuoles. Individual amino acid concentrations were negatively affected with increasing NO (i.e. asparagine and glutamine with low N availability, serine and proline with high N availability). In conclusion, this study provides first evidence that NO affects N uptake and metabolism in a conifer.     

Kinetic parameters of nitrate uptake by different catch crop species: effects of low temperatures or previous nitrate starvation

The pollution of aquifers by NO^?₃ in temperate environments is aggravated by farming practices that leave the ground bare during winter. The use of catch crops during this time may decrease nitrate loss from the soil. Nitrate uptake by several catch crop species (Brassica napus L., Sinapis alba L., Brassica rapa L., Raphanus sativus L., Trifolium alexandrinum L., Trifolium incarnatum L., Phacelia tanacetifolia Benth., Lolium perenne L., Lolium multiflorum Lam. and Secale cereale L.) was here studied in relation to transpiration rate and low temperatures applied to the whole plant or to roots only. The Michaelis constant (K_m), maximum uptake rate (V_max), time of induction and contributions of inducible and constitutive mechanisms were estimated from measurements of NO^?₃ depletion in the uptake medium. There were large differences between species, with K_m (μM) values ranging between 5.12 ± 0.64 (Trifolium incarnatum) and 36.4 ± 1.97 (Lolium perenne). Maximum NO^?₃ uptake rates expressed per unit root weight were influenced by ageing, temperature and previous NO^?₃ nutrition. They were also closely correlated with water flow through the roots and with shoot/root ratio of these species. The combined results from all species and treatments showed that V_max increased with shoot/root ratio, suggesting a regulatory role for the shoots in NO^?₃ uptake. Overall, the results showed a great diversity in NO^?₃ uptake characteristics between species in terms of kinetic parameters, contribution of the constitutive system (100% of total uptake in ryegrass, nil in Fabaceae) and time of induction.     

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.     

13C AND 15N UPTAKE BY MARINE PHYTOPLANKTON. I. INFLUENCE OF NITROGEN SOURCE AND CONCENTRATION IN LABORATORY CULTURES OF DIATOMS1

Two species of marine diatoms, Skeletonema costatum (Grev.) Cleve and Phaeodactylum tricornutum Bohlin were grown in batch and continuous cultures on four different nitrogen compounds (nitrate, nitrite, ammonium, urea). Carbon and nitrogen uptake were measured simultaneously with the stable isotopes ¹³C and ¹⁵N. Nitrogen uptake generally increased with N concentration in the medium, but no clear difference existed between the N sources. Carbon fixation was decreased for up to 5 h following the addition of the N compound. Nitrite generally had the greatest inhibitory effect on C uptake. Carbon-to-nitrogen uptake ratios decreased with increasing dissolved N concentration, becoming lower than one in nutrient-limited cultures. In contrast, batch cultures exhibited C:N uptake ratios greater than one. These effects are essentially short-term and differ from long-term influences of the N source on the cellular chemical composition.     

Estimation of residual N effect of faba bean and pea on two succeeding cereals using15N methodology

A field experiment was conducted using¹⁵N methodology to study the effect of cultivation of faba bean (Vicia faba L.), pea (Pisum sativum L.) and barley (Hordeum vulgare L.) on the N status of soil and their residual N effect on two succeeding cereals (sorghum (Sorghum vulgare) followed by barley). Faba bean, pea and barley took up 29.6, 34.5 and 53.0 kg N ha^–1 from the soil, but returned to soil through roots only 11.3, 10.8 and 5.7 kg N ha^–1, respectively. Hence, removal of faba bean, pea and barley straw resulted in a N-balance of about –18, –24, and –47 kg ha^–1 respectively. A soil nitrogen conserving effect was observed following the cultivation of faba bean and pea compared to barley which was of the order of 23 and 18 kg N ha^–1, respectively. Cultivation of legumes resulted in a significantly higher A_N value of the soil compared to barley. However, the A_N of the soil following fallow was significantly higher than following legumes, implying that the cultivation of the legumes had depleted the soil less than barley but had not added to the soil N compared to the fallow. The beneficial effect of legume cropping also was reflected in the N yield and dry matter production of the succeeding crops. Cultivation of legumes led to a greater exploitation of soil N by the succeeding crops. Hence, appreciable yield increases observed in the succeeding crops following legumes compared to cereal were due to a N-conserving effect, carry-over of N from the legume residue and to greater uptake of soil N by the succeeding crops when previously cropped to legumes.