Nitrogen uptake by rhizomes of the clonal sedge Carex bigelowii: a previously overlooked nutritional benefit of rhizomatous growth

Studies on the rhizomatous clonal sedge Carex bigelowii , at a fellfield site in subarctic Swedish Lapland examined the ecological potential of rhizomes as nitrogen uptake systems. Direct application of a solution of ¹⁵NH₄¹⁵NO₃ to the rhizomes of C. bigelowii significantly enriched the ¹⁵N content of the plant tissue. The pattern of enrichment indicated movement of labelled nitrogen into rhizomes and adjoining tissue (including roots and shoots), showing both uptake and translocation via the rhizome system. There was a gradient of decreasing ¹⁵N enrichment with increasing distance from the point of labelling, and estimation of the total mass of nitrogen taken up via labelled rhizomes showed low levels and rates of uptake. Quantification of the size of rhizome and root systems of C. bigelowii at the study site indicates that 80% of the living biomass may be below ground, and that the surface area of the rhizome system of C. bigelowii is approx. two-thirds that of the roots. The rhizome system of C. bigelowii can therefore act as a route for nitrogen uptake, with the potential to exploit almost as great a volume of soil as the root system. This mechanism of nitrogen uptake may play an important role in the Arctic, where many species have a clonal, rhizomatous growth form. In addition, plant growth in many arctic ecosystems is limited by low soil nutrient availability, a result of low temperatures (leading to slow soil decomposition rates) and patchy resource distribution. Nitrogen uptake via rhizomes may provide plants with the capacity to take advantage of transient nutrient supplies, and may partly compensate for the cost of developing and maintaining persistent rhizome networks in ecosystems where nutrient resources are in short supply.     

Metabolism of nitrate and ammonium in seedlings of Norway spruce (Picea abies) measured by in vivo 14N and 15N NMR spectroscopy

In vivo ¹⁵N and ¹⁴N nuclear magnetic resonance spectroscopy was used to investigate the assimilation of nitrate and ammonium in seedlings of Norway spruce (Picea abies [L.] Karst.). The main objective was to study accumulation of free NH+₄ and examine to what extent the nitrogen source affects the composition of the free amino acid pools in roots, stems and needles. NH+₄ concentrations in plants growing in the presence of 0.5–50 mM ammonium were quantified using ¹⁴N NMR. The NH+₄ values in tissues ranged from 6 to 46 μmol (g fresh weight)^?1. with highest concentrations in roots and needles. The tissue NH+₄ peaked at 5.0 mM NH+₄ in the medium. and failed to increase when NH+₄ in the medium was increased to 50 mM, indicating metabolic control of the concentration of this cation in tissues. The ¹⁴N NMR spectra were used to estimate pH of the NH+₄ storage pools. Based on the pH sensitivity of the quintet of ¹⁴NH+₄ resonance, we suggest that the pH of the ammonium storage compartments in the roots and stems should be 3.7–3.8, and in needles 3.4–3.5, representing extremely low pH values of the tissue. ¹⁵N from nitrate or ammonium was first incorporated into the amide group of glutamine and then into α-amino groups, confirming that the glutamine synthetase/ glutamate synthase cycle is the major route of nitrogen assimilation into amino acids and thus plays a role in lowering the levels of NH+₄ in the cytoplasm. NH+₄ can also be assimilated in roots in plants growing in darkness. The main ¹⁵N-labelled amino acids were glutamine. arginine and alanine. Almost no ¹⁵N signals from needles were observed. Double labelling (δN + w, wN) of arginine is consistent with the operation of the ornithine cycle, and enrichment indicates that this cycle is a major sink of newly assimilated nitrogen. Nitrogen assimilation in roots in the presence of added methionine sulphoximine and glutamate indicated the catabolic action of glutamate dehydrogenase. The ¹⁵N NMR spectra of plants grown on ¹⁵N-urea showed a marked increase in the labelling of ammonium and glutamine. indicating high urease activity. Amino acids were also quantified using high pressure liquid chromatography. Arginine was found to be an important transport form of nitrogen in the stem.     

Natural 15N enrichment of amide-exporting legume nodules

The natural ¹⁵N abundance of amide-exporting nodules was compared to that of shoots in 12 plant species. Nodules were statistically less abundant in ¹⁵N than shoots in one of three cultivars of Pisum sativum L., in Vicia faba L. and in Medicago sativa L., but the ¹⁵N depletion of nodules was very samall. Nodules were statistically more abundant in ¹⁵N than shoots in Trifolium pratense L., depending on time during the growing season, Cyamopsis tetragonaloba L. Taub. and 7 Lupinus species, but the enrichment was small except for C. tetragonalova and 6 Lupinus species. Nodules of 3 Lupinus species infected with Rhizobium lupini isolated from Lupinus subcarnosa Hook, were only slightly enriched in ¹⁵N, but nodules of two of these species were substantially enriched in ¹⁵N when infected with a mix of other Rhizobium lupini strains. The third species, L. texensis Hook., was not infected by this mix of strains. Differences in ¹⁵N abundance between nodules and other tissues of amide-exporting and ureide-exporting nodules from several studies are tabulated. All ureide-exporting nodules in this tabulation are enriched in ¹⁵N. Amide-exporting nodules are considerably more variable in this regard. These results confirm that events associated with ureide synthesis or transport cannot be the sole cause of the substantial ¹⁵N enrichment seen in nodules.     

Nitrate and ammonium influxes in soybean (Glycine max) roots: direct comparison of 13N and 15N tracing

We compared influxes and internal transport in soybean plants (Glycine max cv. Kingsoy) of labelled N from external solutions where either ammonium or nitrate was labelled with the stable isotope¹⁵N and the radioactive isotope¹³N. The objective was to see whether mass spectrometric determinations of tissue ¹⁵N content were sufficiently sensitive to measure influxes accurately over short time periods. Our findings were as follows. (1) There was a close quantitative correspondence between estimates of N influx of individual plants using ¹⁵N or ¹³N measurements with either NO_3/^? or NH₄⁺ at 4 or 2 mol^?3, respectively in the external solution. (2) Transport to the shoot of N from NO₃ absorbed over a 5–15 min period could be monitored when the external NO₃^? concentration ranged from 0–05 to 4 mol m^?3. NH₄⁺ as the N source labelled shoot tissue more slowly, and estimates of the transport between root and shoot could be made only with ¹³N. (3) Influx of NO₃^? into root tissue could be measured by ¹⁵N enrichment after 5–10 min at concentrations approaching the probable K_M of the high-affinity transport system. (4) There was some indication of isotope discrimination, especially with respect to the movement of labelled N to the shoot, when NO₃^? is the N source. For many purposes, ¹⁵N tracing can be used satisfactorily to estimate influxes of both NO₃^? and NH₄⁺ in soybean roots. Use of the short-lived radio nuclide ¹³N remains the method of choice for more refined measurements of internal distribution and assimilation.     

15N natural abundance of vascular rainforest epiphytes: implications for nitrogen source and acquisition

The foliar natural abundance of ¹⁵N was analysed to compare the potential nitrogen sources of vascular rainforest epiphytes and associated soil-rooted trees. Leaves of epiphytes collected from six rainforest communities in Brazil, Australia and the Solomon Islands were depleted in ¹⁵N relative to the trees at each site. Epiphyte δ¹⁵N was as low as -6.4%_o, while trees were generally enriched in ¹⁵N (0.7 to 3.5%_o). These results indicate either that epiphytes use nitrogen sources depleted in ¹⁵N or that discrimination against ¹⁵N is an intrinsic function of epiphyte physiology. At three sites, epiphytes could be grouped into those having both low δ¹⁵N and low leaf-nitrogen content and those possessing both high δ¹⁵N and high leaf-nitrogen content. The second group had δ¹⁵N values in the range sometimes attributable to N₂ fixation (-2 to 0%o). There was no correlation between growth form and δ¹⁵N. It is concluded that epiphytes may utilize ¹⁵N-depleted nitrogen from atmospheric deposition and N₂ fixation.     

Traffic exposure increases natural 15N and heavy metal concentrations in mosses

Mosses have been used as biomonitors of atmospheric pollution for some years, but few studies have been carried out on the effect of NO_x emissions from traffic on moss tissue N. Eight species of moss (102 samples) growing on walls or roofs next to roads exposed to different traffic densities were collected from urban and rural sites in the UK. The shoots were sampled for total N, their stable isotope ¹⁵N/¹⁴N content (δ¹⁵N) and heavy metal content (Pb, Zn). There was a lack of correlation between tissue total N and traffic exposure, but a very good correlation between traffic exposure and tissue δ¹⁵N. Plants collected near motorways or busy urban roads had δ¹⁵N values ranging between +6 and −1‰, while in rural areas with hardly any traffic these ranged from −2 to −12‰. In a separate survey of mosses, the average δ¹⁵N of shoots from busy roadsides in London was +3.66‰, whereas from samples collected from farm buildings near poultry or cattle pens it was −7.8‰. This indicates that the two main atmospheric N sources, NO_x and NH_x, have different δ¹⁵N signatures, the former tending to be positive and the latter negative. Tissue concentrations of both Pb and Zn show a strong positive correlation with traffic exposure, with Zn in particular being greater than Pb. The results are discussed with regard to the use of moss tissue Zn as a means for monitoring or mapping pollution from vehicles, and of δ¹⁵N as an aid to distinguish between urban (NO_x) and rural (NH_x) forms of N pollution.     

Widespread foliage δ15N depletion under elevated CO2: inferences for the nitrogen cycle

Leaf ¹⁵N signature is a powerful tool that can provide an integrated assessment of the nitrogen (N) cycle and whether it is influenced by rising atmospheric CO₂ concentration. We tested the hypothesis that elevated CO₂ significantly changes foliage δ¹⁵N in a wide range of plant species and ecosystem types. This objective was achieved by determining the δ¹⁵N of foliage of 27 field‐grown plant species from six free‐air CO₂ enrichment (FACE) experiments representing desert, temperate forest, Mediterranean‐type, grassland prairie, and agricultural ecosystems. We found that within species, the δ¹⁵N of foliage produced under elevated CO₂ was significantly lower (P<0.038) compared with that of foliage grown under ambient conditions. Further analysis of foliage δ¹⁵N by life form and growth habit revealed that the CO₂ effect was consistent across all functional groups tested. The examination of two chaparral shrubs grown for 6 years under a wide range of CO₂ concentrations (25–75 Pa) also showed a significant and negative correlation between growth CO₂ and leaf δ¹⁵N. In a select number of species, we measured bulk soil δ¹⁵N at a depth of 10 cm, and found that the observed depletion of foliage δ¹⁵N in response to elevated CO₂ was unrelated to changes in the soil δ¹⁵N. While the data suggest a strong influence of elevated CO₂ on the N cycle in diverse ecosystems, the exact site(s) at which elevated CO₂ alters fractionating processes of the N cycle remains unclear. We cannot rule out the fact that the pattern of foliage δ¹⁵N responses to elevated CO₂ reported here resulted from a general drop in δ¹⁵N of the source N, caused by soil‐driven processes. There is a stronger possibility, however, that the general depletion of foliage δ¹⁵N under high CO₂ may have resulted from changes in the fractionating processes within the plant/mycorrhizal system.     

Assessing post-anthesis nitrogen uptake, distribution and utilisation in grain protein synthesis in barley (Hordeum vulgare L.) using 15N fertiliser and 15N proteinogenic and non-proteinogenic amino acids

We investigated the effects of nitrogen (N) availability during the vegetative phase on (a) post‐anthesis N uptake and (b) its translocation into ears in barley plants grown in a greenhouse at two levels of N: low (50 mg N kg^?1 sand) and optimal N supply (150 mg N kg^?1 sand). Plants in the two N treatments were fertilised with the same amount of labelled ¹⁵N [50 mg ¹⁵N kg^?1 sand at 10% ¹⁵N_exc (N_excess, i.e. N_exc, is defined as the abundance of enriched stable isotope minus the natural abundance of the isotope) applied as ¹⁵NH₄¹⁵NO₃] 10 days after anthesis (daa). In a separate experiment, the uptake and transport into ears of proteinogenic and non‐proteinogenic amino acids were studied to determine whether a relationship exists between amino acid transport into ears and their proteinogenic nature. Plants were fed with either ¹⁵N‐α‐alanine, a proteinogenic amino acid, or ¹⁵N‐α‐aminoisobutyric acid, a non‐proteinogenic amino acid. Both these amino acids were labelled at 95.6% ¹⁵N_exc. Results showed that N accumulations in stems, leaves and especially in ears were correlated with their dry matter (dm) weights. The application of 150 mg N kg^?1 sand significantly increased plant dm weight and total N accumulation in plants. During their filling period, ears absorbed N from both external (growth substrate) and internal (stored N in plants) sources. Nitrogen concentration in ears was higher in optimal N‐fed plants than in low N‐fed plants until 10 daa, but from 21 to 35 daa, differences were not detected. Conversely, ¹⁵N_exc in ears, leaves and stems was higher in low N‐fed plants than in optimal N‐fed plants. Ears acted as strong sink organ for the post‐anthesis N taken up from the soil independently of pre‐anthesis N nutrition: on average, 87% of the N taken up from the soil after anthesis was translocated and accumulated in ears. Low N‐fed plants continued to take up N from the post‐anthesis N fertiliser during the later grain‐filling period. The increase of pre‐anthesis N supply rate led to a decrease in the contribution of nitrogen derived from post‐anthesis ¹⁵N‐labelled fertiliser (N_dff) to total N in all aboveground organs, especially in ears where 44% and 22% of total N originated from post‐anthesis N uptake in low N‐fed and optimal N‐fed plants, respectively. The experiment with labelled amino acids showed that there was greater transport of proteinogenic amino acid into the ear (50% of total ¹⁵N) than non‐proteinogenic amino acid (39%). However, this transport of the non‐proteinogenic amino acids into ear suggested that the transport of N compounds from source (leaves) to sink organs (ear) might not be intrinsically regulated by their ability to be incorporated into storage protein of ears.     

Effect of digestion on the δ13C and δ15N of fish-gut contents

Gut contents of sand goby Pomatoschistus minutus showed higher C and N isotope values than the food before consumption. This enrichment was more pronounced in the hindgut than in the foregut, probably because of preferential assimilation of ¹²C and ¹⁴N along the gastro-intestinal tract. The results indicated that the shift towards higher values in the alimentary canal occurs in the first 2 h after feeding.     

Natural abundance of 13C and 15N in C3 and C4 vegetation of southern Africa: patterns and implications

The mean annual rainfall in southern Africa is found to explain over half of the observed variance in the stable nitrogen (N) isotopic signatures of C₃ vegetation in southern Africa (r²=0.54, P<0.01). The inverse relationship between the stable N isotopic signatures of foliar samples from C₃ vegetation and long‐term southern African rainfall is found on a scale larger than previously observed. A modest relationship is found between stable carbon (C) isotopic signatures of C₃ vegetation and rainfall across the region (r²=0.20, P<0.01). No such relationship is found between stable C and N isotopic signatures of C₄ vegetation and rainfall. The explanation of the relationship between ¹⁵N in C₃ vegetation and the mean annual rainfall presented here is that nutrient availability varies inversely with water availability. This suggests that water‐limited systems in southern Africa are more open in terms of nutrient cycling and therefore the resulting natural abundance of foliar ¹⁵N in these systems is enriched. The use of this relationship may be of value to those researchers modeling both the dynamics of vegetation and biogeochemistry across this region. The use of the isotopic enrichment in C₃ vegetation as a function of rainfall may provide an insight into nutrient cycling across the semi‐arid and arid regions of southern Africa. This finding has implications for the study of global change, especially as it relates to vegetation responses to changing regional rainfall regimes over time.