Elevated CO2 levels enhance the uptake and metabolism of organic nitrogen

The effects of elevated CO₂ (eCO₂) on the relative uptake of inorganic and organic nitrogen (N) are unclear. The uptake of different N sources by pak choi (Brassica chinensis L.) seedlings supplied with a mixture of nitrate, glycine and ammonium was studied using ¹⁵N‐labelling under ambient CO₂ (aCO₂) (350 ppm) or eCO₂ (650 ppm) conditions. ¹⁵N‐labelled short‐term uptake and ¹⁵N‐gas chromatography mass spectrometry (GC–MS) were applied to measure the effects of eCO₂ on glycine uptake and metabolism. Elevated CO₂ increased the shoot biomass by 36% over 15 days, but had little effect on root growth. Over the same period, the N concentrations of shoots and roots were decreased by 30 and 2%, respectively. Elevated CO₂ enhanced the uptake and N contribution of glycine, which accounted for 38–44% and 21–40% of total N uptake in roots and shoots, respectively, while the uptake of nitrate and ammonium was reduced. The increased glycine uptake resulted from the enhanced active uptake and enhanced metabolism in the roots. We conclude that eCO₂ may increase the uptake and contribution of organic N forms to total plant N nutrition. Our findings provide new insights into plant N regulation under eCO₂ conditions.     

Glycine uptake in heath plants and soil microbes responds to elevated temperature, CO2 and drought

Temperate terrestrial ecosystems are currently exposed to climatic and air quality changes with increased atmospheric CO₂, increased temperature and prolonged droughts. The responses of natural ecosystems to these changes are focus for research, due to the potential feedbacks to the climate. We here present results from a field experiment in which the effects of these three climate change factors are investigated solely and in all combinations at a temperate heath dominated by heather (Calluna vulgaris) and wavy hair-grass (Deschampsia flexuosa).Climate induced increases in plant production may increase plant root exudation of dissolved organic compounds such as amino acids, and the release of amino acids during decomposition of organic matter. Such free amino acids in soil serve as substrates for soil microorganisms and are also acquired as nutrients directly by plants. We investigated the magnitude of the response to the potential climate change treatments on uptake of organic nitrogen in an in situ pulse labelling experiment with ¹⁵N¹³C₂-labelled glycine (amino acid) injected into the soil.In situ root nitrogen acquisition by grasses responded significantly to the climate change treatments, with larger ¹⁵N uptake in response to warming and elevated CO₂ but not additively when the treatments were combined. Also, a larger grass leaf biomass in the combined T and CO₂ treatment than in individual treatments suggest that responses to combined climate change factors cannot be predicted from the responses to single factors treatments.The soil microbes were superior to plants in the short-term competition for the added glycine, as indicated by an 18 times larger ¹⁵N recovery in the microbial biomass compared to the plant biomass. The soil microbes acquired glycine largely as an intact compound (87%), with no effects of the multi factorial climate change treatment through one year.     

Uptake of amino acids by the aquatic resurrection plant Chamaegigas intrepidus and its implication for N nutrition

Chamaegigas intrepidus is a poikilohydric aquatic plant that lives in rock pools on granitic outcrops in Central Namibia. The pools are filled intermittently during the summer rains, and the plants may pass through up 20 rehydration/dehydration cycles during a single wet season. Rehydrated plants also have to cope with substantial diurnal fluctuations in the pH and extreme nutrient deficiency. Ammonium concentrations are normally around 30 μM. Additional nitrogen sources are amino acids. Total free amino acids are up to 15 μM with glycine and serine as the predominant amino acids. Experiments on uptake of radiolabelled amino acids into roots of C. intrepidus showed high␣affinity (K _M= 16 μM) and low-affinity (K _M= 159 μM) uptake systems. The K _M of the high-affinity system is well in accordance with the free amino acid concentration found in the water of the pools. We conclude that amino acids, predominantly glycine and serine, can be utilised by C. intrepidus in its natural habitat. Since glycine uptake showed a strong reduction at pH 10, nitrogen uptake from glycine or serine should occur mainly in the morning when the pH of the pool water is slightly acid. Further experiments with ¹⁵N-labelled ammonium in combination with non-labelled glycine demonstrated high ¹⁵N values in plant tissues. Under experimental conditions C. intrepidus preferred ammonium as a nitrogen source. The implication of amino acids for nitrogen nutrition of C. intrepidus may depend on the relation of inorganic and organic nitrogen available in the pool water and the preferential utilisation of one or the other nitrogen source may change during the day corresponding with pH changes in the water. Received: 28 January 1998 / Accepted: 24 July 1998     

Off-season uptake of nitrogen in temperate heath vegetation

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

Preferential uptake of soil nitrogen forms by grassland plant species

In this study, we assessed whether a range of temperate grassland species showed preferential uptake for different chemical forms of N, including inorganic N and a range of amino acids that commonly occur in temperate grassland soil. Preferential uptake of dual-labelled (¹³C and ¹⁵N) glycine, serine, arginine and phenylalanine, as compared to inorganic N, was tested using plants growing in pots with natural field soil. We selected five grass species representing a gradient from fertilised, productive pastures to extensive, low productivity pastures (Lolium perenne, Holcus lanatus, Anthoxanthum odoratum, Deschampsia flexuosa, and Nardus stricta). Our data show that all grass species were able to take up directly a diversity of soil amino acids of varying complexity. Moreover, we present evidence of marked inter-species differences in preferential use of chemical forms of N of varying complexity. L. perenne was relatively more effective at using inorganic N and glycine compared to the most complex amino acid phenylalanine, whereas N. stricta showed a significant preference for serine over inorganic N. Total plant N acquisition, measured as root and shoot concentration of labelled compounds, also revealed pronounced inter-species differences which were related to plant growth rate: plants with higher biomass production were found to take up more inorganic N. Our findings indicate that species-specific differences in direct uptake of different N forms combined with total N acquisition could explain changes in competitive dominance of grass species in grasslands of differing fertility.     

High potential, but low actual, glycine uptake of dominant plant species in three Australian land-use types with intermediate N availability

The traditional view of the nitrogen (N) cycle has been challenged since the discovery that plants can compete with microbes for low molecular weight (LMW) organic N. Despite a number of studies that have shown LMW organic N uptake by plants, there remains a debate on the overall ecological relevance of LMW organic N uptake by plants across ecosystems with different N availabilities. We here report patterns of glycine N uptake by plants from three different Australian land-use types with intermediate N availability and low inherent glycine concentrations in the soil. Using ¹⁵N labeled tracers, we tested the potential of these plants to acquire glycine in ex-situ laboratory experiments and attempted to validate these results in the field by determining actual uptake of glycine by plants directly from the soil. We found in the ex-situ experiments that plants from all three land-use types were able to take up significant amounts of glycine. In contrast, glycine uptake directly from the soil was minimal in all three land-use types and ¹⁵N tracers were largely immobilized in the soil organic N pool. Our study confirms that the potential for LMW organic N uptake by plants is a widespread phenomenon. However, our in-situ experiments show that in the three land-use types tested here plants are inferior competitors for LMW organic N and rely on NH ₄ ⁺ as their main N source. In contrast to several previous studies in arctic, alpine and even temperate ecosystems, our study suggests that in ecosystems with intermediate N availability, mineral N is the plants’ main N source, while LMW organic N is of less ecological relevance to plant N nutrition.     

Warming decreased and grazing increased plant uptake of amino acids in an alpine meadow

Organic nitrogen (N) uptake by plants has been recognized as a significant component of terrestrial N cycle. Several studies indicated that plants have the ability to switch their preference between inorganic and organic forms of N in diverse environments; however, research on plant community response in organic nitrogen uptake to warming and grazing is scarce. Here, we demonstrated that organic N uptake by an alpine plant community decreased under warming with ¹³C–¹⁵N‐enriched glycine addition method. After 6 years of treatment, warming decreased plant organic N uptake by 37% as compared to control treatment. Under the condition of grazing, warming reduced plant organic N uptake by 44%. Grazing alone significantly increased organic N absorption by 15%, whereas under warming condition grazing did not affect organic N uptake by the Kobresia humilis community on Tibetan Plateau. Besides, soil NO₃–N content explained more than 70% of the variability observed in glycine uptake, and C:N ratio in soil dissolved organic matter remarkably increased under warming treatment. These results suggested warming promoted soil microbial activity and dissolved organic N mineralization. Grazing stimulated organic N uptake by plants, which counteracted the effect of warming.     

Understorey plant and soil responses to disturbance and increased nitrogen in boreal forests

Question: How do N fertilization and disturbance affect the understorey vegetation, microbial properties and soil nutrient concentration in boreal forests? Location: Kuusamo (66°22′N; 29°18′E) and Oulu (65°02′N; 25°47′E) in northern Finland. Methods: We conducted a fully factorial experiment with three factors: site (two levels), N fertilization (four levels) and disturbance (two levels). We measured treatment effects on understorey biomass, vegetation structure, and plant, soil and microbial N and C concentrations. Results: The understorey biomass was not affected by fertilization either in the control or in the disturbance treatment. Fertilization reduced the biomass of deciduous Vaccinium myrtillus. Disturbance had a negative effect on the biomass of V. myrtillus and evergreen Vaccinium vitis‐idaea and decreased the relative proportion of evergreen species. Fertilization and disturbance increased the biomass of grass Deschampsia flexuosa and the relative proportion of graminoids. The amount of NH₄⁺ increased in soil after fertilization, and microbial C decreased after disturbance. Conclusions: Our results suggest that the growth of slow‐growing Vaccinium species and soil microbes in boreal forests are not limited by N availability. However, significant changes in the proportion of dwarf shrubs to graminoids and a decrease in the biomass of V. myrtillus demonstrate the susceptibility of understorey vegetation to N enrichment. N enrichment and disturbance seem to have similar effects on understorey vegetation. Consequently, increasing N does not affect the rate or the direction of recovery after disturbance. Moreover, our study demonstrates the importance of understorey vegetation as a C source for soil microbes in boreal forests.     

Amino acid uptake by temperate tree species characteristic of low- and high-fertility habitats

The relationship between inorganic nitrogen (N) cycling and plant productivity is well established. However, recent research has demonstrated the ability of plants to take up low molecular weight organic N compounds (i.e., amino acids) at rates that often rival those of inorganic N forms. In this study, we hypothesize that temperate forest tree species characteristic of low-fertility habitats will prefer amino acids over species characteristic of high-fertility habitats. We measured the uptake of ¹⁵N-labeled amino acids (glycine, glutamine, arginine, serine), ammonium (NH₄ ⁺), and nitrate (NO₃ ⁻) by four tree species that commonly occur in eastern North America, where their abundances have been correlated with inorganic N availability. Specific uptake rates of amino acids were largely similar for all tree species; however, high-fertility species took up NH₄ ⁺ at rates more than double those of low-fertility species, rendering amino acid N relatively more important to the N nutrition of low-fertility species. Low-fertility species acquired over four times more total N from arginine compared to NH₄ ⁺ and NO₃ ⁻; high-fertility species acquired the most N from NH₄ ⁺. Arginine had the highest uptake rates of any amino acid by all species; there were no significant differences in uptake rates of the remaining amino acids. Our results support the idea that the dominant species in a particular habitat are those best able to utilize the most available N resources.     

Comparison of nitrogen uptake in the roots and rhizomes of <Emphasis Type="Italic">Leymus chinensis</Emphasis>

Leymus chinensis (Trin.) Tzvel is a rhizomatous grass species in the Eastern Eurasian steppe zone that is often limited by low soil nitrogen availability. Although a previous study showed that the rhizomes of L. chinensis have the capacity to take up nitrogen, the importance of such uptake for nitrogen nutrition is unclear. Moreover, little is known regarding the inorganic nitrogen uptake kinetics of roots and rhizomes in response to nitrogen status. Here, we first found that ammonium is preferred over nitrate and glycine for L. chinensis growth. Using the ¹⁵N-labelling method, we found that the rate of ion influx into roots was approximately five-fold higher than into rhizomes under the same nitrogen content, and the ion influxes into roots and rhizomes under 0.05 mM N were greater than in the presence of 3 mM N, especially in the form of NH₄⁺. Using a non-invasive micro-test technique, we characterised the patterns of NH₄⁺ and NO₃^– fluxes in the root mature zone, root tip, rhizome mature zone, and rhizome tip following incubation in the solution with different N compounds and different N concentrations. These results suggest a dynamic balance between the uptake, utilisation, and excretion of nitrogen in L. chinensis.     

Uptake and assimilation of nitrogen from solutions containing multiple N sources

We assessed the extent to which plants can acquire amino acids when supplied as single N-sources or when plants have access to a mixture of amino- and inorganic N sources. Because the uptake of different N-sources is temperature-dependent, the effects of temperature on amino-N uptake were also tested. Lolium perenne (perennial rye-grass) was grown hydroponically at 11 °C or 21 °C. Uptake of N was determined using ¹⁵N tracers at the growth temperature from solutions containing either nitrate, ammonium or glycine as single N sources and from a mixture containing all three N-forms. Estimates of the relative importance of amino acids such as glycine to the total N budget of plants will have been underestimated in studies where uptake was determined in single source solutions compared with those from solutions containing a mixture of N-forms. The proportion of total N acquired from the mixed N source as ammonium increased as temperature was reduced. Regarding the uptake and initial metabolism of glycine, uptake was probably the rate limiting step at 11 °C whilst it was the metabolism of glycine to serine at 21 °C. Although ¹⁵N incorporation into the plant amino-N pool was generally in proportion to the abundance of individual amino acids, its incorporation into the glycine pool was sometimes significantly less than predicted.     

Nitrogen-addition effects on leaf traits and photosynthetic carbon gain of boreal forest understory shrubs

Boreal coniferous forests are characterized by fairly open canopies where understory vegetation is an important component of ecosystem C and N cycling. We used an ecophysiological approach to study the effects of N additions on uptake and partitioning of C and N in two dominant understory shrubs: deciduous Vaccinium myrtillus in a Picea abies stand and evergreen Vaccinium vitis-idaea in a Pinus sylvestris stand in northern Sweden. N was added to these stands for 16 and 8 years, respectively, at rates of 0, 12.5, and 50 kg N ha^?1 year^?1. N addition at the highest rate increased foliar N and chlorophyll concentrations in both understory species. Canopy cover of P. abies also increased, decreasing light availability and leaf mass per area of V. myrtillus. Among leaves of either shrub, foliar N content did not explain variation in light-saturated CO₂ exchange rates. Instead photosynthetic capacity varied with stomatal conductance possibly reflecting plant hydraulic properties and within-site variation in water availability. Moreover, likely due to increased shading under P. abies and due to water limitations in the sandy soil under P. sylvestris, individuals of the two shrubs did not increase their biomass or shift their allocation between above- and belowground parts in response to N additions. Altogether, our results indicate that the understory shrubs in these systems show little response to N additions in terms of photosynthetic physiology or growth and that changes in their performance are mostly associated with responses of the tree canopy.     

Biomass distribution of two subalpine dwarf‐shrubs in relation to soil moisture and nutrient content

Question: Do soil water content and/or soil nitrogen (N) content and/or soil phosphorus (P) content affect the biomass of Vaccinium myrtillus and V. vitis‐idaea in a sub‐alpine heath? Location: Dolomites, northern Italy, 1800 m a.s.l. Methods: We determined above‐ground and below‐ground biomass of the shrubs at three sites, each on a different substrate type. At each site, we determined soil N‐ and P‐contents. We also determined leaf water potential (Psi;₁), N‐ and P‐concentrations in plant tissues and litter, as well as δ¹³C and δ¹⁵N in mature leaves. Results: V. myrtillus biomass was highest at the silicate site, V. vitis‐idaea biomass was highest at the carbonate site. Both shrubs had low biomass at the peat site, possibly due to a toxic effect of waterlogging in wet soils. For both species, pre‐dawn Psi;₁ indicated optimal hydration and midday Psi;₁ did not show any sign of water stress. Water use efficiency (WUE) did not differ among sites for any species. Whole‐plant nutrient concentrations showed that, with increasing biomass, N was diluted in V. myrtillus tissues while P was diluted in V. vitis‐idaea tissues. Foliar N‐concentration was higher overall for V. myrtillus. Foliar P‐concentration in V. myrtillus peaked at the silicate site. Foliar N : P ratios suggested that V. myrtillus was primarily P‐limited and V. vitis‐idaea primarily N‐limited. Conclusions: Water content affected the distribution of the two shrubs in a similar way, higher P‐availability in the soil enhanced V. myrtillus rather than V. vitis‐idaea.     

Plant and soil microbial community responses to solid waste leachates diffusion on grassland

Litterfall from trees has been identified as an important pathway for deposition of mercury (Hg) and methylmercury (MeHg) in forested catchments, but very little is known about the role of ground vegetation in deposition and cycling of Hg compounds. This study was conducted to identify the origin of Hg compounds in the ground vegetation, and to estimate the role of its litterfall with respect to pools and fluxes of Hg in a coniferous forest in the German Fichtelgebirge mountains. Above and below ground biomass of the dominant ground vegetation (Vaccinium myrtillus, Deschampsia flexuosa and Calamagrostis villosa) were sampled at several plots successively during the growing season. The fluxes to the soil via litterfall of the ground vegetation were calculated using contents of Hg and MeHg in the annual fractions of aboveground biomass. With fluxes of 0.4 – 7.8 mg Hg_total ha^–1 a^–1 and 0.01 – 0.04 mg MeHg ha^–1 a^–1 (depending on the plant species) this pathway contributes only a few percent to the total deposition of both compounds in the catchment. To identify the uptake pathways of Hg compounds, the same plant species were grown in a pot experiment with addition of isotope labelled Hg compounds (²⁰²Hg²⁺, Me¹⁹⁸Hg) to a clean sand substrate. Only small proportions of ²⁰²Hg and Me¹⁹⁸Hg in the substrate were taken up by the plants, but in all cases the proportion translocated into aboveground biomass after uptake was greater in case of Me¹⁹⁸Hg. Thus, internal recycling in the plant-soil system is a source especially for MeHg in the ground vegetation. However, as compared to the input of Hg compounds by tree litterfall and storage in the forest floor, Hg_total and MeHg in ground vegetation are of minor importance. High volatilization of added Hg isotopes raises the question of a re-emission of Hg compounds by the transpiration flux of the ground vegetation.     

Amino acids as a nitrogen source for tomato seedlings: The use of dual-labeled (C, N) glycine to test for direct uptake by tomato seedlings

Direct uptake of organic nitrogen (ON) compounds, rather than inorganic N, by plant roots has been hypothesized to constitute a significant pathway for plant nutrition. The aim of this study was to test whether tomatoes (Solanum lycopersicum cv. Huying932) can take up ON directly from the soil by using ¹⁵NH₄Cl, K¹⁵NO₃, 1, 2-¹³C₂¹⁵N-glycine labeling techniques. The ¹³C and ¹⁵N in the plants increased significantly indicating that a portion of the glycine-N was taken up in the form of intact amino acids by the tomatoes within 48 h after injection into the soil. Regression analysis of excess ¹³C against excess ¹⁵N showed that approximately 21% of the supplied glycine-N was taken up intact by the tomatoes. Atom% excesses of ¹⁵N and ¹³C in the roots were higher than in any shoots. Results also indicated rapid turnover of amino acids (e.g., glycine) by soil microorganisms, and the poor competitive ability of tomatoes in absorbing amino acids from the soil solution. This implies that tomatoes can take up ON in an intact form from the soil despite the rapid turnover of organic N usually found under such conditions. Given the influence of climatic change and N pollution, further studies investigating the functional ecological implications of ON in horticultural ecosystems are warranted.     

Cycling Dynamics of NH4 + and Amino Acid Nitrogen in Soils of a Deciduous Boreal Forest Ecosystem

Conventional studies of nitrogen (N) cycling in forest ecosystems have focused on inorganic N uptake as the primary source of N for plant metabolism. More recently, however, alternative sources of N for plant nutrition, such as free amino acids, have gained attention, particularly in nutrient-limited systems. Using a multiple stable isotope (¹³C and ¹⁵N) design, that allowed us to simultaneously assess root uptake of ammonium (NH₄ ⁺) and glycine, we compared the cycling dynamics of NH₄ ⁺ and amino acid N within the soils of several interior Alaskan floodplain balsam poplar stands. Our design included multiple sampling periods extending from 45 min to 14 days, which permitted us to study interpool transfers of our carbon (C) and N isotopes over time. Microbial biomass N was the largest sink of both ¹⁵N-ammonium and glycine. Percent recovery of ¹⁵N for this pool was an order of magnitude larger than fine-root ¹⁵N uptake for most sampling periods. Although recovery of ¹⁵N in fine-root biomass was small, amino acid N and NH₄ ⁺ were assimilated at approximately the same rate irrespective of sampling period, and total recovery was still increasing 2 weeks after application. Recovery of ¹⁵N in bulk soil samples did not vary significantly over time for either treatment. However, bulk soil ¹³C declined steadily during the experiment, measuring less than 30% recovery of added label after 14 days. We suspect that the majority of ¹³C lost from our soils was respired. Soil microorganisms strongly outcompeted plants in the short term for both NH₄ ⁺ and amino acid N. However, amino acid N appears to cycle through soil N pools at approximately the same rate as inorganic N forms. The similarity in uptake patterns for inorganic and organic N suggests that these stands are meeting part of their N requirements directly from amino acids.     

Cross-Ecosystem Comparisons of In Situ Plant Uptake of Amino Acid-N and NH4 +

Plant and microbial use of nitrogen (N) can be simultaneously mutualistic and competitive, particularly in ecosystems dominated by mycorrhizal fungi. Our goal was to quantify plant uptake of organic and inorganic N across a broad latitudinal gradient of forest ecosystems that varied with respect to overstory taxon, edaphic characteristics, and dominant mycorrhizal association. Using ¹³C and ¹⁵N, we observed in situ the cycling dynamics of NH₄ ⁺ and glycine through various soil pools and fine roots over 14 days. Recovery of ¹⁵N as soil N varied with respect to N form, forest type, and sampling period; however, there were similarities in the cycling dynamics of glycine and NH₄ ⁺ among all forest types. Microbial immobilization of ¹⁵N was immediately apparent for both treatments and represented the largest sink (~25%) for ¹⁵N among extractable soil N pools during the first 24 h. In contrast, fine roots were a relatively small sink (<10%) for both N forms, but fine root ¹³C enrichment indicated that plants in all forest types absorbed glycine intact, suggesting that plants and microbes effectively target the same labile soil N pools. Relative uptake of amino acid-N versus NH₄ ⁺ varied significantly among sites and approximately half of this variation was explained by mycorrhizal association. Estimates of plant uptake of amino acid-N relative to NH₄ ⁺ were 3× higher in ectomycorrhizal-dominated stands (1.6 ± 0.2) than arbuscular mycorrhizae-dominated stands (0.5 ± 0.1). We conclude that free amino acids are an important component of the N economy in all stands studied; however, in these natural environments plant uptake of organic N relative to inorganic N is explained as much by mycorrhizal association as by the availability of N forms per se.     

Formation of unidentified nitrogen in plants: an implication for a novel nitrogen metabolism

Plants take up inorganic nitrogen and store it unchanged or convert it to organic forms. The nitrogen in such organic compounds is stoichiometrically recoverable by the Kjeldahl method. The sum of inorganic nitrogen and Kjeldahl nitrogen has long been known to equal the total nitrogen in plants. However, in our attempt to study the mechanism of nitrogen dioxide (NO₂) metabolism, we unexpectedly discovered that about one-third of the total nitrogen derived from ¹⁵N-labeled NO₂ taken up by Arabidopsis thaliana (L.) Heynh. plants was converted to neither inorganic nor Kjeldahl nitrogen, but instead to an as yet unknown nitrogen compound(s). We here refer to this nitrogen as unidentified nitrogen (UN). The generality of the formation of UN across species, nitrogen sources and cultivation environments for plants has been shown as follows. Firstly, all of the other 11 plant species studied were found to form the UN in response to fumigation with ¹⁵NO₂. Secondly, tobacco (Nicotiana tabacum L.) plants fed with ¹⁵N-nitrate appeared to form the UN. And lastly, the leaves of naturally fed vegetables, grass and roadside trees were found to possess the UN. In addition, the UN appeared to comprise a substantial proportion of total nitrogen in these plant species. Collectively, all of our present findings imply that there is a novel nitrogen mechanism for the formation of UN in plants. Based on the analyses of the exhaust gas and residue fractions of the Kjeldahl digestion of a plant sample containing the UN, probable candidates for compounds that bear the UN were deduced to be those containing the heat-labile nitrogen–oxygen functions and those recalcitrant to Kjeldahl digestion, including organic nitro and nitroso compounds. We propose UN-bearing compounds may provide a chemical basis for the mechanism of the reactive nitrogen species (RNS), and thus that cross-talk may occur between UN and RNS metabolisms in plants. A mechanism for the formation of UN-bearing compounds, in which RNS are involved as intermediates, is proposed. The important broad impact of this novel nitrogen metabolism, not only on the general physiology of plants, but also on plant substances as human and animal food, and on plants as an integral part of the global environment, is discussed.Abbreviations NO Nitric oxide - NO₂ Nitrogen dioxide - RNS Reactive nitrogen species - UN Unidentified nitrogen - TNNAT, RNNAT, INNAT and UNNAT Total, Kjeldahl, inorganic and unidentified nitrogen in naturally fed plants, respectively - TNNIT, RNNIT, INNIT and UNNIT Total, Kjeldahl, inorganic and unidentified nitrogen derived from nitrate, respectively - TNNO2, RNNO2, INNO2 and UNNO2 Total, Kjeldahl, inorganic and unidentified nitrogen derived from NO₂, respectively     

Site-dependent N uptake from N-form mixtures by arctic plants,soil microbes and ectomycorrhizal fungi

Soil microbes constitute an important control on nitrogen (N) turnover and retention in arctic ecosystems where N availability is the main constraint on primary production. Ectomycorrhizal (ECM) symbioses may facilitate plant competition for the specific N pools available in various arctic ecosystems. We report here our study on the N uptake patterns of coexisting plants and microbes at two tundra sites with contrasting dominance of the circumpolar ECM shrub Betula nana. We added equimolar mixtures of glycine-N, NH₄⁺–N and NO₃⁻–N, with one N form labelled with ¹⁵N at a time, and in the case of glycine, also labelled with ¹³C, either directly to the soil or to ECM fungal ingrowth bags. After 2 days, the vegetation contained 5.6, 7.7 and 9.1% (heath tundra) and 7.1, 14.3 and 12.5% (shrub tundra) of the glycine-, NH₄⁺- and NO₃⁻–¹⁵N, respectively, recovered in the plant–soil system, and the major part of ¹⁵N in the soil was immobilized by microbes (chloroform fumigation-extraction). In the subsequent 24 days, microbial N turnover transferred about half of the immobilized ¹⁵N to the non-extractable soil organic N pool, demonstrating that soil microbes played a major role in N turnover and retention in both tundra types. The ECM mycelial communities at the two tundras differed in N-form preferences, with a higher contribution of glycine to total N uptake at the heath tundra; however, the ECM mycelial communities at both sites strongly discriminated against NO₃⁻. Betula nana did not directly reflect ECM mycelial N uptake, and we conclude that N uptake by ECM plants is modulated by the N uptake patterns of both fungal and plant components of the symbiosis and by competitive interactions in the soil. Our field study furthermore showed that intact free amino acids are potentially important N sources for arctic ECM fungi and plants as well as for soil microorganisms. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Nitrogen fixation associated with non-legumes in agriculture

Summary This review examines the nitrogen cycle in upland agricultural situations where nonlegume N₂-fixation is likely to be important for crop growth. Evidence for associative fixation is adduced from accumulation of N in the top 15 cm soil under grasses, from N balances for crop production obtained from both pot and field experiments, in tropical and temperate environments, measurements of nitrogen (C₂H₂ reduction) activity, uptake of¹⁵N₂ by plants and¹⁵N isotope dilution. Factors influencing the activity such as the provision of carbon substrate by the plant and the efficiency of its utilisation by the bacteria, plant cultivar, soil moisture and N levels, and inoculation with N₂-fixing bacteria are discussed. Crop responses to inoculation withAzospirillum are detailed. The breakdown of crop residues, particularly straw, can support large levels of N₂-fixation. Cyanobacteria as crusts on the soil surface also fix nitrogen actively in many environments. Fixation by the nodulated, non-legume treesCasuarina andParasponia has beneficial effects in some cropping systems in Asia. I conclude that nonlegume N₂-fixation makes a significant contribution to the production of some major cereal crops in both temperate and tropical environments.