Showing papers on "Nitrogen fixation published in 1996"

Growth and nitrogen fixation of the diazotrophic filamentous nonheterocystous cyanobacterium trichodesmium sp. ims 101 in defined media: evidence for a circadian rhythm

Abstract: Trichodesmium sp. IMS 101, originally isolated from coastal western Atlantic waters by Prufert-Bebout and colleagues and maintained in seawater-based media, was successfully cultivated in two artificial media. Its characteristics of growth, nitrogen fixation, and regulation of nitrogen fixation were compared to those of natural populations and Trichodesmium sp. NIBB 1067. Results indicate that the culture grown in artificial media had nitrogen fixation characteristics similar to those when the culture is grown in seawater-based medium and to those of Trichodesmium sp. in the natural habitat. The study provides practical artificial media to facilitate the physiological studies of these important diazotrophic cyanobacteria, as well as the cultivation of other Trichodesmium species in future studies. Manipulations of the light/dark cycle were performed to determine whether or not the daily cycle of nitrogen fixation is a circadian rhythm. Cultures grown under continuous light maintained the cycle for up to 6 days. We demonstrated that the daily cycle of nitrogen fixation in Trichodesmium sp. IMS 101 was at least partially under the control of a circardian rhythm.

Human contributions to terrestrial nitrogen flux

Abstract: This article assesses the sources and fates of anthropogenic fixed nitrogen. Anthropogenic nitrogen from most sources comes together in the common pathways of the agricultural food chain. The topics covered in this article include the following: inputs of newly fixed nitrogen; nitrogen flow in agriculture; calculating production of plant nitrogen; calculating consumption of nitrogen; harvests and grazing and the effect on nitrogen; nitrogen flow through livestock; net trade; nitrogen flows in regions of the United States; riverine discharges. 51 refs., 4 figs., 4 tabs.

Stimulation of Symbiotic N2 Fixation in Trifolium repens L. under Elevated Atmospheric pCO2 in a Grassland Ecosystem.

Abstract: Symbiotic N2 fixation is one of the main processes that introduces N into terrestrial ecosystems. As such, it may be crucial for the sequestration of the extra C available in a world of continuously increasing atmospheric CO2 partial pressure (pCO2). The effect of elevated pCO2 (60 Pa) on symbiotic N2 fixation (15N-isotope dilution method) was investigated using Free-Air-CO2-Enrichment technology over a period of 3 years. Trifolium repens was cultivated either alone or together with Lolium perenne (a nonfixing reference crop) in mixed swards. Two different N fertilization levels and defoliation frequencies were applied. The total N yield increased consistently and the percentage of plant N derived from symbiotic N2 fixation increased significantly in T. repens under elevated pCO2. All additionally assimilated N was derived from symbiotic N2 fixation, not from the soil. In the mixtures exposed to elevated pCO2, an increased amount of symbiotically fixed N (+7.8, 8.2, and 6.2 g m-2 a-1 in 1993, 1994, and 1995, respectively) was introduced into the system. Increased N2 fixation is a competitive advantage for T. repens in mixed swards with pasture grasses and may be a crucial factor in maintaining the C:N ratio in the ecosystem as a whole.

A global two component signal transduction system that integrates the control of photosynthesis, carbon dioxide assimilation, and nitrogen fixation

Abstract: Photosynthesis, biological nitrogen fixation, and carbon dioxide assimilation are three fundamental biological processes catalyzed by photosynthetic bacteria. In the present study, it is shown that mutant strains of the nonsulfur purple photosynthetic bacteria Rhodospirillum rubrum and Rhodobacter sphaeroides, containing a blockage in the primary CO2 assimilatory pathway, derepress the synthesis of components of the nitrogen fixation enzyme complex and abrogate normal control mechanisms. The absence of the Calvin–Benson–Bassham (CBB) reductive pentose phosphate CO2 fixation pathway removes an important route for the dissipation of excess reducing power. Thus, the mutant strains develop alternative means to remove these reducing equivalents, resulting in the synthesis of large amounts of nitrogenase even in the presence of ammonia. This response is under the control of a global two-component signal transduction system previously found to regulate photosystem biosynthesis and the transcription of genes required for CO2 fixation through the CBB pathway and alternative routes. In addition, this two-component system directly controls the ability of these bacteria to grow under nitrogen-fixing conditions. These results indicate that there is a molecular link between the CBB and nitrogen fixation process, allowing the cell to overcome powerful control mechanisms to remove excess reducing power generated by photosynthesis and carbon metabolism. Furthermore, these results suggest that the two-component system integrates the expression of genes required for the three processes of photosynthesis, nitrogen fixation, and carbon dioxide fixation.

Seasonal variations in nitrogen-fixation (acetylene reduction) and sulphate-reduction rates in the rhizosphere of Zostera noltii: nitrogen fixation by sulphate-reducing bacteria

Abstract: Nitrogen-fixation (acetylene reduction) rates were measured over an annual cycle in meadows of the seagrass Zostera noltii Hornem in the Bassin d'Arcachon, south-west France, between March 1994 and February 1995, using both slurry and whole-core techniques. Measured rates using the slurry technique consistently overestimated those determined on whole cores, probably due to the release of labile organic carbon sources as a result of root damage during preparation of the slurries. Thus, the whole-core technique may provide a more accurate estimate of in situ activity, since disturbance of physicochemical gradients of oxygen, sulphide, nutrients and the relationship between the plant roots and the rhizosphere microflora is minimised. Rates measured by the whole-core method were 1.8- to 4-fold greater (dependent upon season) in the light than those measured during dark incubations, indicating that organic carbon diffusing from the plant roots during photosynthesis was an important factor in regulating nitrogen fixation in the rhizosphere. Additions of sodium molybdate, a specific inhibitor of sulphate-reducing bacteria (SRB) inhibited acetylene-reduction activity by >80% as measured by both the slurry and whole-core techniques throughout the year, inferring that SRB were the dominant component of the nitrogen-fixing microflora. A mutualistic relationship between Z. noltii and nitrogen-fixing SRB in the rhizosphere, based on the exchange of organic carbon and fixed nitrogen is proposed. Acetylene- and sulphate-reduction rates showed distinct summer peaks which correlated with a reduced availability of ammonium in the sediment and the annual growth cycle of Z. noltii in the basin. Overall, these data indicate that acetylene reduction (nitrogen fixation) activity in the rhizosphere of Z. noltii was regulated both by the availability of organic carbon from the plant roots and maintenance of a low NH 4 + concentration in the vicinity of the plant roots due to efficient assimilation of NH 4 + by Z. noltii during the growth season. Nitrogenfixation rates determined from acetylene-reduction rates measured using the whole-core technique ranged from 0.1 to 7.3 mg N m-2d-1, depending on season, and were calculated to contribute between 0.4 and 1.1 g N m-2yr-1, or 6.3 to 12% of the annual fixed nitrogen requirement of Z. noltii.

The Genetic and Biochemical Basis for Nodulation of Legumes by Rhizobia

Abstract: Soil bacteria of the genera Azorhizobium, Bradyrhizobium, and Rhizobium are collectively termed rhizobia. They share the ability to penetrate legume roots and elicit morphological responses that le...

Reassessing the nitrogen relations of Arctic plants : a mini-review

Abstract: The Arctic is often assumed to be an NH 4 + -dominated ecosystem. This review assesses the validity of this assumption. It also addresses the question of whether Arctic plant growth is limited by the ability to use the forms of nitrogen that are available. The review demonstrates that several sources of soil nitrogen are available to Arctic plants, including soluble organic nitrogen (e.g. glycine, aspartic acid and glutamic acid), NH 4 + and NO 3 - . In mesic Arctic soils, soluble organic nitrogen is potentially more important than either NH 4 + or NO 3 - . Many Arctic species are capable of taking up soluble organic nitrogen (either directly and/or in association with ectomycorrhizae), with the greatest potential for soluble organic nitrogen uptake being exhibited by deciduous species. The ability to take up soluble organic nitrogen may enable some Arctic plants to avoid nitrogen limitations imposed by the slow rate of organic matter decomposition. NO 3 - is also present in many Arctic soils, especially calcareous soils and soils near flowing water, animal burrows and bird cliffs. Arctic species characteristic of mesic and xeric habitats are capable of taking up and assimilating NO 3 - . Even when present in lower concentrations in soils than NH 4 + , NO 3 - is still an important source of nitrogen for some Arctic plants. Arctic plants therefore have a variety of nitrogen sources available to them, and are capable of using those nitrogen sources. Taken together, these findings demonstrate that the Arctic is not an NH 4 + -dominated ecosystem. Symbiotic fixation of atmospheric N 2 does not appear to be an important source of nitrogen for Arctic plants. The reliance of Arctic plants on internal recycling of nitrogen substantially reduces their dependence on soil nitrogen uptake (this is particularly the case for slow-growing evergreens). Despite the high level of internal nitrogen recycling, Arctic plant growth remains limited by the low levels of available soil nitrogen. However, Arctic plant growth is not limited by an inability to utilize any of the available forms of nitrogen. The potential effects of climatic warming on nitrogen availability and use are discussed. The question of whether the Arctic ecosystem is uniquely different from temperate nitrogen-deficient ecosystems is also assessed.

Diazotrophic endophytes: progress and prospects for nitrogen fixation in monocots

Abstract: The development of nitrogen fixation in maize can be considered the “holy grail” of nitrogen fixation research. As nitrogen fertilization is one of the highest costs of corn production, the development of a symbiosis between diazotrophic bacteria and corn would be of enormous economic value. Such a discovery would also improve human health as it would decrease the amount of nitrate in ground water as well as in corn cultured for human consumption. Several proposals have been made toward this end. These include: a) the transfer of root nodulation genes from a legume to maize; b) the expression of the bacterial nif regulon in maize organelles; and c) the development of corn lines with the ability to accept fixed nitrogen from diazotrophs in the rhizosphere. All of these proposals have enormous technical problems to overcome such that the development of nitrogen-fixing corn in the near term has been considered unlikely. An alternative and less-technically challenging approach may be a thorough study of non-pathogenic bacterial endophytes that already inhabit the corn plant.

Organization and regulation of genes encoding the molybdenum nitrogenase and the alternative nitrogenase in Rhodobacter capsulatus

Abstract: The phototrophic non-sulfur purple bacterium Rhodobacter capsulatus is able to fix atmospheric dinitrogen either via a conventional molybdenum nitrogenase or via an alternative iron-only nitrogenase. At least 53 genes are involved in the synthesis and regulation of these two nitrogenase systems, most of which are clustered in four regions widely spread in the genome. Expression of both nitrogenase systems is regulated at the transcriptional level by NifR1 and NifR2, homologues of NtrC and NtrB, respectively. However, this ntr system is only involved in the regulation of the two nitrogenase systems and the high-affinity molybdenum transport system and is not required for utilization of other N sources such as proline and arginine. In contrast to enteric bacteria, the R. capsulatus NtrC homologue does not act in concert with the alternative sigma factor RpoN (σ54). Nitrogen fixation in R. capsulatus is regulated at the transcriptional level and also at the post-translational level. The draTG gene products are responsible for covalent modification of the dinitrogenase reductases of both nitrogenase systems. In addition, mutations in hvrA, a gene previously described as being responsible for low-light activation of the photosynthetic apparatus, also affect regulation of nitrogen fixation.

Nitrogen fixation in Arctic vegetation and soils from Svalbard, Norway

Abstract: Nitrogen fixation was measured by the acetylene reduction method in a high Arctic ecosystem at Kongsfjorden, Spitsbergen (79°N, 12°E). The most important source of biologically fixed nitrogen was found in cyanobacteria either as free living colonies ofNostoc sp. in wet unvegetated or sparsely vegetated grounds or growing as epiphytes on bryophytes. Fixation associated with plant roots or in soil and peat samples had little or no significance for nitrogen input to the ecosystem. The ability to support an epiphytic flora of nitrogen-fixing cyanobacteria varied greatly between bryophyte species.Calliergon richardsonii andSanionia uncinata seemed especially well adapted for harbouring epiphytic cyanobacteria, but the extent of nitrogen fixation varied with the growing location. The rate of nitrogen fixation was greatly influenced by grazing by geese. In a geese-grazing area values were found with a maximum of 693.6±1.5 nmol C2H4 h−1 g (dry weight)−1 while the maximum value for ungrazed areas was 65.3±16.6 nmol C2H4 h−1 g (dry weight)−1. In the grazed area cyanobacteria were also found fixing nitrogen epiphytically on grass. The high plant productivity, supporting heavy grazing, clearly indicates an effective transfer of fixed nitrogen to the plant community. Under cliffs harbouring colonies of birds, the biological nitrogen fixation was inhibited by bird droppings.

Spatial variability in the potential for symbiotic N2 fixation by woody plants in a subtropical savanna ecosystem

Abstract: 1. Root infection by symbiotic N 2 -fixing Frankia and Rhizobium strains was quantified in relation to light and soil properties for seedlings of 12 woody species from a subtropical savanna in southern Texas, USA. 2. None of four rhamnaceous species nodulated, despite the fact that bioassays with a known actinorhizal species yielded 13 nodules per seedling. Celtis pallida (Ulmaceae), Acacia greggii and Acacia berlandieri (Leguminosae) also failed to nodulate even though field populations of these species were characterized by high (2.7-4.2%) foliar nitrogen concentration. 3. Infective rhizobia occurred in all soils studied regardless of soil depth, distance from a host plant or type of plant cover. Plant growth in N-free media and acetylene reduction activity suggested that all nodules were capable of N 2 -fixation. 4. The extent of nodulation varied by species. However, nodulated seedlings were taller, produced more biomass and allocated less biomass to root systems than their non-nodulated counterparts. 5. Numbers of nodules on seedlings of Prosopis glandulosa, the dominant woody species in this subtropical savanna and throughout the south-western USA, were reduced by low light (15% full sunlight) regardless of soil N level; at medium and full sunlight nodule biomass expressed as a fraction of whole plant biomass decreased with increasing soil N. Nodulation of field-grown P. glandulosa appears to be ephemeral, apparently varying with changes in soil moisture. 6. Nodulation and N 2 fixation among woody legumes in subtropical savannas can occur across a broad range of soil conditions and depths with significant impacts on local and regional N-cycles. 7. Field levels of foliar N in species that failed to nodulate in the laboratory were comparable to or greater than those in species capable of nodulation, suggesting that leaf N is not a reliable indicator of N 2 fixation.

Soil moisture and potassium affect the performance of symbiotic nitrogen fixation in faba bean and common bean

Abstract: Potassium (K) is reported to improve plant's resistance against environmental stress. A frequently experienced stress for plants in the tropics is water shortage. It is not known if sufficient K supply would help plants to partially overcome the effects of water stress, especially that of symbiotic nitrogen fixation which is often rather low in the tropics when compared to that of temperate regions. Thus, the impact of three levels of fertilizer potassium (0.1, 0.8 and 3.0 mM K) on symbiotic nitrogen fixation was evaluated with two legumes under high (field capacity to 25% depletion) and low (less than 50% of field capacity) water regimes. Plants were grown in single pots in silica sand under controlled conditions with 1.5 mM N (15N enriched NH4NO3). The species were faba bean (Vicia faba L.), a temperate, amide producing legume and common bean (Phaseolus vulgaris L.), a tropical, ureide producing species. In both species, 0.1 mM K was insufficient for nodulation at both moisture regimes, although plant growth was observed. The supply of 0.8 or 3.0 mM K allowed nodulation and subsequent nitrogen fixation which appeared to be adequate for respective plant growth. High potassium supply had a positive effect on nitrogen fixation, on shoot and root growth and on water potential in both water regimes. Where nodulation occurred, variations caused by either K or water supply had no consequences on the percentage of nitrogen derived from the symbiosis. The present data indicate that K can apparently alleviate water shortage to a certain extent. Moreover it is shown that the symbiotic system in both faba bean and common bean is less tolerant to limiting K supply than plants themselves. However, as long as nodulation occurs, N assimilation from the symbiotic source is not selectively affected by K as opposed to N assimilation from fertilizer.

An assessment of nitrogen fixation as a source of nitrogen to the North Atlantic Ocean

Abstract: The role of nitrogen fixation in the nitrogen cycle of the North Atlantic basin was re-evaluated because recent estimates had indicated a far higher rate than previous reports. Examination of the available data on nitrogen fixation rates and abundance of Trichodesmium, the major nitrogen fixing organism, leads to the conclusion that rates might be as high as 1.09 × 1012 mol N yr-1. Several geochemical arguments are reviewed that each require a large nitrogen source that is consistent with nitrogen fixation, but the current data, although limited, do not support a sufficiently high rate. However, recent measurements of the fixation rates per colony are higher than the historical average, suggesting that improved methodology may require a re-evaluation through further measurements. The paucity of temporally resolved data on both rates and abundance for the major areal extent of the tropical Atlantic, where aeolian inputs of iron may foster high fixation rates, represents another major gap.

Seasonal nitrogen fixation dynamics in a marine microbial mat: Potential roles of cyanobacteria and microheterotrophs

Abstract: Diel rates of nitrogen (N,) fixation (acetylene reduction) and primary production (14C0, fixation) were examined seasonally on a North Carolina Atlantic coastal, intertidal, benthic microbial mat community dominated by the filamentous, nonheterocystous cyanobacterial genera Microcoleus and Lyngbya. Highest hourly and daily rates of Nz and CO2 fixation were observed during spring through fall. During this period, an inverse temporal relationship was noted between these processes, with CO, fixation closely tracking irradiance and Nz fixation rates remaining low during daylight and becoming maximal at night. Under the influence of the photosynthetic (PS 2) inhibitor 3-(3,4 dichlorophenyl)- 1,l dimethylurea (DCMU), daytime N, fixation was enhanced, indicating in situ 0, inhibition of Nz fixation. The most pronounced DCMU stimulation of daytime N2 fixation was in spring-fall. Both N2 and CO, fixation rates were lower in winter. Winter patterns of diel N, fixation were the reverse of those in summer, with maximum rates at midday. The reversal was related to seasonal changes in daily and hourly photosynthetic rates, leading to differential 0, suppression of N, fixation. Seasonal changes in cyanobacterial community composition and bacterial diazotrophy may have played additional roles in determining diel rates and patterns of N2 fixation and mat production. Results indicate a more important role for bacteria in the dynamics of mat Nz fixation than has been previously recognized.

Microscale physiological and ecological studies of aquatic cyanobacteria: Macroscale implications

Abstract: Cyanobacteria have had a profound and unparalleled biogeochemical impact on the earth's biosphere. As the first oxygenic phototrophs, cyanobacteria were responsible for the transition from anaerobic to aerobic life. Ironically, molecular oxygen (O2) is inhibitory to critical components of cyanobacterial metabolism, including photosynthesis and nitrogen fixation. Cyanobacteria have developed a great variety of biochemical, structural, and biotic adaptations ensuring optimal growth and proliferation in diverse oxic environments to counter this difficult situation. Structurally, cyanobacteria reveal remarkable diversity, including the formation of highly differentiated, O2-deplete cells (heterocysts), multicellularity as trichomes, and aggregates, that, among N2-fixing genera, facilitate division of labor between aerobic and anaerobic processes. Cyanobacteria enjoy unique consortial and symbiotic associations with other microorganisms, higher plants, and animals, in which O2 consumption is closely coupled in time and space to its production. Because as prokaryotes they are devoid of O2-consuming organelles (e.g., mitochondria), cyanobacteria have developed alternative strategies for locally protecting O2-sensitive processes, including consortial relationships with other microorganisms. Specific organic compounds released by cyanobacteria are capable of chemotactically attracting bacterial consorts, which in turn attach to the host cyanobacteria, consume O2, and recycle inorganic nutrients within the cyanobacterial “phycosphere.” Multicellularity and aggregation lead to localized O2 gradients and hypoxic/anoxic microzones in which O2-sensitive processes can coexist. Microscale partitioning of O2-producing and O2-inhibited processes promotes contiguous and effective metabolite and nutrient exchange between these processes in oxygenated waters, representing a bulk of the world's oceanic and freshwater ecosystems. © 1996 Wiley-Liss, Inc.

Toward a new concept of the evolution of symbiotic nitrogen fixation in the Leguminosae

Abstract: It is generally believed that only the nodulating species of the Leguminosae fix atmospheric nitrogen; however, anatomical, ecological and taxonomic considerations indicate that non-nodulating legume species may also fix nitrogen. To test whether nitrogen-fixing symbioses in the Leguminosae might extend to the non-nodulating species, a survey of the Leguminosae was conducted: living plants of non-nodulating species were assayed using acetylene reduction. Ethylene evolution, indicating apparent nitrogenase activity, was detected in non-nodulating species representing the major taxonomic groups of Caesalpinioideae as well as in non-nodulating species of the Papilionoideae and Mimosoideae. Non-nodules nitrogen fixation appears to have provided evolutionary precursors for the nodular symbiosis in the Leguminosae.

15N-determined dinitrogen fixation capacity of common bean (Phaseolus vulgaris) cultivars under water stress

Abstract: The effect of water stress on nitrogen fixation in seven common bean (Phaseolus vulgaris L.) genotypes was investigated in Celaya, Gto., Mexico, in 1991. Beans were grown under four moisture regimes: (1) well-irrigated, control, (2) with water stress during the vegetative stage, (3) with water stress during the reproductive stage and (4) with water stress during the whole growing cycle. Biological nitrogen fixation was measured by 15N-isotope dilution using sorghum as a reference crop. Nodulation and N2-fixation data showed genotypic differences in response to water stress. Under non-stressed conditions, cv. Bayocel fixed the most nitrogen (85 kg/ha) and cultivar Flor de Mayo Baji'o the least (33 kg/ha). Under water stress at the reproductive stage, these cultivars fixed 9 and 6 kg N/ha, respectively. Water stress during the reproductive stage reduced nodulation by an average of 43% with no recovery after rewatering. Water stress during the reproductive stage had a greater effect on N2-fixation than on grain yield; in comparison to the control, N2-fixation was reduced to one sixth while grain yield was only reduced by 50%.

Nodulation and estimation of symbiotic nitrogen fixation by herbaceous and shrub legumes in Guinea savanna in Nigeria.

Abstract: Twelve herbaceous and shrub legume species were grown in pot and field experiments in five sites representing three agroecological zones in moist savanna in Nigeria. The objectives were to: (1) assess natural nodulation of the legumes and characterize their indigenous rhizobia, (2) determine their need for rhizobia inoculation and (3) estimate the amount of N2 fixed by each of these legumes. At 4 weeks after planting (WAP), Crotolaria verrucosa was not nodulated at any of the sites while Centrosema pascuorum had the highest number of nodules in all sites. At 8 WAP, all legumes were nodulated, with Mucuna pruriens having the least number of nodules and Stylosanthes hamata the highest. The number of nodules, however, was inversely correlated to the mass of nodules. Significant differences in nodulation of the legume species grown in the field also occurred between and within sites. Mucuna pruriens and Lablab purpureus produced more shoot and nodule biomass than the other legumes in all sites. Growth of most of these legumes responded to fertilizer application, except for C. verrucosa and Aeschynomene histrix. Except for C. verrucosa, average proportion of N2 fixed was about 80% and this was reduced by about 20% with N fertilizer application. The majority of rhizobia isolates (60%) were slow growing, belonging to the Bradyrhizobia spp. group. Selected rhizobia isolates evaluated on Cajanus cajan, C. pascuorum, M. pruriens and Psophocarpus palustris varied from ineffective to highly effective in Leonard jar conditions. However, only growth of M. pruriens responded to inoculation in potted soils, whereas it was lower than that obtained with N fertilizer application. This indicated the need to screen more rhizobia in order to improve N2 fixation and growth of legume species such as M. pruriens when it is introduced in soils deficient in N.

Subsoil nitrate uptake and symbiotic dinitrogen fixation by alfalfa

Abstract: Alfalfa (Medicago saliva L.) is a deeply rooted perennial that may be able to remove NO 3 from the subsoil beneath the rooting zone of annual crops. Our main objectives were to characterize the potential of two alfalfa cultivars (Agate and Ineffective Agate, an ineffectively nodulated near isoline of Agate) to remove NO 3 from the subsoil and to evaluate the influence of available subsoil NO 3 on symbiotic N 2 fixation. Low ( 0.3 mM) or high (20 mM) NO 3 -N concentrations were supplied through a subsoil irrigation system installed in a Hubbard loamy sand soil (sandy, mixed Udorthentic Haploboroll) at Beeker, MN. Nitrate uptake and N 2 fixation were evaluated during two regrowth periods using 15 N. During fall regrowth in the establishment year, supply of 20 mM 15 N-enriched NO 3 through subsoil irrigation did not decrease symbiotic N 2 fixation; during summer regrowth of the first production year, symbiotic N 2 fixation decreased only 40% (from 7.77 to 4.60 g N m -2 ). Even though it had higher yields, Agate alfalfa removed less subsoil NO 3 -N (1.52 and 5.51 g N m -2 ) than did Ineffective Agate (3.12 and 7.13 g N m -2 ) in each of two harvests where 15 N was applied. Root length and mass after 2 yr were not altered by NO 3 treatment or cultivar. This is the first time that direct measurements of subsoil nitrate removal have been made in the field for an N 2 -fixing crop. Because it is unlikely that differences in NO3 uptake efficiency could account for such cultivar differences, we propose that assimilation of atmospheric N 2 reduced the capacity of the fixing crop to absorb NO 3 . These results suggest that non-N 2 -fixing alfalfa cultivars would be more useful for bioremediation of nitrate-contaminated sites than standard cultivars.

Cycling of amino compounds in symbiotic lupin

Abstract: The composition of amino acids was determined in the xylem and phloem sap of symbiotic lupins grown under a variety of treatments designed to alter the rate of nitrogen fixation. Asparagine was the major amino acid in both xylem and phloem with glutamine, glutamate and aspartate also major components. GABA had a high concentration in the xylem while valine was a major component in the phloem. Exposure to combined nitrogen in the form of either ammonium or nitrate caused a reduction in specific nitrogenase activity and was associated with subsequent changes in both of the translocated saps. Inhibiting nitrogen fixation by exposing nodules to oxygen produced a lower amide to amine ratio in the xylem sap (1.3 :1) compared with control and nitrate ratios (2.6 :1) and ammonium ratios (7.1 :1). Similar ratios for amide :amine were also observed in the phloem sap. Labelling studies using 15 N 2 to follow nitrogen fixation, ammonium assimilation and amino acid transport have shown rapid accumulation of label into glutamine with subsequent enrichment in glutamate, aspartate, alanine, and GABA. Asparagine was found in high concentrations in nodules and became slowly enriched. Labelled nitrogen fixed and assimilated in nodules was detected 40 min later in stem xylem extracts, largely as the amides glutamine and asparagine. These experiments provide evidence that large amounts of nitrogenous compounds are cycled through the root nodules of symbiotic plants (contributing approximately 50% of xylem N) and that differences in the composition of the phloem sap may influence nodule growth and activity.

Biological Nitrogen Fixation Associated with Rice Production

Abstract: Research on biological nitrogen fixation began in Western Europe during the nineteenth century, under conditions where a mere increase in nitrogen fertilization inevitably increased yields: it was the beginning of the triumphal era of fertilizers. In the thirties began the era of legume inoculation: and this again was due to a very simplistic situation in Western countries: the absence of bacterial symbionts adapted to crops such as soybeans. Biological nitrogen fixation (BNF) appeared as an extension of nitrogen fertilization, with the same effects on farmers' incomes. The amount of nitrogen available was the limiting factor of the farmer' s income, whatever its origin: mineral nitrogen from soil or fertilizers as wen as nitrogen derived from biological fixation. In a way, this very clear-cut situation allowed for the rapid development of our knowledge about BNF, and its use by farmers. Nevertheless, when the time came to extrapolate to tropical countries, some difficulties arose. Some were due to a lack of knowledge about BNF systems in warm countries. Other difficulties were due to the interference of many yield-limiting factors other than nitrogen. But the main difficulty resulted from a misunderstanding about the objectives: the goal of developing BNF is not to achieve the maximum nitrogen input, it is really to achieve the maximum income (money and/or food) for farmers. In many tropical countries, the farmer' s income is not directly proportional to nitrogen availability. We, as scientists, are confined to scientific objectives (maximum nitrogenase activity) whereas countries, such as Bangladesh, must aim at a maximum farming efficiency, biological science being largely secondary to other disciplines such as sociology or economics.

Free-living nitrogen-fixing bacteria in temperate cropping systems: Influence of nitrogen source

Abstract: Sustainable cropping systems rely on a minimum of external inputs. In these systems N is largely acquired in animal manures and leguminous green manures. Little is known of how these organic forms of N fertilizer influence the presence and activity of free-living N2-fixing bacteria. High concentrations of inorganic N in soil inhibit N2-fixation in cyanobacteria and Azotobacter spp. It is likely that manure and fertilizer applications would result in concentrations of inorganic N capable of inhibiting N2 fixation and, ultimately, the presence of these organisms. We investigated the effect of synthetic and organic N fertilizer sources on the populations and N2-fixation potential of free-living N2-fixing bacteria in the Farming Systems Trial at the Rodale Research Institute. Field plots received the following N treatments prior to corn (Zea mays L.) production: (1) Legume rotations and green manures supplying about 165 kg N ha-1; (2) beef cattle manure applied at a rate of 220 kg N ha-1 (plus 60 kg N ha-1 from 1994 hay plow-down); or (3) fertilizer N (urea and NH4NO3) applied at a rate of 145 kg N ha-1. Soil samples were collected at two depths from corn plots four times during the growing season, and analyzed for soil moisture, soil pH, numbers of N2-fixing cyanobacteria and Azotobacter spp., extractable NH inf4 sup+ and NO inf3 sup- , and potentially mineralizable N. Soil samples collected in mid-July were analyzed for nitrogenase activity (by C2H2 reduction) and total C and N. Populations of Azotobacter spp. and cyanobacteria were influenced only slightly by treatment; however, cyanobacteria species composition was notably influenced by treatment. Nitrogenase activity in surface soils was greatest in legume-N plots and in subsurface plots levels were greatest in fertilizer-N plots. Populations and activity of free-living N-fixing bacteria appeared to be somewhat reduced in all plots as a result of low soil pH levels and high concentrations of inorganic N across all treatments. Annual applications of N to all plots resulted in high levels of potentially mineralizable N that in turn may have reduced non-symbiotic N2-fixation in all plots.

Control of nitrogen fixation by oxygen in purple nonsulfur bacteria

Abstract: Some members of the facultatively phototrophic bacteria are able to grow diazotrophically in the presence of oxygen. As in other diazotrophs, the nitrogenase of the phototrophic bacteria is highly sensitive to oxygen; therefore, both the function and the expression of nitrogenase are strictly controlled by oxygen. This review focuses on the different levels of oxygen control in the two most extensively studied facultatively phototrophic bacteria, Rhodospirillum rubrum and Rhodobacter capsulatus. Current data show that oxygen controls nitrogen fixation at least at the levels of (1) transcription of nif genes, (2) the accumulation of the three different nitrogenase polypeptides, (3) the cellular activity of nitrogen fixation. In Rba. capsulatus, activation of the nifH promoter is the least oxygen-sensitive step, and nitrogen fixation is the most oxygen-sensitive step. ADP-Ribosylation of nitrogenase, occurring under conditions of ammonium-dependent inactivation of the enzyme, is not observed when Rba. capsulatus is exposed either suddenly or at a steady state to increased oxygen concentrations. Future research is required to understand the mechanisms of protection of nitrogenase against oxygen damage, and also the mechanisms by which oxygen controls the formation and activity of nitrogenase; this will add significantly to the biologically important question of how cells deal with the presence of toxic oxygen.

Nodulation, biomass production, and nitrogen fixation in alfalfa under drought

Abstract: The effect of drought on nodulation, biomass production, and symbiotic nitrogen fixation was evaluated for seven strains of Rhizobium meliloti Dang, and three alfalfa (Medicago sativa L. and M. falcata L.) accessions. Alfalfa plants were grown for 10 weeks in plastic pots containing a sterilized mixture of sand and peat moss maintained at ‐0.03, ‐0.5, and ‐1.0 MPa. Number of nodules, total plant dry weight, and shoot nitrogen pool declined as drought increased from ‐0.03 to ‐1.0 MPa. Rhizobial strains varied markedly in their ability to form nodules and fix nitrogen under drought. Plants inoculated with strains UL 136, UL 210, and UL 222 formed nodules at ‐0.5 and ‐1.0 MPa and had plants with greater total dry weight and shoot nitrogen pool than plants with no nitrogen. These three rhizobial strains may be beneficial for increasing alfalfa production under water‐limited conditions.

Nodulation and nitrogen fixation by subterranean clover in acid soils as influenced by lime application, toxic aluminium, soil mineral N, and competition from annual ryegrass

Abstract: The effects of low soil pH and lime addition on nodulation and nitrogen fixation in subterranean clover ( Trifolium subterraneum L.) were examined in field and glasshouse experiments in south-west Australia. Field data from a number of broadacre pastures indicated that soil acidity combined with high soil aluminium reduced clover dependence on nitrogen fixation (% Ndfa) and that addition of lime, generally, promoted modulation and increased % Ndfa. In a detailed study at one site additions of lime increased soil pH, reduced extractable aluminium concentrations below phytotoxic concentrations and increased plant nodulation. Glasshouse experiments also demonstrated positive effects of liming in reducing extractable aluminium and increasing total N 2 fixation and % Ndfa, but beneficial effects on N 2 fixation were partly offset by counteractive effects on symbiosis through increased nitrogen mineralisation at high lime application rates. Where companion reegrass ( Lolium rigidum Gaudin) was pot cultured with subterranean clover soil mineral N released following liming was absorbed preferentially by the grass, resulting in increases in % Ndfa of the associated clover.