Showing papers on "Nitrogen fixation published in 2008"

Nitrogen fixation island and rhizosphere competence traits in the genome of root-associated Pseudomonas stutzeri A1501

Abstract: The capacity to fix nitrogen is widely distributed in phyla of Bacteria and Archaea but has long been considered to be absent from the Pseudomonas genus. We report here the complete genome sequencing of nitrogen-fixing root-associated Pseudomonas stutzeri A1501. The genome consists of a single circular chromosome with 4,567,418 bp. Comparative genomics revealed that, among 4,146 protein-encoding genes, 1,977 have orthologs in each of the five other Pseudomonas representative species sequenced to date. The genome contains genes involved in broad utilization of carbon sources, nitrogen fixation, denitrification, degradation of aromatic compounds, biosynthesis of polyhydroxybutyrate, multiple pathways of protection against environmental stress, and other functions that presumably give A1501 an advantage in root colonization. Genetic information on synthesis, maturation, and functioning of nitrogenase is clustered in a 49-kb island, suggesting that this property was acquired by lateral gene transfer. New genes required for the nitrogen fixation process have been identified within the nif island. The genome sequence offers the genetic basis for further study of the evolution of the nitrogen fixation property and identification of rhizosphere competence traits required in the interaction with host plants; moreover, it opens up new perspectives for wider application of root-associated diazotrophs in sustainable agriculture.

Globally Distributed Uncultivated Oceanic N2-Fixing Cyanobacteria Lack Oxygenic Photosystem II

Abstract: Biological nitrogen (N2) fixation is important in controlling biological productivity and carbon flux in the oceans Unicellular N2-fixing cyanobacteria have only recently been discovered and are widely distributed in tropical and subtropical seas Metagenomic analysis of flow cytometry-sorted cells shows that unicellular N2-fixing cyanobacteria in "group A" (UCYN-A) lack genes for the oxygen-evolving photosystem II and for carbon fixation, which has implications for oceanic carbon and nitrogen cycling and raises questions regarding the evolution of photosynthesis and N2 fixation on Earth

Use of nitrogen-fixing bacteria as biofertiliser for non-legumes: prospects and challenges.

Abstract: The potential of nitrogen-fixing (NF) bacteria to form a symbiotic relationship with leguminous plants and fix atmospheric nitrogen has been exploited in the field to meet the nitrogen requirement of the latter. This phenomenon provides an alternative to the use of the nitrogenous fertiliser whose excessive and imbalanced use over the decades has contributed to green house emission (N2O) and underground water leaching. Recently, it was observed that non-leguminous plants like rice, sugarcane, wheat and maize form an extended niche for various species of NF bacteria. These bacteria thrive within the plant, successfully colonizing roots, stems and leaves. During the association, the invading bacteria benefit the acquired host with a marked increase in plant growth, vigor and yield. With increasing population, the demand of non-leguminous plant products is growing. In this regard, the richness of NF flora within non-leguminous plants and extent of their interaction with the host definitely shows a ray of hope in developing an ecofriendly alternative to the nitrogenous fertilisers. In this review, we have discussed the association of NF bacteria with various non-leguminous plants emphasizing on their potential to promote host plant growth and yield. In addition, plant growth-promoting traits observed in these NF bacteria and their mode of interaction with the host plant have been described briefly.

Nitrogen fixation by phyllosphere bacteria associated with higher plants and their colonizing epiphytes of a tropical lowland rainforest of Costa Rica.

Abstract: Leaf surfaces (phyllospheres) have been shown to provide appropriate conditions for colonization by microorganisms including diazotrophic bacteria that are able to fix atmospheric nitrogen (N(2)). In this study, we determined leaf-associated N(2) fixation of a range of rainforest plants in Costa Rica, under different environmental conditions, by tracing biomass N incorporation from (15)N(2). N(2)-fixing bacterial communities of the plant species Carludovica drudei, Grias cauliflora and Costus laevis were investigated in more detail by analysis of the nifH gene and leaf-associated bacteria were identified by 16S rRNA gene analysis. N(2) fixation rates varied among plant species, their growth sites (different microclimatic conditions) and light exposure. Leaf-associated diazotrophic bacterial communities detected on C. drudei and C. laevis were mainly composed of cyanobacteria (Nostoc spp.), whereas on the leaves of G. cauliflora gamma-proteobacteria were dominant in addition to cyanobacteria. The complexity of diazotrophic communities on leaves was not correlated with N(2) fixation activity. 16S rRNA gene sequence analysis suggested the presence of complex microbial communities in association with leaves, however, cyanobacteria showed only low abundance. Our findings suggest that cyanobacteria as well as gamma-proteobacteria associated with leaf-colonizing epiphytes may provide significant nitrogen input into this rainforest ecosystem.

Effects of the Inoculation of Burkholderia vietnamensis and Related Endophytic Diazotrophic Bacteria on Grain Yield of Rice

Abstract: During a survey of endophytic diazotrophic bacteria associated with different rice varieties in Tamilnadu, some “endophytes” were obtained Thirteen bacterial isolates from surface-sterilized roots and shoots were obtained in pure culture, which produced indole acetic acid (IAA) and reduced acetylene to ethylene Polymerase chain reaction (PCR) amplification confirmed the presence of nif-H gene in all the isolates Morphological, biochemical, and molecular characteristics indicated that all of them belonged to the genus Burkholderia One of them, MGK3, was consistently more active in reducing acetylene, and 16S rDNA sequences of isolate MGK3 confirmed its identification as Burkholderia vietnamiensis Colonization of rice root was confirmed by strain MGK3 marked with gusA gene The inoculated roots showed a blue color, which was most intense at the points of lateral root emergence and at the root tip Transverse sections of roots, 15 days after inoculation, revealed beta-glucuronidase (GUS) activity within many of the cortical intercellular spaces next to the stele and within the aerenchyma Nitrogen fixation was quantified by using 15N isotope dilution method with two different cultivars grown in pot and field experiments Higher nitrogen fixation was observed in variety Ponni than in ADT-43, where nearly 42% (field) and 40% (pot) of the nitrogen was derived from the atmosphere (% Ndfa) Isolate MGK3 was used to inoculate rice seedlings in a comparison with four other diazotrophs, viz, Gluconacetobacter diazotrophicus LMG7603, Herbaspirillum seropedicae LMG6513, Azospirillum lipoferum 4B LMG4348, and B vietnamiensis LMG10929 They were used to conduct two pot and four field inoculation experiments MGK3 alone, and combined with other diazotrophs, performed best under both pot and field conditions: combined inoculation produced yield increases between 95 and 236%, while MGK3 alone increased yield by 56 to 1216% over the uninoculated control treatment

Plant growth-promoting rhizobacteria for improving nodulation and nitrogen fixation in the common bean (Phaseolus vulgaris L.)

Abstract: A greenhouse experiment was performed to evaluate the effects of plant growth-promoting rhizobac- teria (PGPR) on nodulation, biological nitrogen fixation (BNF) and growth of the common bean (Phaseolus vul- garis L. cv. Tenderlake). Single and dual inoculation treatments of bean with Rhizobium and/or PGPR were administered to detect possible changes in the levels of and interactions between the phytohormones IAA and cytoki- nin. Bean plants cv. Tenderlake were grown in pots containing Fluvic Neosol eutrophic (pH 6.5). Fourteen kilogram aliquots of soil contained in 15-l pots were autoclaved. Bean seeds were surface sterilized and inocu- lated with Rhizobium tropici (CIAT 899-standard strain) alone and in combination with one of the PGPR strains: Bacillus endophyticus (DSM 13796), B. pumilus (DSM 27), B. subtilis (DSM 704), Paenibacillus lautus (DSM 13411), P. macerans (DSM 24), P. polymyxa (DSM 36), P. polymyxa (Loutit L.) or Bacillus sp.(65E180). The experimental design was randomized block design with three replications. Beans co-inoculated with Rhizobium tropici (CIAT899) and Paenibacillus polymyxa (DSM 36) had higher leghemoglobin concentrations, nitrogenase activity and N2 fixation efficiency and thereby formed associations of greater symbiotic efficiency. Inoculation with Rhizobium and P. polymyxa strain Loutit (L) stimu- lated nodulation as well as nitrogen fixation. PGPR also stimulated specific-nodulation (number of nodules per gram of root dry weight) increases that translated into higher levels of accumulated nitrogen. The activities of phytohormones depended on their content and interactions with Rhizobium tropici and Paenibacillus and/or Bacillus (PGPR) strains which affect the cytokinin in content in the common bean.

Azospirillum amazonense inoculation: effects on growth, yield and N2 fixation of rice (Oryza sativa L.)

Abstract: Bacteria of the genus Azospirillum are considered to be plant-growth promoting bacteria (PGPR) and stimulate plant growth directly either by synthesising phyto-hormones or by promoting nutrition by the process of biological nitrogen fixation (BNF). Although this genus extensively studied, the effects of inoculation and the possible BNF contribution of the Azospirillum amazonense specie are very scarce. The aim of this study was to isolate, characterise and evaluate auxin production and nitrogenase activity of this species and to select, by inoculation of rice plants, promising isolates based on their ability to improve plant growth, yield and the BNF contribution. One hundred and ten isolates obtained from rice were characterised and grouped according to colony features. Forty-two isolates, confirmed as A. amazonense by the fluorescent in situ hybridization (FISH) technique, were tested for auxin production and nitrogenase activity in vitro. Subsequently plant growth promotion related to plant nutrition effect was evaluated, in vitro and in greenhouse experiments. The BNF contribution to plant growth was evaluated using the 15N isotope dilution technique. All A. amazonense strains tested produced indoles, but only 10% of them showed high production, above 1.33 μM mg protein−1. The nitrogenase activity also was variable and only 9% of isolates showed high nitrogenase activity and the majority (54%) exhibited a low potential. The inoculation of selected strains in rice under gnotobiotic conditions reduced the growth of root and aerial part when compared to the control, showing the negative effects of excess of phytohormone accumulation in the medium. However, in the greenhouse experiment, inoculation of strains of A. amazonense increased grain dry matter accumulation (7 to 11.6%), the number of panicles (3 to 18.6%) and nitrogen accumulation at grain maturation (3.5 to 18.5%). BNF contributions up to 27% were observed for A. amazonense Y2 (wild type strain). The data presented here is the first report that the PGPR effect of A. amazonense for rice plants grown under greenhouse conditions is mainly due the BNF contribution as measured by 15N isotope dilution technique, in contrast to the hormonal effect observed with other Azospirillum species studied.

Uptake of molybdenum and vanadium by a nitrogen-fixing soil bacterium using siderophores

Abstract: Biological availability of molybdenum and vanadium is facilitated by siderophores that are produced by cultures of the bacterium Azotobacter vinelandii during the fixation of atmospheric nitrogen. This suggests that the production of strong binding compounds may be a widespread strategy for metal acquisition by bacteria and implies that the availability of molybdenum and vanadium may be critical for the nitrogen cycle of terrestrial ecosystems. Nitrogen fixation, the reaction that transforms atmospheric nitrogen into bioavailable ammonia and is responsible for the supply of nitrogen to Earth’s ecosystems, is mediated by the enzyme nitrogenase. This reaction requires molybdenum (Mo) or vanadium (V) in addition to iron (Fe) (refs 1, 2). Therefore, the availability of these trace metals may control the Earth’s nitrogen cycle3,4. Many bacteria release strong iron-binding compounds (siderophores) for iron acquisition5,6, but the effect of these compounds on Mo and V availability to nitrogen-fixing organisms is not well understood. Here, we show that the siderophores produced in cultures of Azotobacter vinelandii while fixing atmospheric nitrogen under limitation by Mo or V form strong complexes with molybdate and vanadate, and that these complexes are available for uptake. We also show that addition of these siderophores rapidly reverses the effect of other natural binding compounds that make Mo and V unavailable for uptake. Our results resolve the long-standing debate regarding the existence of bacterial ‘molybdophores’7,8,9, as well as the corollary question regarding ‘vanadophores’. We conclude that the production of strong binding compounds may be a widespread strategy for metal acquisition by bacteria, implying that the availability of Mo and V may be critical for the nitrogen cycle of terrestrial ecosystems.

Ecological occurrence of Gluconacetobacter diazotrophicus and nitrogen-fixing Acetobacteraceae members: their possible role in plant growth promotion.

Abstract: Gluconacetobacter diazotrophicus has a long-standing history of bacterial-plant interrelationship as a symbiotic endophyte capable of fixing atmospheric nitrogen. In low nitrogen fertilized sugarcane fields it plays a significant role and its occurrence was realised in most of the sugarcane growing countries. In this mini review, the association of G. diazotrophicus with sugarcane, other crop plants and with various hosts is discussed. The factors affecting survival in the rhizosphere and the putative soil mode of transmission are emphasized. In addition, other N2-fixing Acetobacteraceae members, including Gluconacetobacter azotocaptans, Gluconacetobacter johannae and Swaminathania salitolerans, occurring in coffee, corn and rice plants are also covered. Lastly, the plant-growth-promoting traits identified in this group of bacteria, including N2 fixation, phytohormone synthesis, P and Zn solubilization and biocontrol, are analysed.

Perspectives in Biological Nitrogen Fixation Research

Abstract: Nitrogen fixation, along with photosynthesis is the basis of all life on earth. Current understanding suggests that no plant fixes its own nitrogen. Some plants (mainly legumes) fix nitrogen via symbiotic anaerobic microorganisms (mainly rhizobia). The nature of biological nitrogen fixation is that the dinitrogenase catalyzes the reaction-splitting triple-bond inert atmospheric nitrogen (N2) into organic ammonia molecule (NH3). All known nitrogenases are found to be prokaryotic, multi-complex and normally oxygen liable. Not surprisingly, the engineering of autonomous nitrogen-fixing plants would be a long-term effort because it requires the assembly of a complex enzyme and provision of anaerobic conditions. However, in the light of evolving protein catalysts, the anaerobic enzyme has almost certainly been replaced in many reactions by the more efficient and irreversible aerobic version that uses O2. On the other hand, nature has shown numerous examples of evolutionary convergence where an enzyme catalyzing a highly specific, O2-requiring reaction has an oxygen-independent counterpart, able to carry out the same reaction under anoxic conditions. In this review, I attempt to take the reader on a simplified journey from conventional nitrogenase complex to a possible simplified version of a yet to be discovered light-utilizing nitrogenase.

Comparative genomic analysis of carbon and nitrogen assimilation mechanisms in three indigenous bioleaching bacteria: predictions and validations.

Abstract: Carbon and nitrogen fixation are essential pathways for autotrophic bacteria living in extreme environments. These bacteria can use carbon dioxide directly from the air as their sole carbon source and can use different sources of nitrogen such as ammonia, nitrate, nitrite, or even nitrogen from the air. To have a better understanding of how these processes occur and to determine how we can make them more efficient, a comparative genomic analysis of three bioleaching bacteria isolated from mine sites in Chile was performed. This study demonstrated that there are important differences in the carbon dioxide and nitrogen fixation mechanisms among bioleaching bacteria that coexist in mining environments. In this study, we probed that both Acidithiobacillus ferrooxidans and Acidithiobacillus thiooxidans incorporate CO2 via the Calvin-Benson-Bassham cycle; however, the former bacterium has two copies of the Rubisco type I gene whereas the latter has only one copy. In contrast, we demonstrated that Leptospirillum ferriphilum utilizes the reductive tricarboxylic acid cycle for carbon fixation. Although all the species analyzed in our study can incorporate ammonia by an ammonia transporter, we demonstrated that Acidithiobacillus thiooxidans could also assimilate nitrate and nitrite but only Acidithiobacillus ferrooxidans could fix nitrogen directly from the air. The current study utilized genomic and molecular evidence to verify carbon and nitrogen fixation mechanisms for three bioleaching bacteria and provided an analysis of the potential regulatory pathways and functional networks that control carbon and nitrogen fixation in these microorganisms.

Transcription profiling of soybean nodulation by Bradyrhizobium japonicum.

Abstract: Legumes interact with nodulating bacteria that convert atmospheric nitrogen into ammonia for plant use. This nitrogen fixation takes place within root nodules that form after infection of root hairs by compatible rhizobia. Using cDNA microarrays, we monitored gene expression in soybean (Glycine max) inoculated with the nodulating bacterium Bradyrhizobium japonicum 4, 8, and 16 days after inoculation, timepoints that coincide with nodule development and the onset of nitrogen fixation. This experiment identified several thousand genes that were differentially expressed in response to B. japonicum inoculation. Expression of 27 genes was analyzed by quantitative reverse transcriptase-polymerase chain reaction, and their expression patterns mimicked the microarray results, confirming integrity of analyses. The microarray results suggest that B. japonicum reduces plant defense responses during nodule development. In addition, the data revealed a high level of regulatory complexity (transcriptional, post-transcriptional, translational, post-translational) that is likely essential for development of the symbiosis and adjustment to an altered nutritional status.

Expression of a Class 1 Hemoglobin Gene and Production of Nitric Oxide in Response to Symbiotic and Pathogenic Bacteria in Lotus japonicus

Abstract: Symbiotic nitrogen fixation by the collaboration between leguminous plants and rhizobia is an important system in the global nitrogen cycle, and some molecular aspects during the early stage of host-symbiont recognition have been revealed. To understand the responses of a host plant against various bacteria, we examined expression of hemoglobin (Hb) genes and production of nitric oxide (NO) in Lotus japonicus after inoculation with rhizobia or plant pathogens. When the symbiotic rhizobium Mesorhizobium loti was inoculated, expression of LjHb1 and NO production were induced transiently in the roots at 4 h after inoculation. In contrast, inoculation with the nonsymbiotic rhizobia Sinorhizobium meliloti and Bradyrhizobium japonicum induced neither expression of LjHb1 nor NO production. When L. japonicus was inoculated with plant pathogens (Ralstonia solanacearum or Pseudomonas syringae), continuous NO production was observed in roots but induction of LjHb1 did not occur. These results suggest that modulation of NO levels and expression of class 1 Hb are involved in the establishment of the symbiosis.

Diversity of nifH gene pools in the rhizosphere of two cultivars of sorghum (Sorghum bicolor) treated with contrasting levels of nitrogen fertilizer

Abstract: The diversity of nitrogen-fixing bacteria was assessed in the rhizospheres of two cultivars of sorghum (IS 5322-C and IPA 1011) sown in Cerrado soil amended with two levels of nitrogen fertilizer (12 and 120 kg ha(-1)). The nifH gene was amplified directly from DNA extracted from the rhizospheres, and the PCR products cloned and sequenced. Four clone libraries were generated from the nifH fragments and 245 sequences were obtained. Most of the clones (57%) were closely related to nifH genes of uncultured bacteria. NifH clones affiliated with Azohydromonas spp., Ideonella sp., Rhizobium etli and Bradyrhizobium sp. were found in all libraries. Sequences affiliated with Delftia tsuruhatensis were found in the rhizosphere of both cultivars sown with high levels of nitrogen, while clones affiliated with Methylocystis sp. were detected only in plants sown under low levels of nitrogen. Moreover, clones affiliated with Paenibacillus durus could be found in libraries from the cultivar IS 5322-C sown either in high or low amounts of fertilizer. This study showed that the amount of nitrogen used for fertilization is the overriding determinative factor that influenced the nitrogen-fixing community structures in sorghum rhizospheres cultivated in Cerrado soil.

Flux capacities and acclimation costs in Trichodesmium from the Gulf of Mexico

Abstract: Phytoplankton function and acclimation are driven by catalytic protein complexes that mediate key physiological transformations, including generation of photosynthetic ATP and reductant, and carbon and nitrogen fixation. Quantitation of capacities for these processes allows estimation of rates for key ecosystem processes, and identification of factors limiting primary productivity. We herein present molar quantitations of PSI, PSII, ATP synthase, RuBisCO and the Fe protein of nitrogenase of Trichodesmium collected from the Gulf of Mexico, in comparison to determinations for a range of cyanobacteria growing in culture. Using these measurements, estimates were generated for Trichodesmium capacities for carbon fixation of 1–3.4 g C g chl a −1 h−1 and nitrogen fixation of 0.06–0.17 g N g chl a −1 h−1, with diel variations in capacities. ATP synthase levels show that ATP synthesis capacity is sufficient to support these levels of carbon and nitrogen fixation, and that ATP synthase levels change over the day in accordance with the ATP demands of nitrogenase and RuBisCO activity. Levels of measured complexes indicate that Trichodesmium manifests n-type diel light acclimation through rapid changes in RuBisCO:PSII, supported by significant investment of cellular nitrogen. The plasticity in the levels and stoichiometry of these core complexes show that changes in the abundance of core protein complexes are an important component of acclimation and regulation of metabolic function by Trichodesmium populations.

Influence of different Sinorhizobium meliloti inocula on abundance of genes involved in nitrogen transformations in the rhizosphere of alfalfa (Medicago sativa L.)

Abstract: Summary Inoculation of leguminous seeds with selected rhizobial strains is practised in agriculture to ameliorate the plant yield by enhanced root nodulation and nitrogen uptake of the plant. However, effective symbiosis between legumes and rhizobia does not only depend on the capacity of nitrogen fixation but also on the entire nitrogen turnover in the rhizosphere. We investigated the influence of seed inoculation with two indigenous Sinorhizobium meliloti strains exhibiting different efficiency concerning plant growth promotion on nitrogen turnover processes in the rhizosphere during the growth of alfalfa. Quantification of six target genes (bacterial amoA, nirK, nirS, nosZ, nifH and archaeal amoA) within the nitrogen cycle was performed in rhizosphere samples before nodule formation, at bud development and at the late flowering stage. The results clearly demonstrated that effectiveness of rhizobial inocula is related to abundance of nifH genes in the late flowering phase of alfalfa. Moreover, other genes involved in nitrogen turnover had been affected by the inocula, e.g. higher numbers of amoA copies were observed during flowering when the more effective strain had been inoculated. However, the respective gene abundances differed overall to a greater extent between the three plant development stages than between the inoculation variants.

Chapter 4 – Nitrogen Fixation in the Marine Environment

Abstract: Publisher Summary This chapter first summarizes the recent advances and insights relative to nitrogen (N2) fixation in benthic marine environments, move on to the water column, then summarizes controls on N2 fixation and comments on the broader, biogeochemical impacts of N2 fixation. In benthic environments, ranging from the rhizosphere of shallow water macrophyte communities such as Zostera, Thalassia, and Spartina hundreds of different diazotrophic strains have been isolated. These diazotrophs make significant contributions to the nitrogen economy of their respective plant communities. In the pelagic zone, iron and phosphorus availability is a major constraint on N2 fixation. In the north Atlantic, positive nitrogen anomalies in the subeuphotic zone, resulting from Aeolian dust deposition of Fe corresponds to areas in which P limits growth of diazotrophs. There is precious little information on the pathways of fixed N from pelagic diazotrophs to other members of the plankton and nekton. Viral lysis may be a major pathway as is the leakage of fixed N from healthy Trichodesmium cells. The spectrum of organisms recognized to contribute to N2 fixation in the sea has been greatly expanded, and there has been a virtual revolution in understanding of its quantitative significance, the interplay between N2 fixation and the C cycle and the major controls on this process.

Growth and carbon content of three different‐sized diazotrophic cyanobacteria observed in the subtropical north pacific1

Abstract: To develop tools for modeling diazotrophic growth in the open ocean, we determined the maximum growth rate and carbon content for three diazotrophic cyanobacteria commonly observed at Station ALOHA (A Long-term Oligotrophic Habitat Assessment) in the subtropical North Pacific: filamentous nonheterocyst-forming Trichodesmium and unicellular Groups A and B. Growth-irradiance responses of Trichodesmium erythraeum Ehrenb. strain IMS101 and Crocosphaera watsonii J. Waterbury strain WH8501 were measured in the laboratory. No significant differences were detected between their fitted parameters (±CI) for maximum growth rate (0.51 ± 0.09 vs. 0.49 ± 0.17 d(-1) ), half-light saturation (73 ± 29 vs. 66 ± 37 μmol quanta · m(-2) · s(-1) ), and photoinhibition (0 and 0.00043 ± 0.00087 [μmol quanta · m(-2) · s(-1) ](-1) ). Maximum growth rates and carbon contents of Trichodesmium and Crocosphaera cultures conformed to published allometric relationships, demonstrating that these relationships apply to oceanic diazotrophic microorganisms. This agreement promoted the use of allometric models to approximate unknown parameters of maximum growth rate (0.77 d(-1) ) and carbon content (480 fg C · μm(-3) ) for the uncultivated, unicellular Group A cyanobacteria. The size of Group A was characterized from samples from the North Pacific Ocean using fluorescence-activated cell sorting and real-time quantitative PCR techniques. Knowledge of growth and carbon content properties of these organisms facilitates the incorporation of different types of cyanobacteria in modeling efforts aimed at assessing the relative importance of filamentous and unicellular diazotrophs to carbon and nitrogen cycling in the open ocean.

Measurement of asymbiotic N2 fixation in Australian agriculture

Abstract: A wide range of bacteria capable of nitrogen fixation (free-living and associative) can be found in all agricultural soils across Australia, however measurement of their effectiveness in N2 fixation has proved to be problematic because rates are low compared to symbiotic systems and quantitative methodologies barely adequate. It is generally believed that associative N2 fixation rates may be greater than free-living N2 fixation rates in ecosystems where grasses (including cereals) dominate, although this has not been unequivocally proven. Conditions promoting asymbiotic N2 fixation are reduced availability of oxygen, high temperature and soil water, and large amounts of microbially available C in the soil. The most direct measure of N2 fixation, incorporation of 15N2, has rarely been used in undisturbed systems, and we can find no examples of its field application in Australia. Nitrogen balance calculations, based on long-term changes in total soil N of systems and crop N removal, have been used to infer asymbiotic N2 fixation, but do not measure it directly. Such N balance studies can thus only give an indication of potential asymbiotic N2 fixation over long periods of time, but cannot confirm it. There are no robust N balances published for Australian ecosystems. The acetylene reduction assay for nitrogenase activity has been used in Australia to study responses of both free-living and associative N2 fixation systems to regulating factors. These studies have highlighted the importance of C supply, high soil water content and temperature in increasing asymbiotic N2 fixation in soils. However significant methodological limitations do not allow field scale quantification using this assay. On balance we would concur with the authors of several earlier global reviews of this topic and conclude that (in Australia) contributions of nitrogen to crop growth from asymbiotic N2 fixation are likely to be

Screening of nitrogen fixers from rhizospheric bacterial isolates associated with important desert plants

Abstract: Free living nitrogen fixers represent a range of microorganisms including bacteria living on plant residues (saprophytes), bacteria which live entirely within plants (endophytes) and bacteria living in close association with the plant roots (rhizobacteria). We have isolated a number of rhizosphere associated bacteria from two of the hot arid zone plants and explored these in terms of nitrogen fixing ability both in solid and liquid culture conditions. The maximum coloring zone was developed in T-1 (22 mm) whereas minimum was in T-3 (4 mm) in case of the C. polygonoides associated bacterial community. The coloring zone was found maximum in TS-2 (27 mm) and minimum in TS-1 (11 mm), in case of isolates associated with rhizosphere of L. sindicus. The highest and lowest acetylene reduction activity (ARA) was detected in TS-13 (8303 n moles / 24 h) and T-10 (1658 n moles / 24 h), respectively.

Biological nitrogen fixation of summer legumes and their residual effects on subsequent rainfed wheat yield

Abstract: Biological nitrogen fixation is the most important biochemical reaction for life on earth. Phosphorus and rhizobium inoculation increased N2-fixation by legumes. Legumes in rotation with cereals contribute to the total N pool in soil and improved cereals yield. In view of importance of grain legumes and the role they can play in maintaining soil productivity and succeeding cereal yield, rotational field experiments were conducted on mung bean (Vigna radiata) and mash bean (Vigna mungo) during summer of 2002 and 2003 followed by wheat (Triticum aestivum) in each year at Research Farm of University of Arid Agriculture, Rawalpindi, to assess N2-fixation by beans and their residual effect on subsequent wheat yield. Bean seeds were inoculated at sowing with effective brady rhizobia and grown with and without Phosphorus fertilizer. Sorghum (Var.YSS-98) was sown as non-legume crop with 100kg N ha -1 . Xylem ureide method has been employed for estimation of N2-fixation. Nodulation, shoot dry matter, grain yield and N concentration of both beans were increased by phosphorus fertilization. Both beans showed excellent nodulations i.e., 4, which showed excellent potential for nitrogen fixation. Estimates of nitrogen derived from atmosphere (%Pfix) ranged from 49-71% during 2 nd year and up to 60% increase was observed from 1 st year. Average N2-fixed ranged between 33-55 kg ha -1 during both years and mash bean proved better N2-fixer. Water use efficiency (WUE) based on grain yield were 23-33% higher with phosphorus fertilization. Values of WUE for N2-fixation ranged between 0.22 and 1.00 kg ha -1 mm -1 and declined with declining %Pfix. Total NO3-N was between 56-67 kg ha -1 for legumes and between 40-45 kg ha -1 for non-legume sorghum. Additional residual soil N under legumes, relative to adjacent sorghum was in the range of 16-22 kg ha -1 . Beans with phosphorus fertilization increased grain yield of succeeding wheat by 20% over sorghum. It was concluded that phosphorus fertilizer with inoculation enhanced N2-fixation and rotational results confirmed that legume-cereal sequence increased biomass and grain yield of subsequent wheat.