Showing papers on "Nitrogen fixation published in 2013"

Transport and metabolism in legume-rhizobia symbioses

Abstract: Symbiotic nitrogen fixation by rhizobia in legume root nodules injects approximately 40 million tonnes of nitrogen into agricultural systems each year. In exchange for reduced nitrogen from the bacteria, the plant provides rhizobia with reduced carbon and all the essential nutrients required for bacterial metabolism. Symbiotic nitrogen fixation requires exquisite integration of plant and bacterial metabolism. Central to this integration are transporters of both the plant and the rhizobia, which transfer elements and compounds across various plant membranes and the two bacterial membranes. Here we review current knowledge of legume and rhizobial transport and metabolism as they relate to symbiotic nitrogen fixation. Although all legume-rhizobia symbioses have many metabolic features in common, there are also interesting differences between them, which show that evolution has solved metabolic problems in different ways to achieve effective symbiosis in different systems.

The human gut and groundwater harbor non-photosynthetic bacteria belonging to a new candidate phylum sibling to Cyanobacteria

Abstract: Cyanobacteria were responsible for the oxygenation of the ancient atmosphere; however, the evolution of this phylum is enigmatic, as relatives have not been characterized. Here we use whole genome reconstruction of human fecal and subsurface aquifer metagenomic samples to obtain complete genomes for members of a new candidate phylum sibling to Cyanobacteria, for which we propose the designation 'Melainabacteria'. Metabolic analysis suggests that the ancestors to both lineages were non-photosynthetic, anaerobic, motile, and obligately fermentative. Cyanobacterial light sensing may have been facilitated by regulators present in the ancestor of these lineages. The subsurface organism has the capacity for nitrogen fixation using a nitrogenase distinct from that in Cyanobacteria, suggesting nitrogen fixation evolved separately in the two lineages. We hypothesize that Cyanobacteria split from Melainabacteria prior or due to the acquisition of oxygenic photosynthesis. Melainabacteria remained in anoxic zones and differentiated by niche adaptation, including for symbiosis in the mammalian gut. DOI:http://dx.doi.org/10.7554/eLife.01102.001.

Key role of symbiotic dinitrogen fixation in tropical forest secondary succession

Abstract: In tropical moist forests, nitrogen-fixing tree species can supply a large proportion of the nitrogen required for net forest growth in the first 12 years of recovery after human or natural perturbation, with nitrogen-fixing trees accumulating carbon up to nine times faster per individual than non-fixing trees, and species-specific differences in the amount and timing of fixation. The capacity of tropical forests to act as carbon sinks is limited by the availability of fixed nitrogen, especially where a forest is recovering from human or natural perturbations. This study sets out to estimate the extent to which biological N2 fixation can overcome this constraint. In randomly chosen intact and recovering forest plots in Panama, tree species capable of symbiotic N2 fixation provided more than half of the 50,000 kg of plant carbon produced per hectare during the first 12 years of forest recovery. Biodiversity among nitrogen fixers was critical for maintaining nitrogen availability, with potential implications for policy-makers devising programmes for carbon mitigation through reforestation and conservation efforts. Forests contribute a significant portion of the land carbon sink, but their ability to sequester CO2 may be constrained by nitrogen1,2,3,4,5,6, a major plant-limiting nutrient. Many tropical forests possess tree species capable of fixing atmospheric dinitrogen (N2)7, but it is unclear whether this functional group can supply the nitrogen needed as forests recover from disturbance or previous land use1, or expand in response to rising CO2 (refs 6, 8). Here we identify a powerful feedback mechanism in which N2 fixation can overcome ecosystem-scale deficiencies in nitrogen that emerge during periods of rapid biomass accumulation in tropical forests. Over a 300-year chronosequence in Panama, N2-fixing tree species accumulated carbon up to nine times faster per individual than their non-fixing neighbours (greatest difference in youngest forests), and showed species-specific differences in the amount and timing of fixation. As a result of fast growth and high fixation, fixers provided a large fraction of the nitrogen needed to support net forest growth (50,000 kg carbon per hectare) in the first 12 years. A key element of ecosystem functional diversity was ensured by the presence of different N2-fixing tree species across the entire forest age sequence. These findings show that symbiotic N2 fixation can have a central role in nitrogen cycling during tropical forest stand development, with potentially important implications for the ability of tropical forests to sequester CO2.

Taxon-specific response of marine nitrogen fixers to elevated carbon dioxide concentrations

Abstract: Marine cyanobacteria supply much of the nitrogen that supports open ocean food webs and biogeochemical cycles. An experimental study suggests that the relationship between nitrogen fixation and carbon dioxide concentration varies significantly between cyanobacterial strains.

MALDI mass spectrometry‐assisted molecular imaging of metabolites during nitrogen fixation in the Medicago truncatula–Sinorhizobium meliloti symbiosis

Abstract: Summary Symbiotic associations between leguminous plants and nitrogen-fixing rhizobia culminate in the formation of specialized organs called root nodules, in which the rhizobia fix atmospheric nitrogen and transfer it to the plant. Efficient biological nitrogen fixation depends on metabolites produced by and exchanged between both partners. The Medicago truncatula–Sinorhizobium meliloti association is an excellent model for dissecting this nitrogen-fixing symbiosis because of the availability of genetic information for both symbiotic partners. Here, we employed a powerful imaging technique – matrix-assisted laser desorption/ionization (MALDI)/mass spectrometric imaging (MSI) – to study metabolite distribution in roots and root nodules of M. truncatula during nitrogen fixation. The combination of an efficient, novel MALDI matrix [1,8–bis(dimethyl-amino) naphthalene, DMAN] with a conventional matrix 2,5–dihydroxybenzoic acid (DHB) allowed detection of a large array of organic acids, amino acids, sugars, lipids, flavonoids and their conjugates with improved coverage. Ion density maps of representative metabolites are presented and correlated with the nitrogen fixation process. We demonstrate differences in metabolite distribution between roots and nodules, and also between fixing and non-fixing nodules produced by plant and bacterial mutants. Our study highlights the benefits of using MSI for detecting differences in metabolite distributions in plant biology.

Moss-cyanobacteria associations as biogenic sources of nitrogen in boreal forest ecosystems

Abstract: The biological fixation of atmospheric nitrogen (N) is a major pathway for available N entering ecosystems. In N-limited boreal forests, a significant amount of N2 is fixed by cyanobacteria living in association with mosses, contributing up to 50 % to the total N input. In this review, we synthesize reports on the drivers of N2 fixation in feather moss-cyanobacteria associations to gain a deeper understanding of their role for ecosystem-N-cycling. Nitrogen fixation in moss-cyanobacteria associations is inhibited by N inputs and therefore, significant fixation occurs only in low N-deposition areas. While it has been shown that artificial N additions in the laboratory as well as in the field inhibit N2 fixation in moss-cyanobacteria associations, the type, as well as the amounts of N that enters the system, affect N2 fixation differently. Another major driver of N2 fixation is the moisture status of the cyanobacteria-hosting moss, wherein moist conditions promote N2 fixation. Mosses experience large fluctuations in their hydrological status, undergoing significant natural drying and rewetting cycles over the course of only a few hours, especially in summer, which likely compromises the N input to the system via N2 fixation. Perhaps the most central question, however, that remains unanswered is the fate of the fixed N2 in mosses. The cyanobacteria are likely to leak N, but whether this N is transferred to the soil and if so, at which rates and timescales, is unknown. Despite our increasing understanding of the drivers of N2 fixation, the role moss-cyanobacteria associations play in ecosystem-N-cycling remains unresolved. Further, the relationship mosses and cyanobacteria share is unknown to date and warrants further investigation.

Role of Some of Mineral Nutrients in Biological Nitrogen Fixation

Abstract: Atmospheric nitrogen fixation probably contributes at most about 10% of the total annual yield of fixed nitrogen. The most important source of fixed nitrogen derives from the activity of certain soil bacteria that absorb atmospheric N2 gas and convert it into ammonium. The process of biological nitrogen fixation offers an economical attractive and ecological advantage by of reducing external nitrogen input and improving the quality and quantity of internal resources. Mineral nutrients may influence N2 fixation in legumes and nonlegumes at various stages of the symbiotic process: infection and nodule development, nodule function, and host plant growth. Here, review the basic concepts of mineral nutrition, as well as the importance of mineral nutrients specifically for biological nitrogen fixation in the legume-rhizobia symbiosis. For healthy and vigorous growth, intact plants need to take up from the soil: relatively large amounts of some inorganic elements: ions of nitrogen (N), potassium (K), calcium (Ca), phosphorus (P) and sulphur (S); and, small quantities of other elements: iron (Fe), nickel (Ni), chlorine (Cl), manganese (Mn), zinc (Zn), boron (B), copper (Cu), and molybdenum (Mo). The enhancing effect of low levels of combined nitrogen on N2 fixation in legumes is related to the lag phase between root infection and the onset of N2 fixation. Phosphorus (P) is second only to nitrogen as an essential mineral fertilizer for crop production. At any given time, a substantial component of soil P is in the form of poorly soluble mineral phosphates. A high phosphorus supply is needed for nodulation. When legumes dependent on symbiotic nitrogen receive an inadequate supply of phosphorus, they may therefore also suffer from nitrogen deficiency. Potassium and sulphur are not usually limiting nutrients for nodulated legumes, although a K+ supplement for osmoadaptation has to be considered for growth in saline soils. Among mineral nutrients, B and Ca are undoubtedly the nutrients with a major effect on legume symbiosis. Both nodulation and nitrogen fixation depend on B and Ca2+, with calcium more necessary for early symbiotic events and B for nodule maturation.

Evidence and a conceptual model for the co-occurrence of nitrogen fixation and denitrification in heterotrophic marine sediments

Abstract: Marine waters are often nitrogen (N) limited. Denitrification, the microbial conver- sion of nitrate to dinitrogen (N2) gas, is responsible for significant N removal from the coastal ocean. In contrast, nitrogen fixation, the microbial transformation of N2 to ammonium, is typically regarded as an inconsequential N source. The imbalance between these 2 processes is responsi- ble, at least in part, for N limitation in the coastal ocean. Organic matter quality and quantity has been shown to determine rates of these critical N cycling processes. We hypothesized that the tim- ing of organic matter deposition to the benthos might also be important in determining which pro- cess dominates. We tested this hypothesis using a coupled biogeochemical-molecular approach. We report directly measured net sediment denitrification rates and corresponding expression of nirS, a gene in the denitrification pathway, with the simultaneous expression of nifH, a gene asso- ciated with nitrogen fixation. The timing of organic matter deposition determined the magnitude of the net sediment N2 fluxes. Highest rates of denitrification occurred soon after deposition, and the lowest rates occurred over 200 d after the last deposition event concomitant with increased nifH expression. Phylogenetic evidence suggests that sulfur and sulfate reducers are responsible for the nitrogen fixation. Globally, warming water temperatures, changes in light, and reduced nutrient loads through management intervention have been linked to decreases and/or altered phenology of water column productivity. Based on a conceptual model developed here, we sug- gest that in these systems, heterotrophic sediment nitrogen fixation may become an important component of the nitrogen budget.

Regulation of legume nodulation by acidic growth conditions.

Abstract: Legumes represent some of the most important crop species worldwide. They are able to form novel root organs known as nodules, within which biological nitrogen fixation is facilitated through a symbiotic interaction with soil-dwelling bacteria called rhizobia. This provides legumes with a distinct advantage over other plant species, as nitrogen is a key factor for growth and development. Nodule formation is tightly regulated by the plant and can be inhibited by a number of external factors, such as soil pH. This is of significant agricultural and economic importance as much of global legume crops are grown on low pH soils. Despite this, the precise mechanism by which low pH conditions inhibits nodule development remains poorly characterized.

Local inhibition of nitrogen fixation and nodule metabolism in drought-stressed soybean

Abstract: Drought stress is a major factor limiting symbiotic nitrogen fixation (NF) in soybean crop production. However, the regulatory mechanisms involved in this inhibition are still controversial. Soybean plants were symbiotically grown in a split-root system (SRS), which allowed for half of the root system to be irrigated at field capacity while the other half remained water deprived. NF declined in the water-deprived root system while nitrogenase activity was maintained at control values in the well-watered half. Concomitantly, amino acids and ureides accumulated in the water-deprived belowground organs regardless of transpiration rates. Ureide accumulation was found to be related to the decline in their degradation activities rather than increased biosynthesis. Finally, proteomic analysis suggests that plant carbon metabolism, protein synthesis, amino acid metabolism, and cell growth are among the processes most altered in soybean nodules under drought stress. Results presented here support the hypothesis of a local regulation of NF taking place in soybean and downplay the role of ureides in the inhibition of NF.

How Nitrogen Is Lost

Abstract: As in the back garden, productivity in the ocean is often limited by the availability of nutrients, principally nitrogen (N) and phosphorus (P). On a global scale, the rates of nitrogen fixation (input) and nitrogen loss (output) are believed to be roughly equal. Both rates are, however, very uncertain. Nitrogen loss processes occur in subsurface water and sediments and depend on the supply of organic matter, derived from primary production in surface waters. Reports in the last few years have changed our understanding of the controls and pathways responsible for nitrogen loss.

Transfer of fixed-N from N 2 -fixing cyanobacteria associated with the moss Sphagnum riparium results in enhanced growth of the moss

Abstract: Despite the general assumption that nitrogen fixed by associated cyanobacteria will be readily utilised for growth by the Sphagnum, no empirical evidence is available in the literature. Therefore the effects of nitrogen transfer from cyanobacteria associated with S. riparium were investigated. Cultivation of S. riparium with and without cyanobacteria was performed under laboratory conditions for 57 days. We show that nitrogen fixation by cyanobacteria associated with Sphagnum mosses, influences moss growth by transfer of fixed nitrogen to the moss. More than 35 % of the nitrogen fixed by cyanobacteria was transferred to the newly formed moss biomass and resulted in an increase in the growth of Sphagnum biomass compared to the controls. The variation in the increase of nitrogen content explained 76 % of the biomass increment. Hence, nitrogen fixation will have immediate effect on the carbon fixation by Sphagnum. This shows that factors regulating nitrogen fixation will have a direct effect on the role of Sphagnum dominated ecosystems with respect to carbon cycling.

Nitrogen fixation and transfer in grass-clover leys under organic and conventional cropping systems

Abstract: Symbiotic dinitrogen (N2) fixation is the most important external N source in organic systems. Our objective was to compare symbiotic N2 fixation of clover grown in organically and conventionally cropped grass-clover leys, while taking into account nutrient supply gradients. We studied leys of a 30-year-old field experiment over 2 years in order to compare organic and conventional systems at two fertilization levels. Using 15N natural abundance methods, we determined the proportion of N derived from the atmosphere (PNdfa), the amount of Ndfa (ANdfa), and the transfer of clover N to grasses for both red clover (Trifolium pratense L.) and white clover (Trifolium repens L.). In all treatments and both years, PNdfa was high (83 to 91 %), indicating that the N2 fixation process is not constrained, even not in the strongly nutrient deficient non-fertilized control treatment. Annual ANdfa in harvested clover biomass ranged from 6 to 16 g N m−2. At typical fertilizer input levels, lower sward yield in organic than those in conventional treatments had no effect on ANdfa because of organic treatments had greater clover proportions. In two-year-old leys, on average, 51 % of N taken up by grasses was transferred from clover. Both, organically and conventionally cropped grass-clover leys profited from symbiotic N2 fixation, with high PNdfa, and important transfer of clover N to grasses, provided sufficient potassium- and phosphorus-availability to sustain clover biomass production.

Relationships among phosphorus, molybdenum and free-living nitrogen fixation in tropical rain forests: results from observational and experimental analyses

Abstract: Biological nitrogen (N) fixation is the primary source of “new” N to unmanaged ecosystems, and recent analyses suggest that terrestrial N inputs via free-living N fixation may be more important than previously assumed. This may be particularly true in some tropical rain forests, where free-living fixation could outpace symbiotic N fixation to represent the dominant source of new N inputs. However, our understanding of the controls over free-living N fixation in tropical rain forests remains poor, which directly constrains our ability to predict how N cycling will respond to changing environmental conditions. Although both phosphorus (P) and molybdenum (Mo) availability have been shown to limit free-living N fixation rates in the tropics, few studies have simultaneously explored P versus Mo limitation or the potential importance of P × Mo interactions. Here, an archived set of foliar, litter, and soil samples from a Costa Rican tropical rain forest provided an opportunity to simultaneously assess the relative strength of P versus Mo relationships with free-living N fixation rates. We also conducted a short-term, full-factorial (P × Mo) litter incubation experiment to directly assess nutrient limitation, allowing us to explore P and Mo controls over free-living N fixation rates using both observational and experimental approaches. We previously showed that N fixation rates were positively correlated with P concentrations in all substrates and, using the archived samples, we now show that Mo concentrations correlated with N fixation only in canopy leaves (where total Mo concentrations were extremely low). Likewise, fertilization with P alone (and not Mo) stimulated leaf litter N fixation rates. Thus, our results suggest that P availability dominantly controls free-living N fixation at this site, and when taken with data from other studies, our results suggest that attempts to identify “the nutrient” that limits N fixation in “the tropics” may be misguided. Rather, nutrient controls over free-living N fixation appear to be more nuanced—and the true nature of nutrient limitation to N fixation likely varies over a variety of scales across the vast tropical rain forest biome.

Engineering Pseudomonas protegens Pf-5 for nitrogen fixation and its application to improve plant growth under nitrogen-deficient conditions.

Abstract: Nitrogen is the second most critical factor for crop production after water. In this study, the beneficial rhizobacterium Pseudomonas protegens Pf-5 was genetically modified to fix nitrogen using the genes encoding the nitrogenase of Pseudomonas stutzeri A1501 via the X940 cosmid. Pf-5 X940 was able to grow in L medium without nitrogen, displayed high nitrogenase activity and released significant quantities of ammonium to the medium. Pf-5 X940 also showed constitutive expression and enzymatic activity of nitrogenase in ammonium medium or in nitrogen-free medium, suggesting a constitutive nitrogen fixation. Similar to Pseudomonas protegens Pf-5, Pseudomonas putida, Pseudomonas veronii and Pseudomonas taetrolens but not Pseudomonas balearica and Pseudomonas stutzeri transformed with cosmid X940 showed constitutive nitrogenase activity and high ammonium production, suggesting that this phenotype depends on the genome context and that this technology to obtain nitrogen-fixing bacteria is not restricted to Pf-5. Interestingly, inoculation of Arabidopsis, alfalfa, tall fescue and maize with Pf-5 X940 increased the ammonium concentration in soil and plant productivity under nitrogen-deficient conditions. In conclusion, these results open the way to the production of effective recombinant inoculants for nitrogen fixation on a wide range of crops.

Nitrogen-fixing bacteria with multiple plant growth-promoting activities enhance growth of tomato and red pepper.

Abstract: As a suitable alternative to chemical fertilizers, the application of plant growth-promoting rhizobacteria has been increasing in recent years due to their potential to be used as biofertilizers. In the present work, 13 nitrogen-fixing bacterial strains belonging to 11 different genera were tested for their PGP attributes. All of the strains were positive for 1-aminocyclopropane-1-carboxylate deaminase (ACCD), indole-3-acetic acid (IAA), salicylic acid, and ammonia production while negative for cellulase, pectinase, and hydrocyanic acid production. The strains Pseudomonas sp. RFNB3 and Serratia sp. RFNB14 were the most effective in solubilizing both tri-calcium phosphate and zinc oxide. In addition, all strains except Pseudomonas sp. RFNB3 were able to oxidize sulfur, and six strains were positive for siderophore synthesis. Each strain tested in this study possesses at least four PGP properties in addition to nitrogen fixation. Nine strains were selected based on their multiple PGP potential, particularly ACCD and IAA production, and evaluated for their effects on early growth of tomato and red pepper under gnotobiotic conditions. Bacterial inoculation considerably influenced root and shoot length, seedling vigor, and dry biomass of the two crop plants. Three strains that demonstrated substantial effects on plant performance were further selected for greenhouse trials with red pepper, and among them Pseudomonas sp. RFNB3 resulted in significantly higher plant height (26%) and dry biomass (28%) compared to control. The highest rate of nitrogen fixation, as determined by acetylene reduction assay, occurred in Novosphingobium sp. RFNB21 inoculated red pepper root (49.6 nM of ethylene/h/g of dry root) and rhizosphere soil (41.3 nM of ethylene/h/g of dry soil). Inoculation with nitrogen-fixing bacteria significantly increased chlorophyll content, and the uptake of different macro- and micro-nutrient contents enhancing also in red pepper shoots, in comparison with uninoculated controls. The population estimation studies showed that nitrogen-fixing as well as total heterotrophic bacteria were also noticeably increased in soil and plant samples. The findings of this study suggest that certain nitrogen-fixing strains possessing multiple PGP traits could be applied in the development of biofertilizers.

Mixed plantations can promote microbial integration and soil nitrate increases with changes in the N cycling genes

Abstract: Mixed-species plantations of Eucalyptus and legume trees can symbiotically fix nitrogen and potentially improve the soil quality and biomass productivity compared with a conventional Eucalyptus monoculture. In this study, we evaluated changes in the structure and abundance of different microbial groups and nitrogen cycle genes in mixed and pure plantations of Acacia mangium and Eucalyptus urograndis in an experimental area in southeastern Brazil. Soil samples (0–10 cm) collected in two- and three-year-old stands were submitted to chemical characterization and molecular analyses using DGGE and real time-PCR for bacteria (16S rRNA), fungi (ITS), and genes involved in nitrogen cycling (nirK, amoA, nifH). The mixed plantation did not significantly change general soil fertility or total soil C and N content compared with the Eucalyptus monoculture. However, there was a significant increase in available phosphorus and soil nitrate content in both the A. mangium and mixed-species treatments. The multivariate ordination of the DGGE profiles of bacteria, fungi and archaea groups showed distinct community structures in each treatment. Significant differences in the abundance of copies of the target genes were found for fungi, with higher values in the Eucalyptus followed by the mixed and A. mangium plantations. The analysis of nitrogen cycle genes showed no clear difference in the communities of nitrogen fixing bacteria or nitrifying archaea among treatments. The nitrification activity was dominated by archaea because it was not possible to detect the presence of bacterial nitrifiers; the denitrifier community had a distinct profile in the Eucalyptus monoculture. The abundance of archaeal amoA and nirK genes suggests that the A. mangium treatment had higher nitrification and lower denitrification than the other treatments, which would explain the higher soil nitrate levels found in pure A. mangium treatments. Our data suggest that mixed plantations of E. urograndis and A. mangium result in a distinct microbial community relative to the respective monocultures with positive effects on soil phosphorus and nitrate content, which potentially reduces the need for anthropogenic fertilization.

Asparagine: an amide of particular distinction in the regulation of symbiotic nitrogen fixation of legumes.

Abstract: Symbiotic nitrogen fixation is tightly regulated by a range of fine processes at the nodule level, over which the host plant has overall control through the whole life of the plant. The operation of this control at the nodule level is not yet fully understood, but greater knowledge will ultimately lead to a better improvement of N2 fixation through the use of crop legumes and genetic engineering of crop plants for higher performance. It has been suggested that, nodule responses to the nutritional complexity of the rhizosphere environment involve a great deal of coordination of sensing and signal transduction. This regulation can be achieved through several mechanisms, including changes in carbon metabolism, oxygen supply and/or overproduction of reactive oxygen and nitrogen species. Recently, the cycling of amino acids observed between the plant and bacteroid fractions suggests a new and important regulatory mechanism involved in nodule responses. Most of the recent transcriptional findings are consistent with the earlier biochemical and physiological reports. Current research revealed unique advances for nodule metabolism, especially on the regulation of asparagine synthetase gene expression and the control of asparagine (ASN) to N2 fixing activity. A large amount of ASN is found accumulating in the root nodules of the symbiotic plants under restricted environments, such as drought, salinity and nutrient deficiency. Exceptionally, ASN phloem feeding has resulted in an increased concentration of the ASN amide in nodules followed by a remarkable decrease in nodule activity. In this review, recent progress concerning the possible role of ASN in whole-plant-based down-regulation of symbiotic N2 fixation will be reviewed.

Molybdenum and phosphorus limitation of asymbiotic nitrogen fixation in forests of Eastern Canada: Influence of vegetative cover and seasonal variability

Abstract: The limitation of asymbiotic dinitrogen (N 2 ) fixation by phosphorus (P) is well-documented. Studies on Mo limitation of asymbiotic N 2 fixation are, however, scarce. To what extent Mo limits asymbiotic N 2 fixation on the global scale is still unclear and the mechanisms controlling the emergence of Mo limitation remain elusive. The aim of this work was to investigate the effect of nutrient additions (P, Mo and P+Mo) on asymbiotic N 2 fixation activity in leaf litters from Eastern Canadian forests (cold temperate). We specifically tested how different vegetative covers (deciduous versus coniferous) respond to nutrient additions. We also evaluated on one site (coniferous litters) if nutrient (Mo, P) limitation change during the growing season. We report that the vegetative cover has a strong influence on the emergence of Mo and P limitations; while many sites under coniferous cover responded to different combinations of nutrient addition, none of the sites under deciduous cover responded to any nutrient addition. We also observed that nutrient limitation changed during the growing season; asymbiotic N 2 fixation in coniferous litters was limited by P in the early stage of the growing season, by Mo but not P in mid-season and in late season neither P nor Mo were limiting. This Seasonality in nutrient limitation might be an important factor affecting the biological N input in temperate and boreal ecosystems that remain to be fully described. Our results are discussed in the context of litter decomposition and nutrients cycling.

Effects of salt stress and rhizobial inoculation on growth and nitrogen fixation of three peanut cultivars

Abstract: Increasing soil salinity represents a major constraint for agriculture in arid and semi-arid lands, where mineral nitrogen (N) deficiency is also a frequent characteristic of soils. Biological N fixation by legumes may constitute a sustainable alternative to chemical fertilisation in salinity-affected areas, provided that adapted cultivars and inoculants are available. Here, the performance of three peanut cultivars nodulated with two different rhizobial strains that differ in their salt tolerance was evaluated under moderately saline water irrigation and compared with that of N-fertilised plants. Shoot weight was used as an indicator of yield. Under non-saline conditions, higher yields were obtained using N fertilisation rather than inoculation for all the varieties tested. However, under salt stress, the yield of inoculated plants became comparable to that of N-fertilised plants, with minor differences depending on the peanut cultivar and rhizobial strain. Our results indicate that N fixation might represent an economical, competitive and environmentally friendly choice with respect to mineral N fertilisation for peanut cultivation under moderate saline conditions.

Effects of cross host species inoculation of nitrogen‐fixing endophytes on growth and leaf physiology of maize

Abstract: Plants that grow and thrive under abiotic stress often do so with the help of endophytic microorganisms. Although nitrogen-fixing (diazotrophic) endophytes colonize many wild plants, these natural relationships may be disrupted in cultivated crop species where breeding and genotype selection often occur under conditions of intensive fertilization and irrigation. Many energy crops including corn may still benefit from diazotrophic endophyte inoculations allowing for more efficient biomass production with less input of petroleum-derived fertilizer. A selection of diazotrophic endophytes isolated from willow (Salix sitchensis, Sitka willow) and poplar (Populus trichocarpa, black cottonwood) growing in nutrient-poor river sides were used as inoculum in three experiments testing the effect on plant growth and leaf level physiology of a sweet corn variety under various levels of applied nitrogen fertilizer. We report substantial growth promotion with improved leaf physiology of corn plants in response to diazotrophic endophyte inoculations. Significant gains of early biomass with a greater root : shoot ratio were found for plants receiving endophytic inocula over the uninoculated control groups regardless of the nitrogen level. Furthermore, inoculated plants exhibited consistently higher rates of net CO2 assimilation than did those without endophytic inoculation. These results have beneficial implications for enhanced plant growth in a low-input system on nutrient-poor sites. The immediate increase of root mass observed in endophyte inoculated plants has the potential to provide better establishment and early growth in resource-limited environments. The initial results of this study also indicate that the beneficial effect from endophytes isolated from poplar and willow species is not restricted to the species from which they were initially isolated.

11 Dinitrogen-Fixing Prokaryotes

Abstract: Dinitrogen fixation is a key process in the N cycle and only carried out by few prokaryotes Research on dinitrogen fixation includes basic and practical applications: from nif genes to crops, with molecular, genetic, ecological, taxonomic, and agricultural approaches used Nitrogen fixing rhizobia, which have been used in agriculture for over a 100 years, are excellent research models still leading the knowledge of eukaryotebacteria symbioses Other less known symbioses of dinitrogen fixing bacteria are reviewed as well as free-living diazotrophs

Comparison of common bean (Phaseolus vulgaris L.) genotypes for nitrogen fixation tolerance to soil drying

Abstract: Aims Common bean is a major source of protein for many people worldwide. However, the crop is often subjected to drought conditions and its advantage in undertaking symbiotic nitrogen fixation can be severely decreased. The primary objective of this study was to compare the resistance of nitrogen fixation of 12 selected genotypes to soil drying.

Variations in the Rhythms of Respiration and Nitrogen Fixation in Members of the Unicellular Diazotrophic Cyanobacterial Genus Cyanothece

Abstract: In order to accommodate the physiologically incompatible processes of photosynthesis and nitrogen fixation within the same cell, unicellular nitrogen-fixing cyanobacteria have to maintain a dynamic metabolic profile in the light as well as the dark phase of a diel cycle. The transition from the photosynthetic to the nitrogen-fixing phase is marked by the onset of various biochemical and regulatory responses, which prime the intracellular environment for nitrogenase activity. Cellular respiration plays an important role during this transition, quenching the oxygen generated by photosynthesis and by providing energy necessary for the process. Although the underlying principles of nitrogen fixation predict unicellular nitrogen-fixing cyanobacteria to function in a certain way, significant variations are observed in the diazotrophic behavior of these microbes. In an effort to elucidate the underlying differences and similarities that govern the nitrogen-fixing ability of unicellular diazotrophic cyanobacteria, we analyzed six members of the genus Cyanothece. Cyanothece sp. ATCC 51142, a member of this genus, has been shown to perform efficient aerobic nitrogen fixation and hydrogen production. Our study revealed significant differences in the patterns of respiration and nitrogen fixation among the Cyanothece spp. strains that were grown under identical culture conditions, suggesting that these processes are not solely controlled by cues from the diurnal cycle but that strain-specific intracellular metabolic signals play a major role. Despite these inherent differences, the ability to perform high rates of aerobic nitrogen fixation and hydrogen production appears to be a characteristic of this genus.

Low temperature delays timing and enhances the cost of nitrogen fixation in the unicellular cyanobacterium Cyanothece

Abstract: Marine nitrogen-fixing cyanobacteria are largely confined to the tropical and subtropical ocean. It has been argued that their global biogeographical distribution reflects the physiologically feasible temperature range at which they can perform nitrogen fixation. In this study we refine this line of argumentation for the globally important group of unicellular diazotrophic cyanobacteria, and pose the following two hypotheses: (i) nitrogen fixation is limited by nitrogenase activity at low temperature and by oxygen diffusion at high temperature, which is manifested by a shift from strong to weak temperature dependence of nitrogenase activity, and (ii) high respiration rates are required to maintain very low levels of oxygen for nitrogenase, which results in enhanced respiratory cost per molecule of fixed nitrogen at low temperature. We tested these hypotheses in laboratory experiments with the unicellular cyanobacterium Cyanothece sp. BG043511. In line with the first hypothesis, the specific growth rate increased strongly with temperature from 18 to 30 °C, but leveled off at higher temperature under nitrogen-fixing conditions. As predicted by the second hypothesis, the respiratory cost of nitrogen fixation and also the cellular C:N ratio rose sharply at temperatures below 21 °C. In addition, we found that low temperature caused a strong delay in the onset of the nocturnal nitrogenase activity, which shortened the remaining nighttime available for nitrogen fixation. Together, these results point at a lower temperature limit for unicellular nitrogen-fixing cyanobacteria, which offers an explanation for their (sub)tropical distribution and suggests expansion of their biogeographical range by global warming.

Nickel limitation of nitrogen fixation in Trichodesmium

Abstract: I show that the growth of Trichodesmium, the primary diazotrophic phytoplankton in tropical and subtropical oceans, can be limited by Ni availability in both trace metal-defined culture media and natural seawater when the supply of Fe and P is sufficient. I further show that the increase of Ni concentrations elevates cellular superoxide dismutase (SOD) activities and nitrogen fixation rates, suggesting that NiSOD may be involved in the protection of nitrogenase from superoxide inhibition during photosynthesis in this nonheterocystous diazotroph.