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Showing papers on "Nitrogen fixation published in 2011"


Journal ArticleDOI
TL;DR: It is concluded that various tools and indicators are available for developing the ecological engineering of the rhizobial symbiosis, in particular for its beneficial contribution to the bio-geochemical cycle of N, and also P and C.
Abstract: As a major contributor to the reduced nitrogen pool in the biosphere, symbiotic nitrogen fixation by legumes plays a critical role in a sustainable production system. However this legume contribution varies with the physico-chemical and biological conditions of the nodulated-root rhizosphere. In order to assess the abiotic and biotic constrains that might limit this symbiosis at the agroecosystem level, a nodular diagnosis is proposed with common bean as a model grain-legume, and a major source of plant proteins for world human nutrition. The engineering of the legume symbiosis is addressed by participatory assessment of bean recombinant inbred lines contrasting for their efficiency in use of phosphorous for symbiotic nitrogen fixation. With this methodology, in field-sites chosen with farmers of an area of cereal-cropping in the Mediterranean basin, a large spatial and temporal variation in the legume nodulation was found. Soil P availability was a major limiting factor of the rhizobial symbiosis. In order to relate the field measurements with progress in functional genomics of the symbiosis, in situ RT-PCR on nodule sections has been implemented showing that the phytase gene is expressed in the cortex with significantly higher number of transcripts in P-efficient RILs. It is concluded that various tools and indicators are available for developing the ecological engineering of the rhizobial symbiosis, in particular for its beneficial contribution to the bio-geochemical cycle of N, and also P and C.

440 citations


Journal ArticleDOI
12 May 2011-Nature
TL;DR: It is shown that the exosymbiont-derived ornithine-urea cycle, which is similar to that of metazoans but is absent in green algae and plants, facilitates rapid recovery from prolonged nitrogen limitation and represents a key pathway for anaplerotic carbon fixation into nitrogenous compounds that are essential for diatom growth and for the contribution of diatoms to marine productivity.
Abstract: Diatoms dominate the biomass of phytoplankton in nutrient-rich conditions and form the basis of some of the world's most productive marine food webs. The diatom nuclear genome contains genes with bacterial and plastid origins as well as genes of the secondary endosymbiotic host (the exosymbiont), yet little is known about the relative contribution of each gene group to diatom metabolism. Here we show that the exosymbiont-derived ornithine-urea cycle, which is similar to that of metazoans but is absent in green algae and plants, facilitates rapid recovery from prolonged nitrogen limitation. RNA-interference-mediated knockdown of a mitochondrial carbamoyl phosphate synthase impairs the response of nitrogen-limited diatoms to nitrogen addition. Metabolomic analyses indicate that intermediates in the ornithine-urea cycle are particularly depleted and that both the tricarboxylic acid cycle and the glutamine synthetase/glutamate synthase cycles are linked directly with the ornithine-urea cycle. Several other depleted metabolites are generated from ornithine-urea cycle intermediates by the products of genes laterally acquired from bacteria. This metabolic coupling of bacterial- and exosymbiont-derived proteins seems to be fundamental to diatom physiology because the compounds affected include the major diatom osmolyte proline and the precursors for long-chain polyamines required for silica precipitation during cell wall formation. So far, the ornithine-urea cycle is only known for its essential role in the removal of fixed nitrogen in metazoans. In diatoms, this cycle serves as a distribution and repackaging hub for inorganic carbon and nitrogen and contributes significantly to the metabolic response of diatoms to episodic nitrogen availability. The diatom ornithine-urea cycle therefore represents a key pathway for anaplerotic carbon fixation into nitrogenous compounds that are essential for diatom growth and for the contribution of diatoms to marine productivity.

420 citations


Journal ArticleDOI
TL;DR: How variations in the deposition of iron from dust to different ocean basins affects the limiting nutrient for N2 fixation and the distribution of different diazotrophic species is described.
Abstract: Biological nitrogen fixation is an important part of the marine nitrogen cycle, supporting carbon export and sequestration. In this Review, Sohm, Webb and Capone describe the nutrients that limit nitrogen fixation and the distribution of diazotrophic species in the world's oceans.

412 citations


Journal ArticleDOI
TL;DR: The major cyanobacterial groups have different physiological and ecological constraints that result in highly variable geographic distributions, with implications for the marine N-cycle budget.

391 citations


Journal ArticleDOI
TL;DR: This study provides new information on the mechanisms controlling N input into the open ocean by symbiotic microorganisms, which are widespread and important for oceanic primary production and the first demonstration of N transfer from an N2 fixer to a unicellular partner.
Abstract: Many diatoms that inhabit low-nutrient waters of the open ocean live in close association with cyanobacteria. Some of these associations are believed to be mutualistic, where N2-fixing cyanobacterial symbionts provide N for the diatoms. Rates of N2 fixation by symbiotic cyanobacteria and the N transfer to their diatom partners were measured using a high-resolution nanometer scale secondary ion mass spectrometry approach in natural populations. Cell-specific rates of N2 fixation (1.15–71.5 fmol N per cell h−1) were similar amongst the symbioses and rapid transfer (within 30 min) of fixed N was also measured. Similar growth rates for the diatoms and their symbionts were determined and the symbiotic growth rates were higher than those estimated for free-living cells. The N2 fixation rates estimated for Richelia and Calothrix symbionts were 171–420 times higher when the cells were symbiotic compared with the rates estimated for the cells living freely. When combined, the latter two results suggest that the diatom partners influence the growth and metabolism of their cyanobacterial symbionts. We estimated that Richelia fix 81–744% more N than needed for their own growth and up to 97.3% of the fixed N is transferred to the diatom partners. This study provides new information on the mechanisms controlling N input into the open ocean by symbiotic microorganisms, which are widespread and important for oceanic primary production. Further, this is the first demonstration of N transfer from an N2 fixer to a unicellular partner. These symbioses are important models for molecular regulation and nutrient exchange in symbiotic systems.

334 citations


Journal ArticleDOI
TL;DR: The nitrogen cycle in the oceans is an integral feature of the function of ocean ecosystems and will be a central player in how oceans respond during global environmental change.
Abstract: The marine nitrogen (N) cycle controls the productivity of the oceans. This cycle is driven by complex biogeochemical transformations, including nitrogen fixation, denitrification, and assimilation and anaerobic ammonia oxidation, mediated by microorganisms. New processes and organisms continue to be discovered, complicating the already complex picture of oceanic N cycling. Genomics research has uncovered the diversity of nitrogen metabolism strategies in phytoplankton and bacterioplankton. The elemental ratios of nutrients in biological material are more flexible than previously believed, with implications for vertical export of carbon and associated nutrients to the deep ocean. Estimates of nitrogen fixation and denitrification continue to be modified, and anaerobic ammonia oxidation has been identified as a new process involved in denitrification in oxygen minimum zones. The nitrogen cycle in the oceans is an integral feature of the function of ocean ecosystems and will be a central player in how oceans respond during global environmental change.

320 citations


Book
08 Dec 2011
TL;DR: The lectins identified in Anabaena azollae Newton cells are constitutive as well, and their actual role in the symbiosis with Azolla has not been verified yet, but they may be involved in the regulation and control of the development of anabaena in the leaf cavi ty and, poss ibly, trigger the exchange of me taboli tes between the hos t Azolla and the N2-fixing AnabaENA.
Abstract: Cultured isolates of Anabaena azollae provide a suitable tool for studying the metabolism, transport, cell composition and antigenic properties of the cells. The isolates of Anabaena azollae from Azolla caroliniana and Azolla filiculoides exhibit antigenic homology in the quantitative immunoassays employed, suggesting that they may stem from the same parental Anabaena. These isolates were also similar in respect to the fructose supported N2-fixation activity: the fructose is taken up actively by the cells of Anabaena azollae and although it does not support the growth of Anabaena, it carries a series of changes involving cell composition, differentiation and metabolism. Respiration was facilitated, the amount of storage products increased, and the frequency of heterocysts was increased. Sucrose and glucose stimulated respiration, but showed limited effect on N2-fixation. The uptake of fructose, dependent on ATP synthesis, is recovered faster in starved cells than N2-fixation activity. Fructose carrier seems to be constitutive in the cultured isolate of Anabaena azollae grown autotrophically. The lectins identified in Anabaena azollae Newton cells are constitutive as well, and their actual role in the symbiosis with Azolla has not been verified yet. They may be involved in the regulation and control of the development of Anabaena in the leaf cavi ty and, poss ibly, trigger the exchange of me taboli tes between the hos t Azolla and the N2-fixing Anabaena.

204 citations


Journal ArticleDOI
TL;DR: The existence of a nitrate-NO respiration process in nodules that could play a role in the maintenance of the energy status required for nitrogen fixation under oxygen-limiting conditions is proposed.
Abstract: Nitric oxide (NO) is a signaling and defense molecule of major importance in living organisms. In the model legume Medicago truncatula, NO production has been detected in the nitrogen fixation zone of the nodule, but the systems responsible for its synthesis are yet unknown and its role in symbiosis is far from being elucidated. In this work, using pharmacological and genetic approaches, we explored the enzymatic source of NO production in M. truncatula-Sinorhizobium meliloti nodules under normoxic and hypoxic conditions. When transferred from normoxia to hypoxia, nodule NO production was rapidly increased, indicating that NO production capacity is present in functioning nodules and may be promptly up-regulated in response to decreased oxygen availability. Contrary to roots and leaves, nodule NO production was stimulated by nitrate and nitrite and inhibited by tungstate, a nitrate reductase inhibitor. Nodules obtained with either plant nitrate reductase RNA interference double knockdown (MtNR1/2) or bacterial nitrate reductase-deficient (napA) and nitrite reductase-deficient (nirK) mutants, or both, exhibited reduced nitrate or nitrite reductase activities and NO production levels. Moreover, NO production in nodules was found to be inhibited by electron transfer chain inhibitors, and nodule energy state (ATP-ADP ratio) was significantly reduced when nodules were incubated in the presence of tungstate. Our data indicate that both plant and bacterial nitrate reductase and electron transfer chains are involved in NO synthesis. We propose the existence of a nitrate-NO respiration process in nodules that could play a role in the maintenance of the energy status required for nitrogen fixation under oxygen-limiting conditions.

203 citations


Journal ArticleDOI
07 Jun 2011-PLOS ONE
TL;DR: The results demonstrate the occurrence of nitrogen fixation in nutrient-rich coastal upwelling systems and, importantly, within the underlying OMZ and suggest that nitrogen fixation is a widespread process that can sporadically provide a supplementary source of fixed nitrogen in these regions.
Abstract: Nitrogen fixation is an essential process that biologically transforms atmospheric dinitrogen gas to ammonia, therefore compensating for nitrogen losses occurring via denitrification and anammox. Currently, inputs and losses of nitrogen to the ocean resulting from these processes are thought to be spatially separated: nitrogen fixation takes place primarily in open ocean environments (mainly through diazotrophic cyanobacteria), whereas nitrogen losses occur in oxygen-depleted intermediate waters and sediments (mostly via denitrifying and anammox bacteria). Here we report on rates of nitrogen fixation obtained during two oceanographic cruises in 2005 and 2007 in the eastern tropical South Pacific (ETSP), a region characterized by the presence of coastal upwelling and a major permanent oxygen minimum zone (OMZ). Our results show significant rates of nitrogen fixation in the water column; however, integrated rates from the surface down to 120 m varied by ∼30 fold between cruises (7.5±4.6 versus 190±82.3 µmol m−2 d−1). Moreover, rates were measured down to 400 m depth in 2007, indicating that the contribution to the integrated rates of the subsurface oxygen-deficient layer was ∼5 times higher (574±294 µmol m−2 d−1) than the oxic euphotic layer (48±68 µmol m−2 d−1). Concurrent molecular measurements detected the dinitrogenase reductase gene nifH in surface and subsurface waters. Phylogenetic analysis of the nifH sequences showed the presence of a diverse diazotrophic community at the time of the highest measured nitrogen fixation rates. Our results thus demonstrate the occurrence of nitrogen fixation in nutrient-rich coastal upwelling systems and, importantly, within the underlying OMZ. They also suggest that nitrogen fixation is a widespread process that can sporadically provide a supplementary source of fixed nitrogen in these regions.

176 citations


Journal ArticleDOI
TL;DR: It is suggested that canopy legumes closely regulate N2 fixation, leading to large variations in N inputs across the landscape, and low symbiotic fixation in mature forests despite abundant legumes.
Abstract: Symbiotic dinitrogen (N(2)) fixation is often invoked to explain the N richness of tropical forests as ostensibly N(2)-fixing trees can be a major component of the community. Such arguments assume N(2) fixers are fixing N when present. However, in laboratory experiments, legumes consistently reduce N(2) fixation in response to increased soil N availability. These contrasting views of N(2) fixation as either obligate or facultative have drastically different implications for the N cycle of tropical forests. We tested these models by directly measuring N(2)-fixing root nodules and nitrogenase activity of individual canopy-dominant legume trees (Inga sp.) across several lowland forest types. Fixation was substantial in disturbed forests and some gaps but near zero in the high N soils of mature forest. Our findings suggest that canopy legumes closely regulate N(2) fixation, leading to large variations in N inputs across the landscape, and low symbiotic fixation in mature forests despite abundant legumes.

158 citations


Journal ArticleDOI
TL;DR: The findings indicated that the Antarctic microbial nitrogen cycle could be dramatically altered by temperature and nitrogen, and that warming may be detrimental to the ammonia-oxidizing archaeal community.

Journal ArticleDOI
TL;DR: Genome-wide transcription profiling of A. vinelandii cultured under nitrogen-fixing conditions under various metal amendments revealed the discrete complement of genes associated with each nitrogenase system and the extent of cross talk between the systems, which underscored significant differences between Mo-dependent and Mo-independent diazotrophic growth.
Abstract: Most biological nitrogen (N2) fixation results from the activity of a molybdenum-dependent nitrogenase, a complex iron-sulfur enzyme found associated with a diversity of bacteria and some methanogenic archaea. Azotobacter vinelandii, an obligate aerobe, fixes nitrogen via the oxygen-sensitive Mo nitrogenase but is also able to fix nitrogen through the activities of genetically distinct alternative forms of nitrogenase designated the Vnf and Anf systems when Mo is limiting. The Vnf system appears to replace Mo with V, and the Anf system is thought to contain Fe as the only transition metal within the respective active site metallocofactors. Prior genetic analyses suggest that a number of nif-encoded components are involved in the Vnf and Anf systems. Genome-wide transcription profiling of A. vinelandiicultured under nitrogen-fixing conditions under various metal amendments (e.g., Mo or V) revealed the discrete complement of genes associated with each nitrogenase system and the extent of cross talk between the systems. In addition, changes in transcript levels of genes not directly involved in N2fixation provided insight into the integration of central metabolic processes and the oxygen-sensitive process of N2fixation in this obligate aerobe. The results underscored significant differences between Mo-dependent and Mo-independent diazotrophic growth that highlight the significant advantages of diazotrophic growth in the presence of Mo.

Journal ArticleDOI
TL;DR: It is concluded that A. vinelandii is well adapted to fix nitrogen in metal-limited soil environments, with essentially all the cellular Mo and V allocated to the nitrogenase enzymes.
Abstract: Biological nitrogen fixation, the main source of new nitrogen to the Earth's ecosystems, is catalysed by the enzyme nitrogenase. There are three nitrogenase isoenzymes: the Mo-nitrogenase, the V-nitrogenase and the Fe-only nitrogenase. All three types require iron, and two of them also require Mo or V. Metal bioavailability has been shown to limit nitrogen fixation in natural and managed ecosystems. Here, we report the results of a study on the metal (Mo, V, Fe) requirements of Azotobacter vinelandii, a common model soil diazotroph. In the growth medium of A. vinelandii, metals are bound to strong complexing agents (metallophores) excreted by the bacterium. The uptake rates of the metallophore complexes are regulated to meet the bacterial metal requirement for diazotrophy. Under metal-replete conditions Mo, but not V or Fe, is stored intracellularly. Under conditions of metal limitation, intracellular metals are used with remarkable efficiency, with essentially all the cellular Mo and V allocated to the nitrogenase enzymes. While the Mo-nitrogenase, which is the most efficient, is used preferentially, all three nitrogenases contribute to N₂ fixation in the same culture under metal limitation. We conclude that A. vinelandii is well adapted to fix nitrogen in metal-limited soil environments.

Journal ArticleDOI
TL;DR: The results indicate an evolutionary path whereby Mo-dependent nitrogenase emerged within the methanogenic archaea and then gave rise to the alternative forms suggesting that they arose later, perhaps in response to local Mo limitation.
Abstract: Nitrogenase catalyzed nitrogen fixation is the process by which life converts dinitrogen gas into fixed nitrogen in the form of bioavailable ammonia. The most common form of nitrogenase today requires a complex metal cluster containing molybdenum (Mo), although alternative forms exist which contain vanadium (V) or only iron (Fe). It has been suggested that the Mo-independent forms of nitrogenase (V and Fe) was responsible for N2 fixation on early Earth because oceans were Mo-depleted and Fe-rich. Phylogenetic- and structure-based examinations of multiple nitrogenase proteins suggest that such an evolutionary path is unlikely. Rather, our results indicate an evolutionary path whereby Mo-dependent nitrogenase emerged within the methanogenic archaea and then gave rise to the alternative forms suggesting that they arose later, perhaps in response to local Mo limitation. Structural inferences of nitrogenase proteins and related paralogs suggest that the ancestor of all nitrogenase had an open cavity capable of binding metal clusters which conferred reactivity. The evolution of the nitrogenase ancestor and its associated bound metal cluster was controlled by the availability of fixed nitrogen in combination with local environmental factors that influenced metal availability until a point in Earth’s geologic history where the most desirable metal, Mo, became sufficiently bioavailable to bring about and refine the solution (Mo-nitrogenase) we see perpetuated in extant biology.

Book ChapterDOI
01 Jan 2011
TL;DR: Inoculation of legume plants with appropriate inocula containing rhizobia and heavy metal-resistant plant growth-promoting rhizobacteria and/or mycorrhiza has been found as an interesting option to improve plant performance under stressed conditions.
Abstract: Legumes have traditionally been used in soil regeneration, owing to their capacity to increase soil nitrogen due to biological nitrogen fixation. Recently, legumes have attracted attention for their role in remediation of metal-contaminated soils. Legumes accumulate heavy metals mainly in roots and show a low level of metal translocation to the shoot. The main application of these plants is thus in metal phytostabilization. However, high concentrations of heavy metals in soil lead to a decrease in the symbiotic properties of legumes, which could be due to a decrease in the number of rhizobial infections. In order to identify a best legume–Rhizobium partnership for bioremediation purposes, selection of plant varieties and rhizobia resistant to heavy metal is required. Different approaches directed to improve metal bioremediation potential of legumes have been undertaken; from inoculation with rhizosphere bacterial consortia resistant to heavy metals to genetic engineering. Inoculation of legume plants with appropriate inocula containing rhizobia and heavy metal-resistant plant growth-promoting rhizobacteria (PGPR) and/or mycorrhiza has been found as an interesting option to improve plant performance under stressed conditions. The role of Rhizobium–legume symbiosis and approaches employed to genetically engineer legume–Rhizobium interactions in order to improve bioremediation are reviewed and discussed.

Journal ArticleDOI
TL;DR: It is suggested that moss species identity, but not extrinsic environmental conditions, serves as the primary determinant of nitrogen-fixing cyanobacterial communities that inhabit mosses.
Abstract: Recent studies have revealed that nitrogen fixation by cyanobacteria living in association with feather mosses is a major input of nitrogen to boreal forests. We characterized the community composition and diversity of cyanobacterial nifH phylotypes associated with each of two feather moss species (Pleurozium schreberi and Hylocomium splendens) on each of 30 lake islands varying in ecosystem properties in northern Sweden. Nitrogen fixation was measured using acetylene reduction, and nifH sequences were amplified using general and cyanobacterial selective primers, separated and analyzed using density gradient gel electrophoresis (DGGE) or cloning, and further sequenced for phylogenetic analyses. Analyses of DGGE fingerprinting patterns revealed two host-specific clusters (one for each moss species), and sequence analysis showed five clusters of nifH phylotypes originating from heterocystous cyanobacteria. For H. splendens only, N(2) fixation was related to both nifH composition and diversity among islands. We demonstrated that the cyanobacterial communities associated with feather mosses show a high degree of host specificity. However, phylotype composition and diversity, and nitrogen fixation, did not differ among groups of islands that varied greatly in their availability of resources. These results suggest that moss species identity, but not extrinsic environmental conditions, serves as the primary determinant of nitrogen-fixing cyanobacterial communities that inhabit mosses.

Book
30 Sep 2011
TL;DR: The relationship between inputs and outputs of nitrogen in intensive grassland systems has been investigated in this paper, where the response to nitrogen fertilizer from a cut perennial ryegrass pasture in the Scottish uplands relative to efficiency of fertilizer use and provision of herbage for animals.
Abstract: The relationship between inputs and outputs of nitrogen in intensive grassland systems.- The response to nitrogen fertilizer from a cut perennial ryegrass (Lolium perenne L.) pasture in the Scottish uplands relative to efficiency of fertilizer use and provision of herbage for animals.- Efficient use of fertilizer nitrogen on grass swards: effects of timing, cutting management and secondary grasses.- Nitrogen supply and the persistence of grasses.- Sources and transformations of organic nitrogen in intensively managed grassland soils.- Gaseous losses of nitrogen from grassland.- Relationships between soil mineral nitrogen content and denitrification, following application of slurry and calcium nitrate to grassland.- Losses of nitrogen from intensive grassland systems by leaching and surface runoff.- Nitrate loss through leaching and surface runoff from grassland: effects of water supply, soil type and management.

Journal ArticleDOI
TL;DR: A brief introduction to both the biochemical and ecological aspects of these processes are provided and how human activity over the last 100 years has changed the historic balance of the global nitrogen cycle is considered.
Abstract: The nitrogen cycle describes the processes through which nitrogen is converted between its various chemical forms. These transformations involve both biological and abiotic redox processes. The principal processes involved in the nitrogen cycle are nitrogen fixation, nitrification, nitrate assimilation, respiratory reduction of nitrate to ammonia, anaerobic ammonia oxidation (anammox) and denitrification. All of these are carried out by micro-organisms, including bacteria, archaea and some specialized fungi. In the present article, we provide a brief introduction to both the biochemical and ecological aspects of these processes and consider how human activity over the last 100 years has changed the historic balance of the global nitrogen cycle.

Journal ArticleDOI
TL;DR: Both the proliferation of N2 fixers and expression of the nifH gene indicated that bacteria similar to rhizobia may contribute to N2 fixation in sugarcane under the authors' experimental conditions.
Abstract: To explore the presence and expression of the nifH gene of diazotrophic endophytes in sugarcane (Saccharum spp. hybrids) cv. NiF8, we conducted pot experiments for two seasons in a glasshouse from 10 June to 17 September (high temperature and long days) in 2005 and 1 September to 9 December (low temperature and short days) in 2006. The expression of nifH genes in the stems and roots of sugarcane plants at 50 (or 59) and 100 days after transplanting was investigated by reverse transcription (RT)–PCR and by examining the nifH nucleotide sequence diversity. N2 fixation was assessed by the 15N-dilution method. The nifH RNA sequences in the stems and roots of the first experiment were similar to those of Bradyrhizobium sp. and Azorhizobium caulinodans. In the second experiment, nifH expression from these bacteria was not detected in the stems. nifH gene expression in the stems was in accordance with 18–24% N derived from air in the high-temperature season and negligible in the low-temperature season. Both the proliferation of N2 fixers and expression of the nifH gene indicated that bacteria similar to rhizobia may contribute to N2 fixation in sugarcane under our experimental conditions.

Journal ArticleDOI
TL;DR: In this paper, the effects of compaction can be mediated by straw mulch applied at soil surface, and the results supported the hypotheses that soil compaction affects nodulation and nitrogen fixation.
Abstract: Soil compaction affects nodulation and nitrogen fixation of soybean (Glycine max (L.) Merr.). The effects of compaction can be mediated by straw mulch applied at soil surface. To test these hypotheses a field experiment was carried out in 2008 on a Haplic Luvisol (south-eastern Poland). We grew soybean in treatments with different soil compaction levels: non-compacted (NC), moderately compacted (MC) (3 tractor passes) and strongly compacted soil (SC) (5 tractor passes). Each compaction level had treatments without and with surface chopped straw mulch at the rate of 0.5 kg m−2. Nodulation was evaluated by measurements of nodule-size distribution, number, weight, and diameter of nodules and symbiotic nitrogen fixation – by measurement of nitrogenase activity, nitrogen content in leaves, protein content in seeds and seed yield. Soil measurements included bulk density and nitrogen content in the form of N-NO3 and N-NH4. In general, nitrogenase activity decreased with increasing soil compaction level. At every compaction level, the nitrogenase activity was higher in the mulched when compared to the unmulched soil. Soil compaction and mulching enhanced the contribution of large nodules, those greater than 0.41 cm and dry weight of individual nodules. Total nodule number and weight decreased in sequence: moderately, strongly and non-compacted soil. Mulch influenced nitrogenase activity and nodulation to a higher extent than compaction. An optimum soybean response in terms of seed and protein yields was observed under mulched MC and the worst – under unmulched SC. Both N content in leaves and nitrate content in soil were lower in mulched than in unmulched soil, irrespective of compaction. Bulk density and N-NO3 increased with soil compaction level and N-NH4 remained nearly unchanged. Therefore, our results supported the hypotheses that soil compaction affects nodulation and nitrogen fixation, and that compaction can be mediated by straw mulch. Further studies are needed to assess variability related with different soil and weather conditions.

Journal ArticleDOI
TL;DR: Quantitative PCR (and statistical analyses) revealed that, overall, copy number of nifH sequences increased with progressing succession and correlated with changes in physiochemical properties and the recorded nitrogenase activities of the tailings.

Journal ArticleDOI
TL;DR: The use of inoculants appears compulsory in a frame of sustainable agriculture, which seeks to increase crop yields and nutrient-use efficiency while reducing the environmental costs associated with agriculture intensification.
Abstract: Soybean, Glycine max (L.) Merrill, is one of the most important food crops in the world. High soybean yields require large amounts of N fertilizers, which are expensive and can cause environmental problems. The industrial fixation of nitrogen accounts for about 50% of fossil fuel usage in agriculture. In contrast, biological fixation of N2 is a low-cost source of N for soybean cropping through the symbiotic association between the plant and soil bacteria belonging to the genera Bradyrhizobium and Sinorhizobium, which are collectively called “soybean rhizobia“. In general, symbiotic nitrogen fixation in crop legumes not only reduces fertilizer costs but also improves soil fertility through crop rotation and intercropping. Biological nitrogen fixation is due to symbioses between leguminous plants and species of Rhizobium bacteria. Replacing this natural N source by synthetic N fertilizers would cost around 10 billion dollars annually. Moreover, legume seed and foliage have a higher protein content than that of non-legumes, and this makes them desirable protein crops. There is a wide knowledge of the industrial elaboration and use of commercial soybean inoculants based on bradyrhizobia strains. At present, the technology to prepare different types of inoculants, either solid or liquid, is sufficiently developed to meet market requirements, although further research and investments are still required to improve the symbiotic efficacy of rhizobial inoculants. Inoculation of soybeans under field conditions has been successful in the USA, Brazil and Argentina, which are the world leaders in soybean cultivation in terms of acreage and grain yields. There are, however, limitations to a wider use of rhizobial inoculants: the size of indigenous soil rhizobial populations can prevent the successful use of inoculants in some particular areas. For example, many Chinese soils contain more than 105 soybean rhizobia per gram of soil, which imposes a serious barrier for nodule occupancy by the soybean rhizobia used as an inoculant. The use of inoculants based on soil bacteria other than rhizobia has also increased in the last decades. An example is the genus Azospirillum, which can be used for its capacity to increase plant growth and seed yields through different mechanisms, such as the production of plant hormones and the increase in phosphate uptake by roots. In addition, co-inoculation with Azospirillum and rhizobia enhances nodulation and nitrogen fixation. Although less developed, it is expected that inoculants based on mycorrhizal fungi will also play a relevant role in sustainable agriculture and forestry. In spite of any possible limitations, the use of inoculants appears compulsory in a frame of sustainable agriculture, which seeks to increase crop yields and nutrient-use efficiency while reducing the environmental costs associated with agriculture intensification. This review also summarizes some of the most relevant genetic aspects of soybean rhizobia in relation to their symbiosis with soybeans. They can be listed as follows: (1) legume roots exude flavonoids, which are able to activate the transcription of nodulation (nod, nol, noe) genes; (2) expression of nodulation genes results in the production and secretion of lipo-chitin oligosaccharide signal molecules, called LCOs or “Nod factors”, which activate nodule organogenesis in the legume root; (3) LCOs induce numerous responses of the legume roots, such as hair curling and the formation of nodule primordia in the inner or outer cortex; (4) the function of many soybean rhizobia nod genes is known and the chemical structure of the LCOs produced has been determined; (5) in addition to LCOs, different soybean rhizobia surface polysaccharides are required for the formation of nitrogenfixing nodules; (6) surface polysaccharides might act as signal molecules or could prevent plant defense reactions. Cyclic glucans, capsular polysaccharides and lipopolysaccharides appear to play relevant roles in the soybean nodulation process since rhizobial mutants affected in any of these surface polysaccharides are symbiotically impaired. Present knowledge of the molecular bases determining cultivar-strain specificity and nodule occupancy by soybean rhizobia competitors is clearly insufficient. This lack of information is a serious barrier for developing strategies aimed at improving nodulation and symbiotic nitrogen fixation of commercial inoculants. In spite of these difficulties, recent studies have shown that the signaling pathway involved in triggering nodule organogenesis is independent of that operating in bacterial entry through infection thread formation. Theses facts might offer new insights for improving symbiotic nitrogen fixation and also for the feasibility of transferring nodule organogenesis, a first step in expanding this symbiotic interaction into other agriculturally important species.

Journal ArticleDOI
16 Dec 2011-Science
TL;DR: Light is shed on the extent to which human activities have changed nitrogen availability, with implications for ecosystems around the world.
Abstract: For most of the history of the biosphere, nitrogen has swirled tantalizingly out of reach: In the form of inert N2 gas (78% of the modern atmosphere), it was available only to certain bacteria and cyanobacteria capable of producing the nitrogenase enzyme that breaks the strong N–N triple bond. Even these nitrogen fixers cannot liberally fix nitrogen because of the high energy costs of running nitrogenase and the high demands for other elements needed to produce nitrogenase. It is for this reason that plant and algal biomass and productivity of many ecosystems are limited by nitrogen ( 1 ) and that the supply of nitrogen plays a major role in structuring plant communities ( 2 ). A report by Holtgrieve et al. on page 1545 of this issue ( 3 ) and other recent studies ( 4 , 5 ) shed light on the extent to which human activities have changed nitrogen availability, with implications for ecosystems around the world.

Journal ArticleDOI
TL;DR: The results imply that crude oil seriously affects rhizosphere microbial growth in legumes and total loss of soil fertility attributable to petroleum hydrocarbon contamination in the Niger Delta ultisol.
Abstract: The effect of crude oil on the growth of legumes (Calopogonium muconoides and Centrosema pubescens) and fate of nitrogen-fixing bacteria in wetland ultisol was investigated using standard cultural techniques. The results revealed observable effects of oil on soil physico-chemistry, plant growth and nodulation as well as on densities of heterotrophic, hydrocarbonoclastic and nitrogen fixing bacteria. The effects however varied with different levels (0.5%, 1%, 5%, 10%, 15% and 20%) of pollution. Ammonium and nitrate levels were high in the unpolluted soil but decreased with increase in pollution levels. Nitrite was not detected in contaminated soil probably due to the reduction in numbers of nitrogen fixers, from 5.26 ± 0.23 × l06cfu/g in unpolluted soil to 9.0 ± 0.12 × 105 and 2.2 ± 0.08 × l05 cfu/g in soils with 5% and 20% levels of pollution respectively. The contaminated soil also exhibited gross reduction in the nodulation of legumes. A range of 13–57 nodules was observed in legumes from polluted soil against 476 nodules recorded for plants cultured on unpolluted soil. The heterogeneity of the microbial loads between oil-polluted and unpolluted soil were statistically significant (p < 0.05, ANOVA). Positive significant relationships were observed between the levels of hydrocarbons and the densities of heterotrophic bacteria (r = 0.91) and that of hydrocarbon utilizing bacteria (r = 0.86). On the other hand, relationships between the densities of nitrogen fixing bacteria and total hydrocarbons content was negative (r = −0.30) while positive relationships were recorded between the densities of different microbial groups and treatment periods except at 15% and 20% pollution levels. The LSD tests revealed highly significant differences (p < 0.001) in the physiological groups of soil microorganisms at all levels of pollution. The results imply that crude oil seriously affects rhizosphere microbial growth in legumes. Among the bacterial species isolated, Clostridium pasteurianum, Bacillus polymyxa and Pseudomonas aeruginosa exhibited greater ability to degrade hydrocarbons than Azotobacter sp, Klebsiella pneumoniae and Derxia gummusa while Nitrosomonas and Nitrobacter had the least degradability. A continuous monitoring of the environment is advocated to prevent extinction of nitrogen-fixing bacteria and total loss of soil fertility attributable to petroleum hydrocarbon contamination in the Niger Delta ultisol.

Journal ArticleDOI
Yan-gui Su1, Xin Zhao1, Ai-xia Li1, Xin-rong Li1, Gang Huang1 
TL;DR: It is demonstrated that BSCs are the main nitrogen input source in this temperate desert ecosystem, and cyanobacterial–algal crusts may mediate the majority of nitrogen fixation to facilitate soil nitrogen accumulation in these infertile soils.

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TL;DR: The findings suggest that N fixation improvement in lentils and peas may be addressed most effectively by breeding crops for greater N fixation hosting capacity.
Abstract: Increasing nitrogen fixation in legume crops could increase cropping productivity and reduce nitrogen fertilizer use. Studies have found that crop genotype, rhizobial strain, and occasionally genotype-specific interactions affect N fixation, but this knowledge has not yet been used to evaluate or breed for greater N fixation in US crops. In this study five USDA varieties of lentils (Lens culinaris Medik.) and five varieties of peas (Pisum sativum L.) were tested with 13 to 15 commercially available strains of Rhizobium leguminoserum bv. viciae to identify the better N fixing rhizobial strains, crop varieties, and specific pairings. Peas and lentils inoculated with individual strains were grown in growth chambers for 6 week. Plants received (15NH4)2 SO4 (5 at.%) starter fertilizer to measure N fixation by isotope dilution. Below- and above-ground biomass, numbers of nodules, and the proportion of plant N supplied by fixation (PNF) were determined. The percent of N fixed was significantly affected by crop variety and significantly correlated with number of nodules in both lentils and peas. This implies that one strategy for enhancing crop N fixation is developing varieties that have higher rhizobium infection rates. Total N fixation in lentils was significantly influenced by both crop variety and rhizobial strain. Eston variety lentil and Shawnee variety pea had the highest PNF of 80.8% and 91.3%, respectively. The different strains of R. leguminoserum affected PNF in lentils but not in peas. These findings suggest that N fixation improvement in lentils and peas may be addressed most effectively by breeding crops for greater N fixation hosting capacity.

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TL;DR: Evidence concerning the possible role of GABA in whole-plant-based up regulation of symbiotic nitrogen fixation will be reviewed and work indicating that there may be a flow of nitrogen to bacteroids is discussed in light of hypothesis that such a flow may be important to nodule function.
Abstract: The ability to regulate the rates of metabolic processes in response to changes in the internal and/or external environment is a fundamental feature which is inherent in all organisms. This adaptability is necessary for conserving the stability of the intercellular environment (homeostasis) which is essential for maintaining an efficient functional state in the organism. Symbiotic nitrogen fixation in legumes is an important process which establishes from the complex interaction between the host plant and microorganism. This process is widely believed to be regulated by the host plant nitrogen demand through a whole plant N feedback mechanism in particular under unfavorable conditions. This mechanism is probably triggered by the impact of shoot-borne, phloem-delivered substances. The precise mechanism of the potential signal is under debate, however, the whole phenomenon is probably related to a constant amino acid cycling within the plant, thereby signaling the shoot nitrogen status. Recent work indicating that there may be a flow of nitrogen to bacteroids is discussed in light of hypothesis that such a flow may be important to nodule function. Large amount of γ-aminobutyric acid (GABA) are cycled through the root nodules of the symbiotic plants. In this paper some recent evidence concerning the possible role of GABA in whole-plant-based upregulation of symbiotic nitrogen fixation will be reviewed.

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TL;DR: Evidence is provided that organic management was a strong driver of the rhizobia genotype present in two organically and two conventionally managed fields, and further understanding of Rhizobia ecology and function as related to specific organic management practices remains a priority.

Journal Article
TL;DR: In this paper, the interaction between mycorrhizal fungus, Glomus mosseae, and salinity stress in relation to plant growth, nitrogen fixation, and nutrient accumulation was evaluated in Cicer arietinum (L.) (chickpea).
Abstract: Most legumes in natural conditions form a symbiosis with arbuscular mycorrhizal (AM) fungi. AM fungi in saline soils have been reported to improve salinity tolerance and growth in plants. In the present study, interaction between mycorrhizal fungus, Glomus mosseae, and salinity stress in relation to plant growth, nitrogen fixation, and nutrient accumulation was evaluated in Cicer arietinum (L.) (chickpea). Two genotypes of chickpea (Pusa-329, salt tolerant, and Pusa-240, salt sensitive) were compared under different levels of salinity with and without mycorrhizal inoculations. Salt stress resulted in a noticeable decline in shoot and root dry matter accumulation, resulting in a decline in the shoot-to-root ratio (SRR) in all plants. However, Pusa-329 was found to be more tolerant to salinity than Pusa-240. AM plants exhibited better growth and biomass accumulation under stressed as well as unstressed conditions. Mycorrhizal infection (MI) was reduced with increasing salinity levels, but the mycorrhizal dependency (MD) increased, which was more evident in Pusa-240. Salinity resulted in a marked decline in the nodule dry weights, whereas a surge in the nodule number was recorded. Nitrogenase activity was reduced with increasing salt concentrations. AM plants had considerably higher nodule numbers, dry weights, and nitrogenase activity under both saline and nonsaline environments. Pusa-329 had a comparatively lower Na+ concentration and higher K+ and Ca2+ concentrations than Pusa-240. Although nitrogen (N) and phosphorus (P) contents declined with increasing salinity, Pusa-329 had higher levels of N and P as compared with Pusa-240. Plants inoculated with Glomus mosseae had better plant growth and nitrogen fixation under salt stress.