Showing papers on "Nitrogen fixation published in 2001"

Nitrogen fixation in tropical cropping systems.

Abstract: Tropical environments - climates, soils and cropping systems nitrogen fixing organisms in the tropics nitrogen fixation process and its role in the tropics tropical crops and cropping systems - cereal crops and grasses, wetland rice, grain legumes, legumes as animal fodder, plantation crops, agroforestry optimizing contributions from nitrogen fixation - mixed farming systems, environmental constraints, past approaches, realizing potential benefits

Unicellular cyanobacteria fix N2 in the subtropical North Pacific Ocean.

Abstract: Fixed nitrogen (N) often limits the growth of organisms in terrestrial and aquatic biomes, and N availability has been important in controlling the CO2 balance of modern and ancient oceans. The fixation of atmospheric dinitrogen gas (N2) to ammonia is catalysed by nitrogenase and provides a fixed N for N-limited environments. The filamentous cyanobacterium Trichodesmium has been assumed to be the predominant oceanic N2-fixing microorganism since the discovery of N2 fixation in Trichodesmium in 1961 (ref. 6). Attention has recently focused on oceanic N2 fixation because nitrogen availability is generally limiting in many oceans, and attempts to constrain the global atmosphere-ocean fluxes of CO2 are based on basin-scale N balances. Biogeochemical studies and models have suggested that total N2-fixation rates may be substantially greater than previously believed but cannot be reconciled with observed Trichodesmium abundances. It is curious that there are so few known N2-fixing microorganisms in oligotrophic oceans when it is clearly ecologically advantageous. Here we show that there are unicellular cyanobacteria in the open ocean that are expressing nitrogenase, and are abundant enough to potentially have a significant role in N dynamics.

Phosphorus limitation of nitrogen fixation by Trichodesmium in the central Atlantic Ocean

Abstract: Marine fixation of atmospheric nitrogen is believed to be an important source of biologically useful nitrogen to ocean surface waters1, stimulating productivity of phytoplankton and so influencing the global carbon cycle2. The majority of nitrogen fixation in tropical waters is carried out by the marine cyanobacterium Trichodesmium3, which supplies more than half of the new nitrogen used for primary production4. Although the factors controlling marine nitrogen fixation remain poorly understood, it has been thought that nitrogen fixation is limited by iron availability in the ocean2,5. This was inferred from the high iron requirement estimated for growth of nitrogen fixing organisms6 and the higher apparent densities of Trichodesmium where aeolian iron inputs are plentiful7. Here we report that nitrogen fixation rates in the central Atlantic appear to be independent of both dissolved iron levels in sea water and iron content in Trichodesmium colonies. Nitrogen fixation was, instead, highly correlated to the phosphorus content of Trichodesmium and was enhanced at higher irradiance. Furthermore, our calculations suggest that the structural iron requirement for the growth of nitrogen-fixing organisms is much lower than previously calculated6. Although iron deficiency could still potentially limit growth of nitrogen-fixing organisms in regions of low iron availability—for example, in the subtropical North Pacific Ocean—our observations suggest that marine nitrogen fixation is not solely regulated by iron supply.

Endophytic Colonization and In Planta Nitrogen Fixation by a Herbaspirillum sp. Isolated from Wild Rice Species

Abstract: Nitrogen-fixing bacteria were isolated from the stems of wild and cultivated rice on a modified Rennie medium. Based on 16S ribosomal DNA (rDNA) sequences, the diazotrophic isolates were phylogenetically close to four genera: Herbaspirillum, Ideonella, Enterobacter, and Azospirillum. Phenotypic properties and signature sequences of 16S rDNA indicated that three isolates (B65, B501, and B512) belong to the Herbaspirillum genus. To examine whether Herbaspirillum sp. strain B501 isolated from wild rice, Oryza officinalis, endophytically colonizes rice plants, the gfp gene encoding green fluorescent protein (GFP) was introduced into the bacteria. Observations by fluorescence stereomicroscopy showed that the GFP-tagged bacteria colonized shoots and seeds of aseptically grown seedlings of the original wild rice after inoculation of the seeds. Conversely, for cultivated rice Oryza sativa, no GFP fluorescence was observed for shoots and only weak signals were observed for seeds. Observations by fluorescence and electron microscopy revealed that Herbaspirillum sp. strain B501 colonized mainly intercellular spaces in the leaves of wild rice. Colony counts of surface-sterilized rice seedlings inoculated with the GFP-tagged bacteria indicated significantly more bacterial populations inside the original wild rice than in cultivated rice varieties. Moreover, after bacterial inoculation, in planta nitrogen fixation in young seedlings of wild rice, O. officinalis, was detected by the acetylene reduction and 15N2 gas incorporation assays. Therefore, we conclude that Herbaspirillum sp. strain B501 is a diazotrophic endophyte compatible with wild rice, particularly O. officinalis.

Segregation of nitrogen fixation and oxygenic photosynthesis in the marine cyanobacterium Trichodesmium.

Abstract: In the modern ocean, a significant amount of nitrogen fixation is attributed to filamentous, nonheterocystous cyanobacteria of the genus Trichodesmium. In these organisms, nitrogen fixation is confined to the photoperiod and occurs simultaneously with oxygenic photosynthesis. Nitrogenase, the enzyme responsible for biological N2 fixation, is irreversibly inhibited by oxygen in vitro. How nitrogenase is protected from damage by photosynthetically produced O2 was once an enigma. Using fast repetition rate fluorometry and fluorescence kinetic microscopy, we show that there is both temporal and spatial segregation of N2 fixation and photosynthesis within the photoperiod. Linear photosynthetic electron transport protects nitrogenase by reducing photosynthetically evolved O2 in photosystem I (PSI). We postulate that in the early evolutionary phase of oxygenic photosynthesis, nitrogenase served as an electron acceptor for anaerobic heterotrophic metabolism and that PSI was favored by selection because it provided a micro-anaerobic environment for N2 fixation in cyanobacteria.

Iron availability, cellular iron quotas, and nitrogen fixation in Trichodesmium

Abstract: Iron availability is suggested to be a primary factor limiting nitrogen fixation in the oceans. This hypothesis is principally based on cost‐benefit analyses of iron quotas in the dominant nitrogen-fixing cyanobacteria,Trichodesmium spp., in the contemporary oceans. Although previous studies with Trichodesmium have indicated that iron availability enhanced nitrogen fixation and photosynthesis, no clear relationship has been reported between cellular iron quotas and nitrogen fixation. We re-examined the proposed link between iron availability and nitrogen fixation in laboratory isolates and natural populations collected from coastal waters north of Australia. In laboratory cultures grown under iron-limiting conditions, we measured a decline in cellular iron quotas, photochemical quantum yields, the relative abundance of photosystem I to photosystem II reaction centers, and rates of nitrogen fixation. Nitrogen fixation displayed a critical threshold of the dissolved sum of total inorganic Fe species ([Fe 9]) of ca. log[Fe9] 52 9.7. Field populations of Trichodesmium, collected during bloom conditions, showed high iron quotas consistent with high nitrogen fixation rates. Using seasonal maps of aeolian iron fluxes and model-derived maps of surface water total dissolved Fe, we calculated the potential of nitrogen fixation by Trichodesmium in the global ocean. Our results suggest that in 75% of the global ocean, iron availability limits nitrogen fixation by this organism. Given present trends in the hydrological cycle, we suggest that iron fluxes will be even more limiting in the coming century.

Growth promotion of chickpea and barley by a phosphate solubilizing strain of Mesorhizobium mediterraneum under growth chamber conditions

Abstract: The efficacy of a strain of Mesorhizobium mediterraneum to enhance the growth and phosphorous content in chickpea and barley plants was assessed in a soil with and without the addition of phospates in a growth chamber. The results obtained show that the strain PECA21 was able to mobilize phosphorous efficiently in both plants when tricalcium phosphate was added to the soil. In barley and chickpea growing in soils treated with insoluble phosphates and inoculated with strain PECA21 the phosphorous content was significantly increased in a 100 and 125%, respectively. Also, the dry matter, nitrogen, potassium, calcium and magnesium content in both plants was significantly increased in inoculated soil added with insoluble phosphate. These results show that the inoculation of a soil with rhizobia should not be based only on the effectiveness of the strains with respect to their nitrogen fixation potential, since these microorganisms can increase the growth of plants by means of other mechanisms, for example the phosphate solubilization. q 2001 Elsevier Science Ltd. All rights reserved.

Baltic Sea nitrogen fixation estimated from the summer increase in upper mixed layer total nitrogen

Abstract: We estimated nitrogen fixation from the increase in total nitrogen (N 2 gas excluded) in the upper 20 m during the summer biomass increase of heterocystous filamentous cyanobacteria at the off-shore Landsort Deep station (BY31, 5 yr) and at 10 more stations in all major basins of the Baltic Sea proper. Estimated fixation rates were 2.3‐5.9 mmol N m 22 d 21 , within the range of reported direct measurements. Estimated total fixation in the Baltic Sea proper, 180‐430 Gg N yr 21 taking nitrogen settling loss and atmospheric deposition into account, was sufficient to sustain 30‐90% of the June‐August pelagic net community production. Filamentous cyanobacteria (mostly Aphanizomenonsp.) had low C : N and C : P ratios in spring 1998, indicating internal storage of both N and P. From early June, when their biomass growth started, ratios rose gradually to the biomass peak in August and early September, when the C : N ratio (6.5 mol/mol) was close to the Redfield ratio, but the C : P ratio reached 420, almost four times Redfield. The C : N ratio of the peak biomass was 1.5 times that in spring, and the C : P ratio was 13 times higher. The high C : P ratio indicates a smaller P demand by filamentous diazotrophs than expected from Redfield ratios. Only a few percent of the P mineralized daily is needed for filamentous cyanobacterial growth in summer. Filamentous cyanobacteria incorporated 16‐41 mmol N m 22 into biomass (C : N 5 6.2) at BY31 in summer 1998. This was less than the estimated nitrogen fixation, suggesting fixed N leaks from growing diazotrophs.

Isolation, partial characterization, and the effect of plant growth-promoting bacteria (PGPB) on micro-propagated sugarcane in vitro

Abstract: We report the isolation of nitrogen fixing, phytohormone producing bacteria from sugarcane and their beneficial effects on the growth of micropropagated sugarcane plantlets. Detection of the nitrogen fixing bacteria by ARA-based MPN (acetylene reduction assay-based most probable number) method indicated the presence of up to 106 bacteria per gram dry weight of stem and 107 bacteria per gram dry weight of root of field-grown sugarcane. Two nitrogen fixing bacterial isolates were obtained from stem (SC11, SC20) and two from the roots (SR12, SR13) of field-grown plants. These isolates were identified as Enterobacter sp. strains on the basis of their morphological characteristics and biochemical tests. The isolate SC20 was further characterized by 16S rRNA sequence analysis, which showed high sequence similarity to the sequence of Enterobacter cloacae and Klebsiella oxytoca. All the isolates produced the phytohormone indoleacetic acid (IAA) in pure culture and this IAA production was enhanced in growth medium containing tryptophan. The bacterial isolates were used to inoculate micro-propagated sugarcane in vitro where maximum increase in the root and shoot weight over control was observed in the plantlets inoculated with strain SC20. By using the15N isotope dilution technique, maximum nitrogen fixation contribution (28% of total plant nitrogen) was detected in plantlets inoculated with isolate SC20.

A possible nitrogen crisis for Archaean life due to reduced nitrogen fixation by lightning

Abstract: Nitrogen is an essential element for life and is often the limiting nutrient for terrestrial ecosystems1,2 As most nitrogen is locked in the kinetically stable form3, N2, in the Earth's atmosphere, processes that can fix N2 into biologically available forms—such as nitrate and ammonia—control the supply of nitrogen for organisms On the early Earth, nitrogen is thought to have been fixed abiotically, as nitric oxide formed during lightning discharge4,5,6 The advent of biological nitrogen fixation suggests that at some point the demand for fixed nitrogen exceeded the supply from abiotic sources, but the timing and causes of the onset of biological nitrogen fixation remain unclear7,8,9,10,11 Here we report an experimental simulation of nitrogen fixation by lightning over a range of Hadean (45–38 Gyr ago) and Archaean (38–25 Gyr ago) atmospheric compositions, from predominantly carbon dioxide to predominantly dinitrogen (but always without oxygen) We infer that, as atmospheric CO2 decreased over the Archaean period, the production of nitric oxide from lightning discharge decreased by two orders of magnitude until about 22 Gyr After this time, the rise in oxygen (or methane) concentrations probably initiated other abiotic sources of nitrogen Although the temporary reduction in nitric oxide production may have lasted for only 100 Myr or less, this was potentially long enough to cause an ecological crisis that triggered the development of biological nitrogen fixation

Cyanobacterial biofertilizers in rice agriculture

Abstract: Floodwater and the surface of soil provide the sites for aerobic phototrophic nitrogen (N) fixation by free-living cyanobacteria and theAzolla-Anabaena symbiotic N2-fixing complex. Free-living cyanobacteria, the majority of which are heterocystous and nitrogen fixing, contribute an average of 20–30 kg N ha-1, whereas the value is up to 600 kg ha-1 for theAzollaAnabaena system (the most beneficial cyanobacterial symbiosis from an agronomic point of view). Synthesis and excretion of organic/growth-promoting substances by the cyanobacteria are also on record. During the last two or three decades a large number of studies have been published on the various important fundamental and applied aspects of both kinds of cyanobacterial biofertilizers (the free-living cyanobacteria and the cyanobacteriumAnabaena azollae in symbiotic association with the water fernAzolla), which include strain identification, isolation, purification, and culture; laboratory analyses of their N2-fixing activity and related physiology, biochemistry, and energetics; and identification of the structure and regulation of nitrogenfixing (nif) genes and nitrogenase enzyme. The symbiotic biology of theAzolla-Anabaena mutualistic N2-fixing complex has been clarified. In free-living cyanobacterial strains, improvement through mutagenesis with respect to constitutive N2 fixation and resistance to the noncongenial agronomic factors has been achieved. By preliminary meristem mutagenesis inAzolla, reduced phosphate dependence was achieved, as were temperature tolerance and significant sporulation/spore germination under controlled conditions. Mass-production biofertilizer technology of free-living and symbiotic (Azolla-Anabaena) cyanobacteria was studied, as were the interacting and agronomic effects of both kinds of cyanobacterial biofertilizer with rice, improving the economics of rice cultivation with the cyanobacterial biofertilizers. Recent results indicate a strong potential for cyanobacterial biofertilizer technology in rice-growing countries, which opens up a vast area of more concerted basic, applied, and extension work in the future to make these self-renewable natural nitrogen resources even more promising at the field level in order to help reduce the requirement for inorganic N to the bare minimum, if not to zero.

Nutritional constraints on root nodule bacteria affecting symbiotic nitrogen fixation: a review.

Abstract: Root nodule bacteria require access to adequate concentrations of mineral nutrients for metabolic processes to enable their survival and growth as free-living soil saprophytes, and in their symbiotic relationship with legumes. Essential nutrients, with a direct requirement in metabolism of rhizobia are carbon, hydrogen, oxygen, nitrogen, phosphorus, sulfur, potassium, calcium, magnesium, iron, manganese, copper, zinc, molybdenum, nickel, cobalt and selenium. Boron does not seem to be required by rhizobia, but is essential for the establishment of effective legume symbioses. Nutrient constraints can affect both free-living and symbiotic forms of root nodule bacteria, but whether they do is a function of a complex series of events and interactions. Important physiological characteristics of rhizobia involved in, or affected by, their mineral nutrition include nutrient uptake, growth rate, gene regulation, nutrient storage, survival, genetic exchange and the viable non-culturable state. There is considerable variation between genera, species and strains of rhizobia in their response to nutrient deficiency. The effects of nutrient deficiencies on free-living rhizobia in the soil are poorly understood. Competition between strains of rhizobia for limiting phosphorus and iron in the rhizosphere may affect their ability to nodulate legumes. Processes in the development of some legume symbioses specifically require calcium, cobalt, copper, iron, potassium, molybdenum, nickel, phosphorus, selenium, zinc and boron. Limitations of phosphorus, calcium, iron and molybdenum in particular, can reduce legume productivity by affecting nodule development and function. The effects of nutrient deficiencies on rhizobia-legume signalling are not understood. The supply of essential inorganic nutrients to bacteroids in relation to nutrient partitioning in nodule tissues and nutrient transport to the symbiosome may affect effectiveness of nitrogen fixation. An integration of molecular approaches with more traditional biochemical, physiological and field-based studies is needed to improve understanding of the agricultural importance of rhizobia response to nutrient stress.

Factors Influencing Nitrogen Fixation and Nitrogen Release in Biological Soil Crusts

Abstract: Nitrogen (N) occurs in the atmosphere as N2, a form that is not useable by vascular plants. N2 must first be “fixed”, or reduced, to ammonia (NH4 +) by prokaryotic organisms such as eubacteria and cyanobacteria. Thus, an important feature of the cyanobacteria and cyanolichens in soil crusts is their ability to fix atmospheric N. As this fixation is an anaerobic process, most cyanobacterial fixation takes place in heterocysts, which are specialized, thick-walled cells with enhanced respiration and no oxygen-producing photosystem II (Paerl 1990). Heterocystic genera commonly occurring in soil crusts include Anabaena, Calothrix, Cylindrospermum, Dicothrix, Hapalosiphon, Nodularia, Nostoc, Plectonema, Schizothrix, and Scytonema (Harper and Marble 1988). Nitrogen fixation has also been demonstrated in non- heterocystous soil genera such as Lyngbya, Microcoleus, Oscillatoria, Phormidium, and Tolypothrix (Rogers and Gallon 1988; Belnap 1996), although this may be a result of associated bacteria (Steppe et al. 1996). Nonheterocystic species can exclude oxygen in several ways: (1) behaviorally by clumping; (2) spatially or chemically within a cell; (3) temporally, by fixing at night when no oxygen is being evolved by photosynthesis; or (4) through a combination of these (Paerl 1978; Rogers and Gallon 1988; Paerl 1990). Bacteria associated with cyanobacteria may also contribute to N inputs by scavenging oxygen (thus creating anaerobic microzones for the cyanobacteria) or by fixing N themselves. This has been demonstrated for Microcoleus vaginatus isolated from soil crusts (Steppe et al. 1996). Soil lichens with cyanobacterial photobionts also fix N. Common N-fixing soil lichens include Nostoc-containing Collema spp. and Peltigera spp. and Scytonema-containing Heppia spp. Cyanobacteria also live as epiphytes on soil mosses and phycolichens; thus, this consortium of organisms can show fixation activity (Peters et al. 1986).

Nitrogen fixation: Nitrogenase genes and gene expression

Abstract: Publisher Summary This chapter discusses nitrogenase genes and gene expressions. The primers used for nifH amplification, the methods used to extract genomic DNA and mRNA, the alignment and analysis of nifH sequences, and the RT-PCR protocol are described. Biological nitrogen fixation is the enzymatic reduction of atmospheric dinitrogen to ammonium. The conventional nitrogenase enzyme is encoded by the nifHDK genes, which are in contiguous arrangement within the genome. Alternative nitrogenases (alternative and second alternative) also contain nifH, but contain a third protein in the counterpart to the Mo protein, which is encoded by nifG (nifDGK). Nitrogenase genes can be detected and characterized by amplification from environmental samples using the polymerase chain reaction (PCR). Amplification of nitrogenase genes indicates that nitrogen-fixing microorganisms are present, but not whether or not they are actively fixing nitrogen. By coupling, the PCR assay with reverse transcription (RT-PCR) microorganisms that are actively expressing the nitrogenase enzyme can be detected. Once genes are amplified, the diversity of sequences can be determined by a number of means, including cloning and sequencing of individual amplification products.

Resource Optimization and Symbiotic Nitrogen Fixation

Abstract: In temperate forests, symbiotic nitrogen (N) fixation is restricted to the early phases of succession despite the persistence of N limitation on production late in succession. This paradox has yet to be explained adequately. We hypothesized that the restriction of N fixation to early stages of succession results from the optimization of resource allocation in the vegetation. Because of this optimization, N fixation should be restricted to periods when fixation is less costly than N uptake. Our analysis differs from others in the way we calculate the cost of N uptake; we assess the cost of N uptake as the amount of carbon (C) that could be assimilated if the resources necessary to acquire one gram of N from the soil were allocated instead to photosynthesis. We then simulate N fixation as an asymptotic function of the difference in cost between N uptake and N fixation and proportional to the abundance of host tissues for the N-fixing symbionts. The factors that contribute to conditions that favor N fixation are (a) elevated-carbon dioxide (CO2) concentrations, (b) an open canopy, (c) low available N in the soil, and (d) a soil volume already well exploited by roots. Our results indicate that changes in the relative cost of uptake vs fixation can explain most of the pattern in fixation through both primary and secondary succession, but that competitive interactions with nonfixing species play a role in the final exclusion of fixation in later stages of succession.

Evidence of nitrogen fixation by non-heterocystous cyanobacteria in the Baltic Sea and re-calculation of a budget of nitrogen fixation

Abstract: Nitrogen fixation rates and related plankton parameters were determined at 8 sta- tions in the Baltic proper and Mecklenburg Bay in 1997 and 1998. Nitrogen fixation was mea- sured with 15 N-tracer method in unenriched samples. Measurable nitrogen fixation rates were found from July to October, with highest rates in August when heterocystous cyanobacteria formed blooms. Nitrogen fixation rates measured during a moderate bloom in 1998 were related to biomass of heterocystous and coccoid (non-heterocystous) cyanobacteria and primary produc- tion. The size fraction <10 µm contributed significantly to total nitrogen fixation both during day and night. It is discussed whether this may be due to small, non-heterocystous cyanobacteria, which were abundant in summer and autumn. They may separate photosynthesis and nitrogen fixation temporally (day and night) and may be especially responsible for the high nitrogen fix- ation rates observed in the dark. As the fraction of pico- and nanoplankton was not considered in earlier studies, a new budget of nitrogen fixation in the Baltic proper has been estimated. On average, daily nitrogen fixation rates of 2.5 mmol N m -2 d -1 (in July/August 1997/1998) and mean annual nitrogen fixation of 125 mmol N m -2 yr -1 were estimated for the Baltic proper. The high variability is discussed. For summer 1998, a budget of the nitrogen cycle at the main station in the Gotland Sea is given.

Sulphate reduction and nitrogen fixation rates associated with roots, rhizomes and sediments from Zostera noltii and Spartina maritima meadows.

Abstract: Sulphate reduction rates (SRR) and nitrogen fixation rates (NFR) associated with isolated roots, rhizomes and sediment from the rhizosphere of the marine macrophytes Zostera noltii and Spartina maritima, and the presence and distribution of Bacteria on the roots and rhizomes, were investigated. Between 1% and 3% of the surface area of the roots and rhizomes of both macrophytes were colonized by Bacteria. Bacteria on the surfaces of S. maritima roots and rhizomes were evenly distributed, while the distribution of Bacteria on Z. noltii roots and rhizomes was patchy. Root- and rhizome-associated SRR and NFR were always higher than rates in the bulk sediment. In particular, nitrogen fixation associated with the roots and rhizomes was 41-650-fold higher than in the bulk sediment. Despite the fact that sulphate reduction was elevated on roots and rhizomes compared with bulk sediment, the contribution of plant-associated sulphate reduction to overall sulphate reduction was small (< or =11%). In contrast, nitrogen fixation associated with the roots and rhizomes accounted for 31% and 91% of the nitrogen fixed in the rhizosphere of Z. noltii and S. maritima respectively. In addition, plant-associated nitrogen fixation could supply 37-1,613% of the nitrogen needed by the sulphate-reducing community. Sucrose stimulated nitrogen fixation and sulphate reduction significantly in the root and rhizome compartments of both macrophytes, but not in the bulk sediment.

Ammonia and amino acid transport across symbiotic membranes in nitrogen-fixing legume nodules

Abstract: Biological nitrogen fixation involves the reduction of atmospheric N2 to ammonia by the bacterial enzyme nitrogenase. In legume-rhizobium symbioses, the nitrogenase-producing bacteria (bacteroids) are contained in the infected cells of root nodules within which they are enclosed by a plant membrane to form a structure known as the symbiosome. The plant provides reduced carbon to the bacteroids in exchange for fixed nitrogen, which is exported to the rest of the plant. This exchange is controlled by plant-synthesised transport proteins on the symbiosome membranes. This review summarises our current understanding of these transport processes, focusing on ammonia and amino acid transport.

Nitrogen fixation and denitrification

Abstract: Publisher Summary The chapter discusses nitrogen fixation, denitrification, and their applications. It discusses the most widely used enzyme-based and direct tracer procedures in the N cycle- N 2 fixation and denitrification. Biological nitrogen fixation is the enzymological capacity of certain bacteria and Archaea to convert gaseous dinitrogen to ammonium on a pathway to amino acid synthesis. In marine systems, the attention is focused on diazotrophic cyanobacteria, because of their quantitative importance in supplying new N to the upper ocean. Nitrogenase activity is routinely determined in many laboratories using the C 2 H 2 reduction procedure. Denitrification is measured conveniently in many microbial systems by addition of C 2 H 2 , which inhibits N 2 O reductase, the last step of the denitrification pathway. The presence of C 2 H 2 results in the accumulation of N 2 O, which is detected selectively and sensitively by electron capture gas chromatography. Many of the procedures and conceptual design of denitrification experiments parallel those of the C 2 H 2 reduction procedure. As for nitrogen fixation, denitrification can also be measured directly by the introduction of enriched 15-N nitrate (or nitrite) into a system, with the determination of progressive enrichment of 15N in the N 2 pool.

Organic matter and nitrogen accumulation and nitrogen fixation during early ecosystem development in Hawaii.

Abstract: We used a chronosequence comprised of 10 y, 52 y and 142 yold `a'a lava flows on Mauna Loa, Hawaii, to determine theaccumulation of organic matter and nitrogen and rates of nitrogenfixation through time. The mass of organic matter (live and deadbiomass and soil organic matter) on the 1984, 1942 and 1852 lavaflows was 0.6, 2.2 and 7.6 kg m− 2, respectively, while total N was 4.8, 10.9 and 85.7 g m− 2. We estimated the total rates of nitrogen fixation for thethree different aged ecosystems using an acetylene reduction assaycalibrated with 15N incubations. While mean rates of total N fixation remained largely constant across the three sites – between2.0 and 3.1 kg ha− 1 y− 1 – the most important sources of N fixation changed. On the 10 y flow, the most important fixer was the pioneering cyanolichen, Stereocaulon vulcani. After 52 years ofecosystem development, the most important N fixer was a cyanoalga,while after 142 years, the predominant N fixers were heterotrophicbacteria associated with leaf litter, twigs and detritus. The totalamount of N accumulated after 52 years of ecosystem development wasequivalent to cumulative inputs through biological N fixation. After 142 years, however, cumulative inputs from N fixation couldonly account for between 27–59% of the total nitrogen accrued. We used fertilizer additions of all essential nutrients otherthan N to test whether the availability of lithophilic nutrientsregulated rates of N fixation in early ecosystem development. Ratesof nitrogen fixation by the lichen, S. vulcani, approximately doubled when fertilized on the 1984 and 1942 flows. Rates of N-fixation by heterotrophic nitrogen fixing bacteria on leaf litter ofMetrosideros polymorpha also increased significantly when fertilized with lithophilic nutrients. These findings suggest that weathering rates of lava in part regulate rates of nitrogen fixation in these young ecosystems.

Rhizobial ecology as affected by the soil environment

Abstract: In this paper we review the influence of various soil factors on the legume–Rhizobium symbiotic relationship. Abiotic factors such as extremes in soil pH (highly acidic or alkaline soils), salinity, tillage, high soil temperature and chemical residues, all of which can occur in crop and pasture systems in southern Australia, generally reduce populations of Rhizobium in the soil. Naturally occurring Rhizobium populations, although often found in high numbers, are generally poor in their ability to fix nitrogen and can compete strongly with introduced Rhizobium inoculant. The introduction of new legume genera as a continuing and essential part of change in farming systems usually requires the need to identify new and specific inoculant Rhizobium strains not found in the soil, but necessary for optimum nitrogen fixation. It is therefore necessary to characterise the specific requirements or limitations in the soil for establishing Rhizobium populations to ensure optimal nitrogen fixation following inoculation of legumes. The ability of the introduced Rhizobium to form effective nodules is rarely linked to a single soil attribute; therefore the study of rhizobial ecology requires an understanding of many soil and environmental factors. This paper reviews current knowledge of the influence of soil factors on rhizobial survival, the nodulation process, and nitrogen fixation by legumes.

Symbiotic specificity of tropical tree rhizobia for host legumes

Abstract: Summary • The host range and specificity is reported of a genetically diverse group of rhizobia isolated from nodules of Calliandra calothyrsus, Gliricidia sepium, Leucaena leucocephala and Sesbania sesban. • Nodule number and nitrogen content was measured in seedlings of herbaceous and woody legume species after inoculation with rhizobial strains isolated from tropical soils, to establish symbiotic effectiveness groups for rhizobial strains and their hosts. • Specificity for nodulation and N2 fixation varied greatly among the legumes. Symbionts of all four legumes exhibited a wide range of promiscuity and symbiotic effectiveness with isolates of S. sesban having the narrowest host range. N2 fixation varied greatly; although some strains fixed large amounts of N2 with more than one host, none was effective with all hosts. Rhizobial isolates of C. calothyrsus, G. sepium and L. leucocephala were able to effectively cross-nodulate each others’ hosts as well as a number of other species. • The complex nature of cross-nodulation relationships between diverse rhizobial strains and legume hosts is highlighted. Host plants inoculated with effective rhizobial strains showed better nitrogen use efficiency than plants supplied solely with mineral nitrogen.

N2 fixation in phototrophs: adaptation to a specialized way of life

Abstract: Phototrophic diazotrophs include the photosynthetic green and purple bacteria, the heliobacteria, many cyanobacteria and the unusual chlorophyll-containing rhizobia that are found in the stem nodules of Aeschynomene spp. In this review, which concentrates on cyanobacteria, the interrelations between photosynthesis and N2 fixation are discussed. Photosynthesis can, in theory, directly provide the ATP and reductant needed to support N2 fixation but the link between these two processes is usually indirect, mediated through accumulated carbon reserves. In cyanobacteria, which possess an oxygenic photosynthesis, this serves to separate the O2 that is produced by photosynthesis from the O2-sensitive nitrogenase. However, in certain circumstances, oxygenic photosynthesis and N2 fixation coexist. Under these conditions, respiratory consumption of photosynthetically generated O2 may have an important role in minimizing O2-damage to nitrogenase.