Showing papers on "Nitrogen fixation published in 2002"

The role of glutamine synthetase and glutamate dehydrogenase in nitrogen assimilation and possibilities for improvement in the nitrogen utilization of crops.

Abstract: This short review outlines the central role of glutamine synthetase (GS) in plant nitrogen metabolism and discusses some possibilities for crop improvement. GS functions as the major assimilatory enzyme for ammonia produced from N fixation, and nitrate or ammonia nutrition. It also reassimilates ammonia released as a result of photorespiration and the breakdown of proteins and nitrogen transport compounds. GS is distributed in different subcellular locations (chloroplast and cytoplasm) and in different tissues and organs. This distribution probably changes as a function of the development of the tissue, for example, GS1 appears to play a key role in leaf senescence. The enzyme is the product of multiple genes with complex promoters that ensure the expression of the genes in an organ- and tissue-specific manner and in response to a number of environmental variables affecting the nutritional status of the cell. GS activity is also regulated post-translationally in a manner that involves 14-3-3 proteins and phosphorylation. GS and plant nitrogen metabolism is best viewed as a complex matrix continually changing during the development cycle of plants. Along with GS, a number of other enzymes play key roles in maintaining the balance of carbon and nitrogen. It is proposed that one of these is glutamate dehydrogenase (GDH). There is considerable evidence for a GDH shunt to return the carbon in amino acids back into reactions of carbon metabolism and the tri-carboxylic acid cycle. Results with transgenic plants containing transferred GS genes suggest that there may be ways in which it is possible to improve the efficiency with which crop plants use nitrogen. Marker-assisted breeding may also bring about such improvements.

The role of legumes as a component of biodiversity in a cross‐European study of grassland biomass nitrogen

Abstract: To investigate how plant diversity loss affects nitrogen accumulation in above-ground plant biomass and how consistent patterns are across sites of different climatic and soil conditions, we varied the number of plant species and functional groups (grasses, herbs and legumes) in experimental grassland communities across seven European experimental sites (Switzerland, Germany, Ireland, United Kingdom (Silwood Park), Portugal, Sweden and Greece). Nitrogen pools were significantly affected by both plant diversity and community composition. Two years after sowing, nitrogen pools in Germany and Switzerland strongly increased in the presence of legumes. Legume effects on nitrogen pools were less pronounced at the Swedish, Irish and Portuguese site. In Greece and UK there were no legume effects. Nitrogen concentration in total above-ground biomass was quite invariable at 1.66 ± 0.03% across all sites and diversity treatments. Thus, the presence of legumes had a positive effect on nitrogen pools by significantly increasing above-ground biomass, i.e. by increases in vegetation quantity rather than quality. At the German site with the strongest legume effect on nitrogen pools and biomass, nitrogen that was fixed symbiotically by legumes was transferred to the other plant functional groups (grasses and herbs) but varied depending on the particular legume species fixing N and the non-legume species taking it up. Nitrogen-fixation by legumes therefore appeared to be one of the major functional traits of species that influenced nitrogen accumulation and biomass production, although effects varied among sites and legume species. This study demonstrates that the consequences of species loss on the nitrogen budget of plant communities may be more severe if legume species are lost. However, our data indicate that legume species differ in their N 2 fixation. Therefore, loss of an efficient N 2 -fixer (Trifolium in our study) may have a greater influence on the ecosystem function than loss of a less efficient species (Lotus in our study). Furthermore, there is indication that P availability in the soil facilitates the legume effect on biomass production and biomass nitrogen accumulation.

Nitrogen Fixation and Nitrogen Isotope Abundances in Zooplankton of the Oligotrophic North Atlantic

Abstract: Deep-water nitrate is a major reservoir of oceanic combined nitrogen and has long been considered to be the major source of new nitrogen supporting primary production in the oligotrophic ocean. 15N:14N ratios in plankton provide an integrative record of the nitrogen cycle processes at work in the ocean, and near-surface organic matter in oligotrophic waters like the Sargasso Sea is characterized by an unusually low 15N content relative to average deep-water nitrate. Herein we show that the low dΔ15N of suspended particles and zooplankton from the tropical North Atlantic cannot arise through isotopic fractionation associated with nutrient uptake and food web processes but are instead consistent with a significant input of new nitrogen to the upper water column by N2 fixation. These results provide direct, integrative evidence that N2 fixation makes a major contribution to the nitrogen budget of the oligotrophic North Atlantic Ocean.

A New Species of Devosia That Forms a Unique Nitrogen-Fixing Root-Nodule Symbiosis with the Aquatic Legume Neptunia natans (L.f.) Druce

Abstract: Rhizobia are the common bacterial symbionts that form nitrogen-fixing root nodules in legumes. However, recently other bacteria have been shown to nodulate and fix nitrogen symbiotically with these plants. Neptunia natans is an aquatic legume indigenous to tropical and subtropical regions and in African soils is nodulated by Allorhizobium undicola. This legume develops an unusual root-nodule symbiosis on floating stems in aquatic environments through a unique infection process. Here, we analyzed the low-molecular-weight RNA and 16S ribosomal DNA (rDNA) sequence of the same fast-growing isolates from India that were previously used to define the developmental morphology of the unique infection process in this symbiosis with N. natans and found that they are phylogenetically located in the genus Devosia, not Allorhizobium or Rhizobium. The 16S rDNA sequences of these two Neptunia-nodulating Devosia strains differ from the only species currently described in that genus, Devosia riboflavina. From the same isolated colonies, we also located their nodD and nifH genes involved in nodulation and nitrogen fixation on a plasmid of approximately 170 kb. Sequence analysis showed that their nodD and nifH genes are most closely related to nodD and nifH of Rhizobium tropici, suggesting that this newly described Neptunia-nodulating Devosia species may have acquired these symbiotic genes by horizontal transfer.

Azoarcus Grass Endophytes Contribute Fixed Nitrogen to the Plant in an Unculturable State

Abstract: The extent to which the N2-fixing bacterial endophyte Azoarcus sp. strain BH72 in the rhizosphere of Kallar grass can provide fixed nitrogen to the plant was assessed by evaluating inoculated plants grown in the greenhouse and uninoculated plants taken from the natural environment. The inoculum consisted of either wild-type bacteria or nifK- mutant strain BHNKD4. In N2-deficient conditions, plants inoculated with strain BH72 (N2-fixing test plants) grew better and accumulated more nitrogen with a lower delta15N signature after 8 months than did plants inoculated with the mutant strain (non-N2-fixing control plants). Polyadenylated or polymerase chain reaction-amplified BH72 nifH transcripts were retrieved from test but not from control plants. BH72 nifH transcripts were abundant. The inocula could not be reisolated. These results indicate that Azoarcus sp. BH72 can contribute combined N2 to the plant in an unculturable state. Abundant BH72 nifH transcripts were detected also in uninoculated plants taken from the natural environment, from which Azoarcus sp. BH72 also could not be isolated. Quantification of nitrogenase gene transcription indicated a high potential of strain BH72 for biological N2 fixation in association with roots. Phylogenetic analysis of nitrogenase sequences predicted that uncultured grass endophytes including Azoarcus spp. are ecologically dominant and play an important role in N2-fixation in natural grass ecosystems.

Rhizobium nod factor perception and signalling.

Abstract: Biological nitrogen fixation is a process that can only be performed by certain prokaryotes. In some cases, such bacteria are able to fix nitrogen in a symbiotic relationship with plants. Bacteria of the genera Azorhizobium , Bradyrhizobium , Mesorhizobium , Rhizobium , and Sinorhizobium (

Quantitative effects of soil nitrate, growth potential and phenology on symbiotic nitrogen fixation of pea (Pisum sativum L.)

Abstract: The influence of soil nitrate availability, crop growth rate and phenology on the activity of symbiotic nitrogen fixation (SNF) during the growth cycle of pea (Pisum sativum cv. Baccara) was investigated in the field under adequate water availability, applying various levels of fertiliser N at the time of sowing. Nitrate availability in the ploughed layer of the soil was shown to inhibit both SNF initiation and activity. Contribution of SNF to total nitrogen uptake (%Ndfa) over the growth cycle could be predicted as a linear function of mineral N content of the ploughed layer at sowing. Nitrate inhibition of SNF was absolute when mineral N at sowing was over 380 kg N ha−1. Symbiotic nitrogen fixation was not initiated unless nitrate availability in the soil dropped below 56 kg N ha−1. However, SNF could no longer be initiated after the beginning of seed filling (BSF). Other linear relationships were established between instantaneous %Ndfa and instantaneous nitrate availability in the ploughed layer of the soil until BSF. Instantaneous %Ndfa decreased linearly with soil nitrate availability and was nil above 48 and 34 kg N ha−1 for the vegetative and reproductive stages, respectively, levels after which no SNF occurred. Moreover, SNF rate was shown to be closely related to the crop growth rate until BSF. The ratio of SNF rate over crop growth rate decreased linearly with thermal time. Maximum SNF rate was about 40 mg N m−2 degree-day−1, equivalent to 7 kg N ha−1, regardless of the N treatment. From BSF to the end of the growth cycle, the high N requirements of the crop were supported by both SNF and nitrate root absorption but, of the two sources, nitrate root absorption seemed to be less affected by the presence of reproductive organs. However, since soil nitrate availability was low at the end of the growth cycle, SNF was the main source of nitrogen acquisition. The onset of SNF decrease at the end of the growth cycle seemed to be first due to nodule age and then associated to the slowing of the crop growth rate.

Effect of mineral nitrogen on nitrogen nutrition and biomass partitioning between the shoot and roots of pea (Pisum sativum L.).

Abstract: The effect of mineral N availability on nitrogen nutrition and biomass partitioning between shoot and roots of pea (Pisum sativum L., cv Baccara) was investigated under adequately watered conditions in the field, using five levels of fertiliser N application at sowing (0, 50, 100, 200 and 400 kg N ha−1). Although the presence of mineral N in the soil stimulated vegetative growth, resulting in a higher biomass accumulation in shoots in the fertilised treatments, neither seed yield nor seed nitrogen concentration was affected by soil mineral N availability. Symbiotic nitrogen fixation was inhibited by mineral N in the soil but it was replaced by root mineral N absorption, which resulted in optimum nitrogen nutrition for all treatments. However, the excessive nitrogen and biomass accumulation in the shoot of the 400 kg N ha−1 treatment caused crop lodging and slightly depressed seed yield and seed nitrogen content. Thus, the presumed higher carbon costs of symbiotic nitrogen fixation, as compared to root mineral N absorption, affected neither seed yield nor the nitrogen nutrition level. However, biomass partitioning within the nodulated roots was changed. The more symbiotic nitrogen fixation was inhibited, the more root growth was enhanced. Root biomass was greater when soil mineral N availability was increased: root growth was greater and began earlier for plants that received mineral N at sowing. Rooting density was also promoted by increased mineral N availability, leading to more numerous but finer roots for the fertilised treatments. However, the maximum rooting depth and the distribution of roots with depth were unchanged. This suggested an additional direct promoting effect of mineral N on root proliferation.

Strategies used by rhizobia to lower plant ethylene levels and increase nodulation.

Abstract: Agriculture depends heavily on biologically fixed nitrogen from the symbiotic association between rhizobia and plants. Molecular nitrogen is fixed by differentiated forms of rhizobia in nodules loc...

Nitrogen fixation at the millennium.

Abstract: 1. Nitrogen fixation - A general overview (K. Fisher, W.E. Newton). 2. Nitrogenase structure (P.M.C. Benton et al.). 3. Spectroscopy of nitrogenase (B.J. Hales). 4. The gene products of the nif regulon (L.M. Rubio, P.W. Ludden). 5. Use of short-chain alkynes to locate the nitrogenase catalytic site (S.M. Mayer et al.). 6. Regulation of Mo nitrogenases (P. Rudnick, C. Kennedy). 7. Actinorhizal symbioses (K. Pawlowski). 8. Alternative nitrogenases (B. Masepohl et al.). 9. Advances towards the mechanism of nitrogenases (R.A. Henderson). 10. A novel nitrogenase superoxide-dependent nitrogen fixation (D. Gadkari). 11. Dinitrogen chemistry (G.J. Leigh). 12. Chemical models for nitrogenase (R.L. Richards). 13. Quantification of nitrogen fixation (M.B. Peoples et al.). 14. Nitrogen fixation and agricultural practice (G.W. O'Hara et al.). 15. Nitrogen fixation in rice (P.M. Reddy et al.). Index.

Haber process for ammonia synthesis

Abstract: Fixed nitrogen from the air is the major ingredient of fertilizers which makes intensive food production possible. During the development of inexpensive nitrogen fixation processes, many principles of chemical and high-pressure processes were clarified and the field of chemical engineering emerged. Before synthetic nitrogen fixation, wastes and manures of various types or their decomposition products, and ammonium sulfate, which is a by-product from the coking of coal, were the primary sources of agricultural nitrogen. Chilean saltpetre, saltpetre from human and animal urine, and later ammonia recovered from coke manufacture were some of the important sources of fixed nitrogen [1]. During the first decade of the twentieth century, the worldwide demand for nitrogen-based fertilizers far exceeded the existent supply. The largest source of the chemicals necessary for fertilizer production was found in a huge guano deposit (essentially sea bird droppings) that was 220 miles in length and five feet thick, located along the coast of Chile. Scientists had long desired to solve the problem of the world’s dependence on this fast disappearing natural source of ammonia and nitrogenous compounds. Priestly and Cavendish passed electric sparks through air and produced nitrates by dissolving the oxides of nitrogen thus formed in alkalis. Commercial development of this process had proved elusive, for much electrical energy was consumed at low efficiency. Nitrogen had been fixed as calcium cyanamide, but the process was too expensive except for producing chemicals requiring the cyanamide configuration. Other processes, such as thermal processing to mixed oxides of nitrogen (NOX), cyanide formation, aluminum nitride formation and decomposition to ammonia, etc., showed little commercial promise although they were technically possible. It was Fritz Haber, along with Carl Bosch, who finally solved this problem. Haber invented a large-scale catalytic synthesis of ammonia from elemental hydrogen and nitrogen gas, reactants Haber Process for Ammonia Synthesis

The Influence of Abiotic Factors on Biological Nitrogen Fixation in Different Types of Vegetation in the High Arctic, Svalbard

Abstract: The influence of environmental factors on the nitrogen fixation activity in soil and vegetation samples from different types of plant communities from the Sassen Valley (78°N, 16°E), Svalbard, Norw...

Below-ground nitrogen cycling in relation to net canopy production in mangrove forests of southern Thailand

Abstract: Rates of accumulation, transformation and availability of sediment nitrogen in four mangrove forests of different age and type in southern Thailand were examined in relation to forest net canopy production. Net ammonification (range: 0.3–2.3 mmol N m–2 day–1), nitrification (range: 0–0.7 mmol N m–2 day–1) and nitrogen fixation (range: 0–0.6 mmol N m–2 day–1) in surface sediments equated to <10% of canopy nitrogen demand (range: 7.5–32 mmol N m–2 day–1). By mass balance, we estimated that most of the nitrogen required for tree growth must be derived from root-associated nitrogen fixation and/or mineralisation processes occurring possibly to the maximum depth of live root penetration (75–100 cm). Denitrification, nitrification, rainfall and tidal exchange were comparatively small components of sediment nitrogen flow. Denitrification (range: 0–3.8 mmol N m–2 day–1) removed 3–6% of total nitrogen input at three Rhizophora forests, but removed 23% of total nitrogen input in a high-intertidal Ceriops forest. Nitrogen burial ranged from 4% to 12% of total nitrogen input, with the greatest burial rates in two forests receiving the least tidal inundation. Inputs of nitrogen to the forests were rapid (range: 11–37 mmol N m–2 day–1), likely originating from upstream sources such as agricultural and industrial lands, sewage and shrimp ponds. Our results indicate that ~70% to 90% of the nitrogen supplied to the forest floor is shunted via the ammonium pool to trees to sustain the rapid rates of net canopy production measured in these forests. Differences in plant–sediment nitrogen relations between the forests appeared to be a function of the interaction between intertidal position and stand age.

Nitrate reduction and nitrogen fixation in symbiotic association Rhizobium-legumes.

Abstract: The inhibitory effect of nitrate on nitrogenase activity in root nodules of legume plants has been known for a long time. The major factor inducing changes in nitrogenase activity is the concentration of free oxygen inside nodules. Oxygen availability in the infected zone of nodule is limited, among others, by the gas diffusion resistance in nodule cortex. The presence of nitrate may cause changes in the resistance to O2 diffusion. The aim of this paper is to review literature data concerning the effect of nitrate on the symbiotic association between rhizobia and legume plants, with special emphasis on nitrogenase activity. Recent advances indicate that symbiotic associations of Rhizobium strains characterized by a high nitrate reductase activity are less susceptible to inhibition by nitrate. A thesis may be put forward that dissimilatory nitrate reduction, catalyzed by bacteroid nitrate reductase, significantly facilitates the symbiotic function of bacteroids.

Codenitrification and Denitrification Are Dual Metabolic Pathways through Which Dinitrogen Evolves from Nitrate in Streptomyces antibioticus

Abstract: The biological process of dinitrogen (N2) gas formation from fixed nitrogen compounds such as nitrate (NO3−) plays an important role in maintaining homeostasis of the global environment. Bacterial denitrification was long considered the sole biological reaction responsible for this condition, and its systems have been characterized in detail (3, 8, 20). Denitrification physiologically functions as anaerobic respiration in which NO3− is used as the terminal electron acceptor when oxygen (O2) is unavailable. Known bacterial denitrifying systems consist of four steps that successively reduce NO3− to N2 and involve nitrite (NO2−), nitric oxide (NO), and nitrous oxide (N2O) as intermediates. Production of the enzymes catalyzing each step is induced by nitrogen oxides and suppressed by O2 (3, 20). During the past decade, denitrifiers have been discovered in a variety of taxa including filamentous fungi (9, 10), yeasts (15), and actinomycetes (1, 11). All of these novel denitrifiers produce N2O as the major denitrification product. Both nitrogen atoms in the N2O product are derived from nitrate (or nitrite) (1, 9-11, 15). The phenomenon is therefore defined as denitrification, since the process should include the formation of an N—N bond (20). This system in several fungi is localized at respiring mitochondria, where it acts as anaerobic respiration, as it does in bacterial systems (5, 13, 16). Another unique feature of the fungal system is the involvement of cytochrome P450 (P450nor) as NO reductase (Nor) (6). Biological processes other than bacterial denitrification evolve N2 from fixed nitrogen compounds, but their molecular mechanisms and physiological significance remain to be elucidated. We identified simultaneous fungal codenitrification and denitrification, in which a hybrid N2 species is formed by combining two nitrogen atoms derived from NO2− and from other nitrogen compounds (10, 14). The denitrifying fungi Fusarium solani and Cylindrocarpon tonkinense evolve hybrid N2 species (10). The fact that the denitrifying fungus Fusarium oxysporum evolves N2O instead of N2 by codenitrification only when a nitrogen compound in addition to NO2− such as azide, salicylhydroxamic acid, or ammonium (NH4+) is available (14) suggests that the mechanisms of this process differ among fungal species. Anammox is a third N2-generating metabolic process that has been identified in the strictly anaerobic chemolithotrophic Planctomycetales (12), in which NH4+ is combined with NO2− to form N2. The actinomycetes form a unique taxon among gram-positive bacteria. Although actinomycetes naturally proliferate in soil and in aqueous environments, little is known about how they accomplish denitrification. Denitrifiers also occur among actinomycetes (1, 11), and the system of Streptomyces thioluteus has been characterized previously (11). All of the actinomycete denitrifiers found in these studies evolve N2O from NO3− or NO2−, and thus, no known actinomycete strains contain a complete denitrifying system that can thoroughly reduce NO3− to N2. The present study continues screening for denitrifying actinomycetes (11) by using a highly sensitive N2 detection system equipped with an isotope mass spectrometer and NO3− labeled with a stable isotope ([15N]NO3−). The results showed that Streptomyces antibioticus B-546 has N2-producing activity and that most of the N2 molecules are formed via intracellular codenitrification that is induced simultaneously with denitrification.

Interorganismal signaling in suboptimum environments: The legume-rhizobia symbiosis

Abstract: During the Rhizobium -legume interactions that lead to the establishment of the nitrogen fixing symbiosis, an exchange of molecular signals regulates the expression of genes essential for infection, nodule development, and function. All stages of nodule formation and function are impacted by stressful environmental conditions and by soil nitrogen levels. Researchers have concluded that these conditions decrease N 2 fixation activity by directly inhibiting the activity of the nitrogenase complex and by suppressing and/or delaying root infection and nodulation. Infection and early nodule development processes in the soybean- Bradyrhizobium japonicum association are sensitive to suboptimal temperatures. The nodABC genes of Sinorhizobium meliloti and Rhizobium leguminosarum are not expressed under acid conditions. Low pHalso reduced growth and multiplication of rhizobia in soil and increased numbers of ineffective rhizobia. High temperatures were found to increase the release of nod gene inducers from seeds during the first 24 h but decrease the nod gene-inducing activity from bean and soybean roots. Combined nitrogen (NO 3 - , NH 4 + , and urea)has been demonstrated to influence symbiotic N 2 fixation from the initial bidirectional signal exchange between symbionts through to nodule senescence. Nitrate affects a broad range of infection events, including decreases in symbiotic signal exchange, root hair deformation, the binding of rhizobia to root hairs, the number of infection threads formed, and an increase in the number of aborted infection events. Water supply has a major effect on nodulation and N 2 fixation. The relationship between soil moisture and nodulation has been recognized in that nodulation increases with soil water content until waterlogging occurs. Water stresses (deficit and excess) decrease the number of infection threads formed and inhibit nodulation. Under high-salinity conditions, bacterial colonization and root hair curling of plants grown at 100 mol m −3 NaCl are both reduced when compared to those of plants grown at 50 mol m −3 and the proportion of root hairs containing infection threads is reduced by about 30%. All of the negative effects of these environmental factors on legume nodulation and N 2 fixation are known to act entirely or partially through early nod gene induction and nodule infection. Specific compounds have been used to preactivate bacterial nod genes or rhizobia prior to their use as inoculants. Soybean inoculated with preactivated B. japonicum increases nodule numbers and weights (about 30%), seasonal levels of N 2 fixation (35%), and yields (10–40%) when compared to conventional inoculants and when soil temperatures were low at seeding time. Inoculation of soybean with preactivated B. japonicum also accelerated root hair infection and nodule development and increased seasonal N 2 fixation and yield of soybean when soils were saline, acidic, or high in available mineral N. Inoculation of pea with preactivated R. leguminosarum almost doubled nodule number and nodule dry matter and enhanced the final grain yield by 10%.

The nitrogen fixation potential of arctic cryptogram species is influenced by enhanced UV-B radiation.

Abstract: Effects of enhanced UV-B (representing a 15% ozone depletion) on cyanobacterial nitrogen fixation were measured at a high arctic site (Adventdalen, 79°N, Svalbard) and a subarctic site (Abisko, 68°N, Sweden). Nitrogen fixation potential (acetylene reduction) by cyanobacteria associated with the moss Sanionia uncinata in vegetation exposed to experimentally enhanced levels of UV-B for 3 and 4 years in the high arctic in Adventdalen was reduced by 50% compared to controls after 3 years. No reduction in nitrogen fixation potential was observed in cyanobacteria associated with the moss Hylocomium splendens when previously exposed to enhanced UV-B in Abisko for a 7-year period. However, in the same experiment a 50% increase in summer precipitation stimulated nitrogen fixation potential by up to 6-fold above the natural precipitation treatments both in cyanobacteria associated with vegetation exposed to natural and enhanced UV-B radiation. In contrast to the lack of UV effect on moss-associated nitrogen fixation at the subarctic site, nitrogen fixation potential by the dominant lichen species Peltigera aphthosa was reduced by 50% when measured after 8 years exposure to elevated UV-B treatment. Evidence from these studies highlights the importance of UV-B radiation for cyanobacterial nitrogen fixation in the Arctic and future impact on nitrogen availability in such plant communities.

Ecological and Biogeochemical Interactions Constrain Planktonic Nitrogen Fixation in Estuaries

Abstract: Many types of ecosystems have little or no N2 fixation even when nitrogen (N) is strongly limiting to primary production. Estuaries generally fit this pattern. In contrast to lakes, where blooms of N2-fixing cyanobacteria are often sufficient to alleviate N deficits relative to phosphorus (P) availability, planktonic N2 fixation is unimportant in most N-limited estuaries. Heterocystic cyanobacteria capable of N2 fixation are seldom observed in estuaries where the salinity exceeds 8 ‐10 ppt, and blooms have never been reported in such estuaries in North America. However, we provided conditions in estuarine mesocosms (salinity over 27 ppt) that allowed heterocystic cyanobacteria to grow and fix N2 when zooplankton populations were kept low. Grazing by macrozooplankton at population densities encountered in estuaries strongly suppressed cyanobacterial populations and N2 fixation. The cyanobacteria grew more slowly than observed in fresh waters, at least in part due to the inhibitory effect of sulfate (SO4 2 ), and this slow rate of growth increased their vulnerability to grazing. We conclude that interactions between physiological (bottom‐up) factors that slow the growth rate of cyanobacteria and ecological (top‐ down) factors such as grazing are likely to be important regulators excluding planktonic N2 fixation from most Temperate Zone estuaries.

Nitrogen dynamics in an Alaskan salt marsh following spring use by geese

Abstract: Lesser snow geese (Anser caerulescens caerulescens) and Canada geese (Branta canadensis) use several salt marshes in Cook Inlet, Alaska, as stopover areas for brief periods during spring migration. We investigated the effects of geese on nitrogen cycling processes in Susitna Flats, one of the marshes. We compared net nitrogen mineralization, organic nitrogen pools and production in buried bags, nitrogen fixation by cyanobacteria, and soil and litter characteristics on grazed plots versus paired plots that had been exclosed from grazing for 3 years. Grazed areas had higher rates of net nitrogen mineralization in the spring and there was no effect of grazing on organic nitrogen availability. The increased mineralization rates in grazed plots could not be accounted for by alteration of litter quality, litter quantity, microclimate, or root biomass, which were not different between grazed and exclosed plots. In addition, fecal input was very slight in the year that we studied nitrogen cycling. We propose that trampling had two effects that could account for greater nitrogen availability in grazed areas: litter incorporation into soil, resulting in increased rates of decomposition and mineralization of litter material, and greater rates of nitrogen fixation by cyanobacteria on bare, trampled soils. A path analysis indicated that litter incorporation by trampling played a primary role in the nitrogen dynamics of the system, with nitrogen fixation secondary, and that fecal input was of little importance.

Effects of liming and legume/cereal cropping on populations of indigenous rhizobia in an acid Brazilian Oxisol

Abstract: Given the acid soil conditions in many regions of common bean production in the tropics and the deleterious effects of soil acidity on rhizobia, studies to assess survival of Phaseolus-nodulating rhizobial populations in acidic soils are important to ensure establishment of effective N2-fixing symbioses. The abundance of indigenous Phaseolus-nodulating rhizobia in a Brazilian acidic soil varied with the rate of lime applied more than 6 yr prior to sampling, with the season of sampling and with the previous cropping history and showed a significant interaction between these factors. The abundance of rhizobia was enhanced by both soil liming and by presence of common bean with populations ranging between less than 10 cells g−1 soil and more than 105 cells g−1 soil. In experiments cropped with upland rice, maize and a soyabean/wheat rotation differences between the sizes of rhizobial populations were highly significant where small amounts of lime had been added. Rhizobial populations were smallest in soils continuously cropped with rice compared with those cropped with maize and the soyabean/wheat rotation only when effective Al saturation was high (>20%). Conversely, the numbers of Leucaena-nodulating rhizobia in the soil samples from a field cropped with common bean were small and were not related to lime application. After incubation for 25 months rhizobia in an acidic soil sampled from the common bean field remained at around 90–94% of the initial population in limed soils containing 4.0, 7.0 and 11.0% effective Al saturation, whilst in soils with 36 and 27% effective Al saturation the abundance declined to 3–7% of that initially observed. An effective Al saturation of >20% is critical for survival of Phaseolus-nodulating rhizobia only in the absence of the host plant.

In vitro studies on the effects of herbicides on the growth of rhizobia

Abstract: Aims: To study the possible adverse effect of herbicides on nodulation and nitrogen fixation in legumes by affecting the nitrogen-fixing rhizobia. Methods and Results: Experiments were conducted to study the effect of four herbicides (terbutryn/terbuthylazine, trietazine/simazine, prometryn and bentazone) on the growth of nitrogen-fixing pea rhizobia (Rhizobium leguminosarum) in vitro by measuring optical density. Terbutryn/terbuthylazine, trietazine/simazine and prometryn had an adverse effect on the growth of rhizobia whereas bentazone was safe to rhizobia. Conclusions: The above herbicides could be used in pea at the recommended rates. Significance and Impact of the Study: The adverse effects of herbicides on rhizobia were observed at concentrations not normally expected to occur under field conditions. Further, previously observed adverse effects of these herbicides on nodulation and nitrogen fixation of peas were, possibly, not due to their effects in rhizobia but to their adverse effects on the plant growth itself.

The influence of long-term tillage systems on symbiotic N2 fixation of pea (Pisum sativum L.) and red clover (Trifolium pratense L.)

Abstract: Pea as a grain legume and red clover as a forage legume in the seeding year were cultivated in two long-term differentiated tillage systems on a loess soil in Germany. A continuous conventional tillage system (plow; CT) and a continuous minimum tillage system (rotary harrow; MT) were established in 1970. With pea and red clover dry matter accumulation and N parameters (N accumulation, Ndfa, N-harvest-index, N balance) were investigated in 1998 and 1999. Differences in the N2 fixation of pea due to the tillage system could clearly be shown whereas grain yields and total N accumulation were equal in both tillage systems and years. In both years a significantly (P < 0.05) higher Ndfa in the MT system was found at least in the final harvest (maturity of pea): 1998/1999, 0.42/0.54 in CT, 0.62/0.75 in MT. The differences in N2 fixation of pea may be explained by the delayed soil N supply in MT at the beginning of the vegetative period. Simplified N balances of pea were -18 and −25 kg N ha−1 in CT and −5 and +1 kg N ha−1 in MT for 1998 and 1999, respectively. Red clover showed no significant differences in the DM and N accumulation between both tillage systems but a year dependent effect caused by different stubble and root yields between the years was apparent. With red clover slightly, but also significantly (P < 0.05) increased Ndfa values were found in the MT system compared to the CT system with 0.55/0.62 in CT (1998/1999) and 0.64/0.71 in MT. However, the difference in Ndfa between the tillage systems (9 percentage points) was much smaller with red clover than with pea (20 and 21 percentage points in 1998 and 1999, respectively). Soil N uptake of red clover using the longer growing season reflected the more adjusted N supply in both long-term differentiated tillage systems, whereas pea in using only a short-term vegetative period reacted stronger to the lower N mineralization in the MT system in springtime.

Growth of Alfalfa in Sludge-Amended Soils and Inoculated with Rhizobia Produced in Sludge

Abstract: The efficiency of rhizobial inoculants produced in wastewater sludge used as a growth medium and as a carrier was compared with that of inoculants produced in yeast mannitol broth (YMB) medium and by using peat as a carrier. Alfalfa (Medicago sativa L.) plants were inoculated with solid and liquid Sinorhizobium meliloti inoculants and grown in pots containing two soil types (Kamouraska clay soil and Saint-Andre sandy soil). The effect of various levels of sludge amendment (60 and 120 kg N/ha) and nitrogen fertilizer (60 kg N/ ha) was also studied. The sludge-based inoculants showed the same symbiotic efficiency (nodulation and plant yield) as YMB-based inoculants. The inoculation increased the nodulation indexes from 4-6 to 8-12, and the rhizobial number from 10(3) (uninoculated soils) to 10(6)-10(7) cells/g in inoculated soils. However, the shoot dry weights and the nitrogen contents were not increased significantly by the inoculation. Applying sludge as an amendment enhanced the rhizobial number in soils from 10(3) to 10(4) cells/g and improved significantly the plant growth (shoot dry weights and nitrogen contents). This improvement increased with sludge rate and with the cut (three cuts). Compared with sludge, N fertilizer gave lower plant yields. The nodulation was not affected by sludge and N-fertilizer application. The texture and physico-chemical properties of soil were found to affect the yield and nitrogen content of the plants. In this study, macroelements and heavy metals were at acceptable levels and were not considered to be negative factors.

N2 fixation by Acacia species increases under elevated atmospheric CO2

Abstract: In the present study the effect of elevated CO 2 on growth and nitrogen fixation of seven Australian Acacia species was investigated. Two species from semi-arid environments in central Australia ( Acacia aneura and A. tetragonophylla ) and five species from temperate south-eastern Australia ( Acacia irrorata , A. mearnsii, A. dealbata , A. implexa and A. melanoxylon ) were grown for up to 148 d in controlled greenhouse conditions at either ambient (350 µ mol mol − 1 ) or elevated (700 µ µ µ mol mol − 1 ) CO 2 concentrations. After establishment of nodules, the plants were completely dependent on symbiotic nitrogen fixation. Six out of seven species had greater relative growth rates and lower whole plant nitrogen concentrations under elevated versus normal CO 2 . Enhanced growth resulted in an increase in the amount of nitrogen fixed symbiotically for five of the species. In general, this was the consequence of lower whole-plant nitrogen concentrations, which equate to a larger plant and greater nodule mass for a given amount of nitrogen. Since the average amount of nitrogen fixed per unit nodule mass was unaltered by atmospheric CO 2 , more nitrogen could be fixed for a given amount of plant nitrogen. For three of the species, elevated CO 2 increased the rate of nitrogen fixation per unit nodule mass and time, but this was completely offset by a reduction in nodule mass per unit plant mass.

Competition between inoculant and naturalised Rhizobium leguminosarum bv. trifolii for nodulation of annual clovers in alkaline soils

Abstract: Inoculant rhizobia typically need to compete with naturalised soil populations of rhizobia to form legume nodules. We have used the polymerase chain reaction to test the ability of seed-inoculated rhizobia to compete with naturalised populations of rhizobia and form nodules on clover (Trifolium alexandrinum, T. purpureum, and T. resupinatum) in alkaline soil. Clover rhizobia, Rhizobium leguminosarum bv. trifolii, were identified at the strain level using either a nif-specific RP01 primer or ERIC primers. Analysis of rhizobia isolated from nodules indicated that strain TA1 competed poorly for nodule occupancy at 2 field sites (Roseworthy and Mallala, South Australia), with the exception that it nodulated T. alexandrinum at a level of 39% at the Roseworthy site in the first year of the trial. Strains CC2483g and WSM409 successfully colonised nodules when inoculated onto a particular clover species (T. resupinatum and T. purpureum, respectively) in the first year of inoculation and persisted in the soil to form nodules in the following year. Nodules frequently contained naturalised strains of rhizobia, distinct from introduced commercial strains. Dominant isolates were specific to a field site and nodulated all 3 clover species in both years of the field trial, with each isolate occupying up to 19% of the total nodules at a field site. It was hypothesised that field isolates had a better alkaline soil tolerance conferring a greater ability to nodulate clovers under these edaphic conditions. The results indicate that soil populations of rhizobia may provide a significant constraint to the introduction of current Australian commercial clover rhizobia into alkaline soils, and a more profitable strategy may be to seek rhizobial inoculants that are adapted to these soils.

Molecular Differentiation of the Heterocystous Cyanobacteria, Nostoc and Anabaena, Based on Complete NifD Sequences

Abstract: The segregation of Nostoc and Anabaena into separate genera has been debated for some time. The nitrogen fixation gene nifD was completely sequenced from representatives of these genera and analyzed phylogenetically, by using the representatives of other genera of the heterocystous cyanobacteria as outgroups. We were clearly able to differentiate between Nostoc and Anabaena in all analyses used. Our data suggest that Nostoc and Anabaena should remain as separate genera.

Secondary aerenchyma formation and its relation to nitrogen fixation in root nodules of soybean plants (Glycine max) grown under flooded conditions

Abstract: Soybean (Glycine max (L.) Merr.) is considered to be susceptible to flooding, a major agronomic problem in the world, and nitrogenase activity rapidly declines due to oxygen deficiency in root nodu...