A direct observation technique for studies on rhizoplane and rhizosphere colonization

Summary A slide incubation chamber was described which allowed small plants to be grown from seed and the root systems to be observed microscopically. A fluorescence stain, the ammonium salt of 8-anilino-1-naphthalene sulfonic acid, was applied to the soil in which the roots were growing and the stained microorganisms on the roots and in the rhizosphere were counted. A statistical pattern analysis technique, the two-within-four randomization test, was used to analyze the data obtained from quadrats on the roots. Distinct colonization patterns and colony growth, especially of bacteria, were easily distinguished with the technique.     

Water uptake by barley roots as affected by the osmotic and matric potential in the rhizosphere

Summary The water uptake rates of roots in saline soils are depressed by the simultaneously decreasing matric and osmotic water potentials in the soil surrounding the roots (rhizospheric soil). Unfortunately there are no reliable tools available for direct measurements of the effect of decreasing water potentials in the rhizospheric soil on the uptake rate of soil water by roots. This paper presents some results of a vegetation technique for studying the effect of different combinations of osmotic and matric water potentials in the rhizospheric soil on the water uptake rates of barley roots. Water uptake rates were reduced to a greater extent by decreasing soil matric water potentials than by decreasing soil osmotic water potentials. According to the results of this experiment, there was no relationship between the total soil water potential of a sandy soil and the water uptake rates when the roots were exposed to different combinations of and .     

Growth of bacteria in the rhizoplane and the rhizosphere of rape seedlings

Abstract The growth of 10 isolates of rhizosphere bacteria was compared in the rhizoplane (RP), rhizosphere (RS) and non-rhizosphere soil of a model system with rape seedlings growing in sterile sand. The colonization of the RP differed little among isolates. However, the bacterial isolates differed according to their degree of dependence on the root for growth, as judged by RS:RP and plant:non-plant ratios for CFU. These two ratios were correlated with changes in viability and 'physiological status' (as judged by γ values, Hattori, T. (1983) J. Gen. Appl. Microbiol. 29, 9–16).     

Nitrogen fixation by Methanobacterium formicicum

Abstract: Methanobacterium formicicum utilized molecular nitrogen as the sole nitrogen source for growth as monitored by methane production and culture turbidity measurements. The rate of methane production by the bacteria was correlated to nitrogen gas concentrations. In the absence of nitrogen gas or any other nitrogen source, the bacteria completely stopped growing. The presence of selenium and molybdenum in the culture medium was vital for the growth of the bacteria under nitrogen fixing conditions.     

Effect of inoculation ofAzospirillum lipoferum on nitrogen fixation in rhizosphere soil,their association with root,yield and nitrogen uptake by mustard (Brassica juncea)

Summary A microplot field experiment was conducted in the presence or absence of P and N application to evaluate the influence of the seed inoculation of mustard (cv. Baruna T₅₉) withAzospirillum lipoferum on N₂-fixation in rhizosphere, association of the bacteria with the roots and grain yield and N uptake. Inoculation significantly increased the N content in rhizosphere soil particularly at early stage (40 days) of plant growth, which was accompanied by the increased association of the bacteria (A. lipoferum) in rhizosphere soil, root surface washing and surface-sterilized macerated root. A significant increase in grain yield and N uptake was also observed due to inoculation. Application of P particularly at the 20 kg. ha^–1 level further enhanced the beneficial effect ofAzospirillum lipoferum inoculation, while N addition markedly reduced such an effect.     

Nitrogen fixation and nitrogen metabolism in heliobacteria

Three species of anoxygenic phototrophic heliobacteria, Heliobacterium chlorum, Heliobacterium gestii, and Heliobacillus mobilis, were studied for comparative nitrogen-fixing abilities and regulation of nitrogenase. Significant nitrogenase activity (acetylene reduction) was detected in all species grown photoheterotrophically on N₂, although cells of H. mobilis consistently had higher nitrogenase activity than did cells of either H. chlorum or H. gestii. Nitrogen-fixing cultures of all three species of heliobacteria were subject to switch-off of nitrogenase activity by ammonia; glutamine also served to switch-off nitrogenase activity but only in cells of H. mobilis and H. gestii. Placing photosynthetically grown heliobacterial cultures in darkness also served to switch-off nitrogenase activity. Dark-mediated switch-off was complete in lactate-grown heliobacteria but in pyruvate-grown cells substantial rates of nitrogenase activity continued in darkness. In all heliobacteria examined ammonia was assimilated primarily through the glutamine synthetase/glutamate synthase (GS/GOGAT) pathway although significant levels of alanine dehydrogenase were present in extracts of cells of H. gestii, but not in the other species. The results suggest that heliobacteria, like phototrophic purple bacteria, are active N₂-fixing bacteria and that despite their gram-positive phylogenetic roots, heliobacteria retain the capacity to control nitrogenase activity by a switch-off type of mechanism. Because of their ability to fix N₂ both photosynthetically and in darkness, it is possible that heliobacteria are significant contributors of fixed nitrogen in their paddy soil habitat.     

Predicted microbial biomass in the rhizosphere of barley in the field

Summary Laboratory data for the loss of root material by barley and field data for the growth of barley plants in Syria and in England have been combined to predict the amount of material lost by barley roots during a season, and to predict the resulting microbial biomass in the rhizosphere. The predicted microbial biomass C in the rhizosphere ranged from 10–34% of the total plant biomass C depending mainly upon the value used for rate of loss of root material. Total loss of root material predicted during a season in England constituted 7.7–25.4 percent of C fixed by photosynthesis. The major assumptions made in these calculations are considered, and the predicted values discussed in relation to reported values for soil microbial biomass, CO₂ fluxes from soil and associative nitrogen fixation.     

Nitrogen fixation in hardwood forests of the northeastern United States

Summary Nitrogen-fixing activity in hardwood forests of the northeastern United States occurred in wood litter, greater than 2 cm in diameter. Activity in large dead wood was independent of species, in the case of deciduous wood litter, but was restricted to partially decayed wood with a high moisture content. Maximum rates of activity were observed in the summer months, minimum rates in the winter. Evidence from six stands of varying ages showed that fixation in large wood litter occurred in only 25% of the samples assayed.Fixation was highest in the youngest, 4 years, and oldest, over 200 years, stands; being about 2 kg/ha/yr. The quantity of nitrogen fixed appears to be related to the biomass of dead wood. Large amounts of wood litter in the youngest stands were from slash left after cutting. As the supply of slash is exhausted by decay, nitrogen fixation decreases, with a low around year 20. Fixation then gradually increases as natural thinning adds wood to the litter compartment.Apparently, the amount of nitrogen fixed in dead wood the first 20 years following clearcutting can only replace a modest fraction of the amount lost as a result of the cutting and product removal. Finally, the results indicate that nitrogen fixation in wood litter does not equal nitrogen fixation in a northern hardwood forest calculated using a mass balance approach, suggesting that additional nitrogen inputs exist.     

Nitrogen fixation in lake Mendota, 1971–1973

The contribution of nitrogen fixation to the nitrogen budget of Lake Mendota has been calculated. On average, the equivalent of 1.28 × 10⁵ kg of NH₃ (as determined by the acetylene reduction technique) was added to this eutrophic lake during June, July and August. Diurnal variation (approximately two-thirds of the day's fixation occurs prior to noon) in algal nitrogen fixation, and variation of fixation with depth (3.6% of the fixation in the column occurs in the top decimeter) were characterized as prerequisites to this calculation.     

Nitrogen metabolism of young barley plants as affected by NaCl-salinity and potassium

Summary In a solution culture experiment with 31 days old barley plants (var. Miura) the influence of NaCl-salinization (80 mM) and KCl addition (5 and 10 mM) on the uptake and turnover of labelled nitrogen (¹⁵NH₄ ¹⁵NO₃) was studied. Labelled N was applied for 24 h at the end of a 20 days' salinization period. Salinization impaired growth and incorporation of labelled N into the protein fraction paralleled by accumulation of labelle dinorganic N. All salt effects were much more pronounced in the shoots than in the roots.Potassium addition enhanced N uptake (total¹⁵N-content) and incorporation into protien, reduced the accumulation of inorganic N and improved the growth of salinized plants.The presented data support the point of view that impairment of protein (enzyme) metabolism is an important aspect of salt stress which is probably induced by the disturbance of the K/Na balance of the tissues under saline conditions.This work was supported by a grant from the Alexander von Humboldt foundation.     

Nitrogen fixation by three species of leguminosae in the Canadian High Arctic Tundra

Abstract. Significant levels of nitrogenase activity (nitrogen fixation) were demonstrated in three species of Arctic legumes ( Oxytropis maydelliana, O. arctobia and Astragalus alpinus ) growing in high tundra at Sarcpa Lake, Melville Peninsula, N.W.T. Nitrogenase activity of intact plants was correlated with the number of nodules per plant, with field soil temperatures and limited by water shortage. Activity in freshly detached nodules showed a plateau of maximum activity between 10°C and 25°C and a near linear decline with temperature down to 0°C. Unusually, the segmented nodules of all three species are perennial in which growth and leghaemoglobin production resumes each spring from an overwintering apical meristem. Nodules are most numerous in the warmer soil stratum (2–10 cm. depth). Other studies indicate that the arctic rhizobia belong to a single cold-adapted species which has co-evolved with the legumes of tundra.     

Mobilisation of Cu,Mn and Zn in the soil solutions of barley rhizospheres

Summary We report data showing changes during the growing season in the concentrations of manganese, zinc and copper in the rhizosphere in soil solutions of barley plants. Substantial mobilisation of these trace elements occurred during the early stages of rhizosphere development so that soil solution concentrations increased three fold for the copper and zinc and fifteen fold for manganese.     

Nitrogen fixation by the benthic freshwater cyanobacterium Lyngbya wollei

The ability of the benthic cyanobacterium Lyngbya wollei to fix nitrogen was studied using field samples and axenic cultures. L. wollei was collected and isolated from Lake Okeechobee, Florida, where it forms extensive mats. Rates of acetylene reduction up to 39.1 nmol mg dry wt⁻¹ h⁻¹ were observed for field samples. The maximum observed rate of acetylene reduction in axenic laboratory cultures was 200 nmol mg dry wt⁻¹ h⁻¹. Aerobic conditions limited nitrogen fixation activity, but dark/light cycles promoted the development of activity. Reduced oxygen levels appeared to be required for the development of significant levels of nitrogenase activity. The level of irradiance also had a significant impact on the level of activity. The potential significance of nitrogen fixation to Lyngbya production is discussed.     

The effect of nitrate-nitrogen supply on bacteria and bacterial-feeding fauna in the rhizosphere of different grass species

Summary Microbial growth in the rhizosphere is affected by the release of organic material from roots, so differences in carbon budgets between plants may affect their rhizosphere biology. This was tested by sampling populations of bacteria and bacteriophagous fauna from the rhizosphere of Lolium perenne, Festuca arundinacea, Poa annua, and Poa pratensis, under conditions of high and low nitrate availability. Concentrations of soluble phenolics and lignin varied considerably between the species but were not related to differences in rhizosphere biology. L. perenne and F. arundinacea supported fewer bacteria than the Poa species. There was no significant rhizosphere effect on the groups of protozoa. The major indicators of rhizosphere productivity were the bacterial-feeding nematodes (mainly Acrobeloides spp.), and there was a large positive effect of added nitrate. Nematode biomass was significantly lower in the rhizosphere of the slow-growing P. pratensis compared with the fast-growing P. annua, indicating that the differential allocation of carbon has affects on rhizosphere biology. A large rhizosphere effect on enchytraeid worms was also observed, and their potential importance in the rhizosphere is discussed.     

Nitrogen fixation varies spatially and seasonally in linked stream-lake ecosystems

We performed surveys of nitrogen (N₂)-fixation in three oligotrophic lake-stream systems in the Sawtooth Mountains of central Idaho to address two questions: (1) Which habitat types within linked lake-stream systems (lake pelagic, lake benthic, and stream) exhibit the highest rates of N₂ fixation?, and (2) How does N₂ fixation compare to the hydrologic flux of nitrogen? A seasonal survey showed that N₂ fixation in a single lake and its outlet stream peaked in late summer, when hydrologic N fluxes were lowest. Benthic lake N₂-fixation rates by epiphytes were highest at mid-lake depths, where their percent cover was highest, while rates by epipelon were greatest at shallow lake depths. Pelagic N₂ fixation was below detection. Stream N₂-fixation rates were greatest on rock substrates and in the lake outlet stream. These patterns were supported by a baseflow survey (late July) in three lake-stream ecosystems which confirmed that N₂-fixation rates peaked in the lake benthos at shallow depths and on rock substrates in outlet streams. Scaling N₂-fixation rates to whole lake and stream areas revealed that N₂ fixation could exceed the nitrate, and sometimes the total dissolved nitrogen flux during baseflow in lakes and outlet streams. Despite low rates, total N₂-fixation contributions (kg/day) from lakes were greater because they had far larger surface areas than the stream environments. Fixed nitrogen contributions from stream outlets were also relatively high because of high N₂-fixation rates and despite low surface areas. This study suggests that N₂ fixation could be a seasonally important nitrogen source to nutrient deficient subalpine lake-stream ecosystems. In addition, the frequency and location of lakes could control N₂-fixation contributions to watersheds by providing a large area for within-lake N₂ fixation, and creating conditions favorable for N₂ fixation in outlet streams.     

Nitrogen fixation (C2H2 reduction) in soil samples from rhizosphere of rice grown under alternate flooded and nonflooded conditions

Summary In a greenhouse study the influence of alternate flooded and nonflooded conditions on the N₂-ase activity of rice rhizosphere soil was investigated by C₂H₂ reduction assay. The soil fraction attached to roots represent the rhizosphere soil. Soil submergence always accelerated N₂-ase and this effect was more pronounced in planted system. Moreover, rice plant exhibited phase-dependent N₂-ase with a maximum activity at 60 days after transplanting. The alternate flooded and nonflooded regimes resulted in alterations of the N₂-ase activity. Thus, the N₂-ase activity increased following a shift from nonflooded to flooded conditions, but the activity decreased when the flooded soil was returned to nonflooded condition by draining. However, the differential influence of the water regime on N₂-ase was not marked in prolonged flooded-nonflooded cycles. Microbial analysis indicated the stimulation of different groups of free-living and associative N₂-fixing microorganisms depending on the water regime.     

疏叶骆驼刺与花花柴互作对氮素固定和根际微生物的影响

植物种间的相互关系通过相关微生物直接或间接的影响来实现。豆科与非豆科植物的互作是研究植物种间关系的理想模型,但是对其互作关系中氮素固定和微生态过程尚不明确。以塔南荒漠优势植物疏叶骆驼刺(Alhagi sparsifolia Shap.)（豆科）和花花柴(Karelini acaspia(Pall.) Less)（菊科）为研究对象,研究了在不同生境（自然和小区）下两者互作对氮素固定和根际微生物的影响。结果表明,在自然和小区两种生存环境下疏叶骆驼刺与花花柴都有氮素转移特征,并且这种转移特征在自然生境下更为明显。在自然生境中从疏叶骆驼刺转移到花花柴的氮素占花花柴总氮的50%左右,而在小区生境中只占30%左右。互作改变了花花柴各组织的化学计量比,在互作条件下花花柴叶片氮素含量比重增加。此外,疏叶骆驼刺与花花柴的互作降低了前者的根际细菌群落的Shannon index,并且改变了其根际土壤细菌的基因功能。互作对花花柴根际微生物群落没有显著影响,但在互作条件下疏叶骆驼刺根际土壤细菌中参与氮素转运的相关基因丰度显著高于单独种植,其中对细根根际土壤细菌的多样性及其基因丰度影响最大。且互作降低了疏叶骆驼刺细根的氮含量。因此,疏叶骆驼刺细根可能是疏叶骆驼刺和花花柴互作的关键部位。本研究为荒漠植被保护与恢复提供了科学依据。     

Involvement of CO2 fixation products in the light-dark modulation of nitrate reductase activity in barley leaves

Nitrate reductase (NR, NADH:nitrate oxidoreductase, EC 1.6.6.1) activity from leaves of barley (Hordeum vulgare L. cv. Hassan) is rapidly and reversibly inactivated during a light-dark transition. A hyperbolic correlation exists between in vivo rates of CO₂ fixation and extractable NR activity from the leaves, and feeding hexose and hexosephosphate protects against the dark-inactivation; indicating that carbon-assimilation products are regulatory factors of NR activity mediating both the light-dark modulation and its dependence upon CO₂ fixation. To corroborate this point, the effect of inhibiting CO₂ fixation on NR activity in barley leaves has been analyzed. Glycolaldehyde (50 mM), an inhibitor of the regeneration phase of the Calvin cycle, was fed through the transpiration stream and inhibited CO₂ fixation by more than 80% at the same time as it produced a parallel inhibition of NR light-activation. Feeding mannose (10 mM), inhibited CO₂ fixation by 35% but did not affect NR activity in illuminated leaves and completely protected against dark-inactivation. Interestingly, feeding inorganic phosphate, P_i, (10 mM) alone or together with mannose also protected NR activity against dark-inactivation. The mannose effect could be interpreted in terms of accumulation of mannose 6-phosphate, an analog of glucose 6-phosphate. After feeding either 10 mM glucose or dihydroxyacetone phosphate, NR activity from darkened leaves was significantly higher than that of darkened control leaves fed with water (P< 0.03). These treatments, as well as P_i feeding, also produce some increase in extractable NR activity from illuminated leaves. The results indicate that factors increasing the levels of hexose- and triose-phosphate have positive effects on NR activation, supporting the contention that the NR activation system is sensitive to carbon-assimilation products.     

Nitrogen fixation and primary production in Western Mediterranean

Nitrogen fixation, nitrate assimilation and primary production ((13)C/(15)N method) were investigated during one year and half in the northwestern Mediterranean Sea. Nitrogen fixation was detectable all over the year with rates ranged from 2 to 17 nmol N l(-1) d(-1)(d). Highest values being obtained during spring associated with the phytoplankton bloom. High rates (4-8 nmol N l(-1) d(-1)(d)) were also measured during summer, when primary productivity was very low. Then, diazotrophy process supplies significant new nitrogen during summer oligotrophic periods. This new nitrogen input can balance the annual nitrogen biogeochemical budget in the Mediterranean Sea and should explain the high nitrate/phosphate ratio observed in deep waters.     

Nitrogen fixation in sediments and the rhizosphere of the seagrassZostera capricorni

Rates of nitrogen fixation in seagrass beds (Zostera capricorni) were determined with¹⁵N and reduction of acetylene in intact cores of sediment and seagrass. There was good agreement in the results from the two techniques, with a molar ratio of 3∶1.9 ethylene: ammonia produced. Fixed nitrogen was rapidly utilized by the plants, with significant amounts of¹⁵N found in the roots and rhizomes and 50% of fixed¹⁵N apparently translocated to the leaves. Rates of fixation were high in summer (25 to 40 mg N m⁻² day⁻¹) and lower in winter (10 mg N m⁻² day⁻¹) and were estimated to supply between one-third and one-half of the nitrogen requirements of the seagrass. Rates of nitrogen fixation were greater in the light than in the dark, and in cores of intact seagrass than in defoliated cores, indicating that the bacteria were dependent on organic compounds secreted by the plants.