生物成因次生铁矿物对酸性矿山废水中三价砷的吸附

在常温常压条件下,利用嗜酸性氧化亚铁硫杆菌的促进作用,生物合成了酸性矿山废水中常见的三种次生铁矿物:施氏矿物(schwertmannite)、黄钾铁矾(jarosite)和针铁矿(goethite)。在15℃、25℃、35℃三个不同温度下,通过序批式吸附试验研究,在pH3.0的模拟酸性矿山废水条件下,3种生物成因次生铁矿物对As(Ⅲ)的吸附性能。结果表明:3种生物成因的次生矿物对As(Ⅲ)的吸附属于拟二级反应,可以用Lagergren拟二级速率方程进行拟合,相关系数均在0.97以上。吸附速率常数K分别为施氏矿物0.094 g mg-1min-1,针铁矿0.042 g mg-1min-1,黄钾铁矾0.02 g mg-1min-1。3种生物成因铁矿物对As(Ⅲ)的吸附等温线符合Langmuir方程和Freundlich方程,相关系数均在0.94以上。吸附反应的焓变ΔH分别为11.76、18.40和9.34 kJ mol-1,ΔG均小于0,吸附过程属于吸热的自发反应。     

Trimethylsilylation of commonly occurring primary and secondary minerals in soils

Soil is formed through the weathering of minerals—mainly silicates—and the manner in which these materials weather is related to some extent to the arrangement of silica units within their structures. This has been termed the degree of polymerizafion of the silica and can be quantitatively assessed by preparing organic (trimethylsilyl) derivatives which can be separated and identified. This procedure has been applied to a number of primary and secondary (including clay and highly disordered) minerals occurring in soils and has indicated that: (a) of the primary minerals, the ferro-magnesian are the most susceptible to acid attack; (b) inorganic gels, particularly those high in alumina, are reactive; and (c) crystalline clay minerals, especially kaolinite and illite, are relatively stable.     

Characterization of Andosols in Yakushima Island

We examined soils derived from volcanic ash of Kikai-Akahoya tephra on Yakushima Island, Japan, and classified them according to the Unified Soil Classification System of Japan, 2nd Approximation (USCSJ 2nd) and the World Reference Base for Soil Resources (WRB). Five pedons with horizons showing high (>20%) volcanic glass content were investigated. Soils developed under evergreen broad-leaved forests had high acid oxalate-extractable aluminum (Alo) and acid oxalate-extractable silicon (Sio) concentrations, and low acid oxalate-extractable iron (Fe)/dithionite-citrate-extractable Fe ratio. This indicates a warmer climate and less severe leaching conditions compared with soils developed under coniferous forests dominated by Cryptomeria japonica and grasslands dominated by Pseudosasa owatarii. All soils contained considerable amount of hydroxyl-Al-interlayered 2:1 clay minerals. The surface horizons of the pedons developed under the cool-temperate C. japonica forests contained smectite as a result of podzolization. However, the surface horizon of the pedon developed under cool-temperate P. owatarii grasslands did not contain smectite. All pedons belonged to the Kuroboku soils great group (USCSJ 2nd) and Andosols (WRB). Pedons in mountainous areas did not contain horizons with more than 6?g?kg^?1 of Sio and hence were classified as non-allophanic Andosols. In mountainous areas, it was observed that allophane formation was inhibited by Al leaching due to intense rainfall (>10,000?mm year^?1); Al consumption due to the formation of the Al-humus complex; and Al incorporation into the interlayers of vermiculite. The low soil water pH [pH(H₂O)] and leaching of silicon (Si) in mountainous areas would support these anti-allophanic effects.     

Efficiency of gypsum application to acid Andosols estimated using aluminum release rates and plant root growth

Abstract

A great deal of information on the efficiency of gypsum or phosphogypsum to ameliorate acidity in highly weathered soils is available, but only limited information is available on the efficiency in acid Andosols, which possess large amounts of active aluminum (Al). We examined the effectiveness of gypsum application to non-allophanic Andosols (one humus-rich A horizon and two B horizons poor in humus) using extractable soil Al analyses (batch and continuous extraction methods) and a cultivation test using burdock (Arctium lappa). With gypsum amendment, pH(H₂O) values of the soil decreased from 4.5–4.7 to 4.2–4.4, whereas the treatment made almost no difference to the values of pH(KCl). Total active Al (acid oxalate-extractable Al) was hardly affected by gypsum for all samples. Potassium chloride-extractable Al definitely decreased with the addition of gypsum in all soils; however, the decrease was small (0.1–1.4 cmol_c kg^?1) and the values still exceeded “the threshold of 2 cmol_c kg^?1” for inducing Al toxicity in sensitive plants (4.4–8.6 cmol_c Al kg^?1). The change in Al solubility with gypsum application represented by Al release rates from soils using continuous extraction methods with a dilute acetate buffer solution (10^?3 mol L^?1, pH 3.5) differed greatly among the soil samples: The release rate of one of the B horizon samples decreased by 71%, certainly showing the insolubilization of Al compounds, whereas the release rates of the A horizon sample showed almost no change. These changes in Al solubility were well correlated with the plant root growth. Root growth was improved with gypsum in the B horizon sample, whereas improvement was not observed in the A horizon soil. The decrease in the rate of Al release of another B horizon soil with gypsum treatment was smaller (by 20–34%), possibly because of lower pH values after gypsum application (pH[H₂O] of 4.2–4.3). In the B horizon soil, root growth improved only slightly. Thus, the effectiveness of gypsum application to acid Andosols appeared to be largely influenced by soil humus contents and slight differences in soil pH values, and corresponded to a decrease in Al release rates using the continuous extraction method.     

Structural stability and prebiotic properties of resistant starch type 3 increase bile acid turnover and lower secondary bile acid formation

Microbial metabolism is essential in maintaining a healthy mucosa in the large bowel, preferentially through butyrate specific mechanisms. This system depends on starch supply. Two structurally different resistant starches type 3 (RS3) have been investigated with respect to their resistance to digestion, fermentability, and their effects on the composition and turnover of bile acids in rats. RSA (a mixture of retrograded maltodextrins and branched high molecular weight polymers), which is more resistant than RSB (a retrograded potato starch), increased the rate of fermentation accompanied by a decrease of pH in cecum, colon, and feces. Because they were bound to RS3, less bile acids were reabsorbed, resulting in a higher turnover through the large bowel. Because of the rise of volume, the bile acid level was unchanged and the formation of secondary bile acids was partly suppressed. The results proved a strong relation between RS3, short chain fatty acid production, and microflora. However, butyrate specific benefits are only achieved by an intake of RS3 that result in good fermentation properties, which depend on the kind of the resistant starch structures.     

Clay minerals and base status of strongly acid soils

In Japan, as the climate is temperate and humid, almost all soils show acid reaction. Especially, a good many strongly acid soils were reported in the survey of reclaimed lands (1, 2). The great majority of strongly acid soils are mineral soils derived from Tertiary or Diluvial sediments.     

Aggregates stability and water erosion in Andosols of the Canary Islands

The aim of this paper is to assess the mechanisms of water erosion in andic soils using two tests, which in a certain way simulate the two principal mechanisms of aggregate destruction in the process of water erosion—water dispersion and raindrops impact—and compare them with the aggregation observed in material dettached by inter‐rill erosion (sediments) in experimental plots with natural rain. In accordance with the obtained results, the erosive process in these soils seems to come about through a picking off of surface material of larger aggregates, due to the impact of raindrops. The intensity of pull off and fragment size from larger aggregates depends on the kinetic energy of the drops (rain intensity), but the size generally ranges between 0ċ2 and 0ċ5 mm. Therefore inter‐rill erosion initially proceeds by a washing down of smaller aggregates (<0ċ5 mm) (of less bulk density than larger aggregates 0ċ4 Mg m⁻³ against 0ċ9 Mg m⁻³), enriching the soil in larger sized aggregates which, on being fragmented by picking off of raindrops, supply new material for washing down by inter‐rill erosion. Copyright © 2002 John Wiley & Sons, Ltd.     

Characterization of the perylenequinone pigments in Japanese Andosols and Cambisol

The green fraction of humic acid (Pg) and the chloroform-extractable green fraction (CEGF) are characteristic soil organic matter (SOM) components. These alkaline solutions are green-colored due to the presence of 4,9-dihydroxyperylene-3,10-quinone (DHPQ) chromophore. While both of which are potential indicators for the effect of land use and paleoclimatic environment in the fields of soil science as well as geochemistry, CEGF as well as its relationship with Pg in soils are not yet fully documented. In this study, we firstly investigated the chemical properties of soil CEGF fractions by ultraviolet–visible (UV–Vis) and infrared (IR) method. Two CEGF components were separated by sequential liquid-liquid extraction using aqueous ammonium hydroxide (NH₄OH) followed by aqueous sodium hydroxide (NaOH). Results showed that the UV–Vis spectral shape of NH₄OH-extractable component is very similar to that of DHPQ, except that it is red-shifted. The solubility and UV–Vis spectrum of the NaOH-extractable fraction were completely identical with those of synthesized DHPQ. Their IR spectral shapes were also almost the same. Subsequently, the distribution of CEGF in humic acid (HA), fulvic acid (FA) and humin (HN) from Japanese Andosols and Cambisol was quantitatively evaluated by sequential extraction. Most of CEGF was detected in the HA (60–78%) and HN (22–40%), but not in the FA. While the UV–Vis spectral shape of CEGF extracted from Andosols HAs showed a relatively higher proportion of DHPQ than its derivative, the opposite was observed in Cambisol HA, whose CEGF is similar to that of sclerotium grain (one of the possible origin of CEGF). These results suggest the diversity of CEGF-producing soil fungi. Quantitative data also indicated that 35–49% of Pg consisted of a chloroform-soluble fraction (i.e., CEGF) and the remaining 51–65% of Pg was chloroform-insoluble. Based on these results, we propose that CEGF is composed of DHPQ and DHPQ-derivatives and that CEGF is one of the major fractions of Pg.     

Quantitative determination of minerals in acid forest soils of granite

A procedure for quantifying minerals in particle size fractions of fine earth (< 2 mm) is presented. Mineral fractions of the acid forest soils are characterised by various transition products. Transitional minerals between illite and vermiculite or smectite, polyhydroxy-Al polymers interlayered in smectite and vermiculite, as well as poorly crystalline compounds are present. Normative calculations with standard minerals, generally applied to rocks, cannot be used to quantify precisely minerals of these soil samples. Conventional qualitative and quantitative chemical analyses are therefore completed by microanalyses of well-identifiable minerals. The calculation procedure is based mainly on a system of simultaneous linear equations. These are set up from the chemical contents of a particle size fraction on the one hand and from the corresponding element contents of single minerals on the other hand. The contribution of single minerals to overall parameters can be calculated by programmed algorithms. The methodology was adjusted to different particle size fractions.     

Organic carbon stocks and soil erodibility in Canary Islands Andosols

Soil organic carbon (SOC) plays a key role in the structural stability of soils and in their resistance against erosion. However, and as far as andic soils are concerned, these mechanisms and processes, as well as the influence of the different types of SOC on aggregate stability, are not fully understood. The targets of this paper are: (i) to determine the content and forms of SOC in Andosols under evergreen forest vegetation [laurel (Laurus) and heather (Erica) forest] and (ii) to find out the role of soil organic matter (SOM) in the aggregate stability and in the resistance of Andosols to water erosion. Soil samples have been collected in 80 sites in a 40 km² area under udic soil moisture regime. In them, fulvic and humic acids, Walkley–Black SOC, pyrophosphate-extractable SOC, Fe and Al, potassium sulphate extractable SOC, dissolved SOC, acid oxalate-extractable Fe, Al and Si, USLE K-factor and aggregate stability have been determined. The Andosols over volcanic ash are Aluandic Andosols (non-allophanic Andosols), whereas over basaltic lava flows are Silandic Andosols (allophanic Andosols). The surface (0–30 cm) samples analyzed contain 9.5–30 kg C m^− 2 being significantly higher in allophanic Andosols (p < 0.5). Organic carbon adsorbed onto the mineral fraction (extractable pyrophosphate, C_p) accounts for 35–55% of the total SOC. All samples show a high stability to slaking and raindrop impact, being the first one highly correlated (r = 0.6) with pyrophosphate extractable C (C_p), Fe (Fe_p), and Al (Al_p) in allophanic Andosols, unlike non-allophanic ones. The stability to raindrop impact correlates with pyrophosphate extractable C (C_p) and Fe (Fe_p) in both types of soils (r = 0.3–0.6, p < 0.05). These findings suggest that the high stability to both slaking and water-drop impact is due to the occurrence of allophane–Fe–OC complexes, rather than to the total OC, and the active Fe and Al forms, generated by the weathering of volcanic materials, constitute an essential constituent responsible for C sequestration and resistance to degradation in these soils.     

Occurrence of laminar opaline silica in some Oregon Andosols

Laminar opaline silica was first found in the 0.2 to 5 μ fraction and most abundant in the 0.4 to 2 μ fractions of young Japanese Andosols by Shoji and Masui (1969a, b). It was noted that the A horizon of a profile tends to be relatively rich in opaline silica whereas the B or C horizon, in allophane (Shoji and Masui, 1972a, b). They (I972a) distinguished four types of opaline silica particles such as circular, elliptical, rectangular, and rhombic, of which the circular and elliptical types predominate. It has been suggested that the formation of opaline silica is favored by a plentiful supply of soluble silica in the early weathering stage of Andosols, the supersaturation of silica by surface evaporation of soil solution, and the suppression of aluminum activity in the soil solution by the accumulation of soil organic matter (Shoji and Masui, 1972b; Wada and Harward, 1974). The purpose of the present short communication is to describe the occurrence of laminar opaline silica particles in some Oregon Andosols, U.S.A.     

Extraction of organic phosphorus in Andosols by various methods

The amounts of inorganic (P_i) and organic (P_o) phosphorus extracted by the Truog, Bray II, Olsen, and Citrate methods, in upland soils consisting of three Andosols and a Yellow soil, with different levels of fertilization were determined. In the soils subjected to the application of both chemical fertilizers and organic matter, the amount of Po extracted by the respective extraction methods was much smaller than that of P_i. Especially, in the Andosol with yearly addition of manure for 28 y, the amount of extractable Po did not increase with the increase of the amount of manure applied even though the contents of total carbon, total phosphorus, and extractable P_i linearly increased. In the uncultivated high humic Andosols, on the other hand, P_o was the dominant form extracted by the Olsen and Citrate methods. In order to determine the P_o status, Pain each extract was further hydrolyzed by phosphatases (acid phosphatase, phytase, and alkaline phosphatase), and the amount of P_i released was measured. Citrate-P_o was hydrolyzed to a certain extent (30–40% by phytase, and 12–13% by acid phosphatase), whereas Olsen-P_o was not hydrolyzed by these phosphatases. These results suggest that the chemical structure or molecular size of the P_o extracted by these methods is different. Citrate-P_o may enable to evaluate plant available P to a certain extent, which is particularly important in Andosols with a low fertility.     

Andosols and soils with andic properties in the German soil taxonomy

The presence of soils with andic properties on German territory has been suspected for decades and there are numerous reports of sites where they may potentially occur. Andic properties, however, are not adequately represented by the German soil‐classification system. The German taxonomic category “Lockerbraunerde” has not been revised or reconciled with international taxonomic categories since the year 1957, when it was initially proposed. With this review, we show that there are true Andosols of both the silandic (allophane‐containing) and the aluandic (Al‐Humus‐dominated) type in Germany and that their properties differ substantially from other soils which merely exhibit low bulk density. By (1) comparing soil carbon storage between some German Andosols, Chernozems, and nonandic Cambisols with particularly low bulk density and (2) elucidation of the differential pedogenetic pathways leading to Andosol formation, we further demonstrate that Andosols are important objects of study in research issues of contemporary interest. We propose that appropriate measures be taken to lay the foundations for the protection and conservation of these soils, because they are valuable as archives of natural history and provide opportunities to study unique soil processes.     

Aggregation and organic matter in subarctic Andosols under different grassland management

Quantity and quality of soil organic matter (SOM) affect physical, chemical, and biological soil properties, and are pivotal to productive and healthy grasslands. Thus, we analyzed the distribution of soil aggregates and assessed quality, quantity, and distribution of SOM in two unimproved and improved (two organic and two conventional) grasslands in subarctic Iceland, in Haplic and Histic Andosols. We also evaluated principal physicochemical and biological soil properties, which influence soil aggregation and SOM dynamics. Macroaggregates (>250 µm) in topsoils were most prominent in unimproved (62–77%) and organically (58–69%) managed sites, whereas 20–250 µm aggregates were the most prominent in conventionally managed sites (51–53%). Macroaggregate stability in topsoils, measured as mean weight diameter, was approximately twice as high in organically managed (12–20 mm) compared with the conventionally managed (5–8 mm) sites, possibly due to higher organic inputs (e.g., manure, compost, and cattle urine). In unimproved grasslands and one organic site, macroaggregates contributed between 40% and 70% of soil organic carbon (SOC) and nitrogen to bulk soil, whereas in high SOM concentration sites free particulate organic matter contributed up to 70% of the SOC and nitrogen to bulk soil. Aggregate hierarchy in Haplic Andosols was confirmed by different stabilizing mechanisms of micro- and macroaggregates, however, somewhat diminished by oxides (pyrophosphate-, oxalate-, and dithionite-extractable Fe, Al, and Mn) acting as binding agents for macroaggregates. In Histic Andosols, no aggregate hierarchy was observed. The higher macroaggregate stability in organic farming practice compared with conventional farming is of interest due to the importance of macroaggregates in protecting SOM and soils from erosion, which is a prerequisite for soil functions in grasslands that are envisaged for food production in the future.     

Factors controlling accumulation and decomposition of organic carbon in humus horizons of Andosols

Andosols often accumulate soil organic matter (SOM) in large amounts. To investigate the factors controlling the stability and lability of organic carbon (OC) in humus horizons of Andosols, we selected 19 A horizon samples (surface and subsurface horizons) from the Field Station of Tohoku University including areas where benchmark soil profiles of non-allophanic Andosols are distributed. We determined the soil properties possibly controlling the OC accumulation, such as pH(H₂O), 1 M KCl-extractable aluminum (KCl-Al), pyrophosphate-extractable Al and iron (Al_p, Fe_p), acid oxalate-extractable silicon (Si_o), total OC, water-extractable OC, and humified OC. To evaluate the OC mineralization, we measured the soil respiration rates in a laboratory for non-treated, neutralized (CaCO₃, Ca(OH)₂ and NaOH), and nutrient applied (KH₂PO₄, (NH₄)₂SO₄) soil samples. Statistical analyses, including a path analysis, showed that the Al_p and pH(H₂O) values are directly related to the OC concentration (P?<?0.01 and P?<?0.05, respectively). There was a significant negative correlation (P?<?0.01) between the soil respiration rates of the non-treated samples and the ratios of the humified OC to total OC, showing that the humification of the SOM was definitely related to the OC stability. Effects of the chemical treatments to the soil respiration rates were greater in the surface horizon samples with an abundant labile OC than those in the subsurface samples. Neutralization affected the soil respiration rates more significantly than the nutrient application. Among the neutralization treatments, the liming materials more effectively increased the respiration rates. This was probably due to an increase in the lability of the humified OC by liming.     

Source minerals and formation of partially interlayered vermiculites in Dystrochrepts derived from Tertiary sediments

The formation of partially interlayered vermiculite (PIV) was studied in six Dystrochrepts derived from Tertiary sediments. Mineralogy of silt and clay fractions of gravel and fine earth separated from surface and subsurface samples were determined by X-ray diffraction. PIV, mica, vermiculite and regularly interstratified 1:1 PIV/chlorite (PIV/Ch(l:l)) were the dominant clay minerals. The contents of these minerals were compared between the specific particle-size fractions of gravel and fine earth for each sample and the difference was interpreted in terms of mineral transformation associated with soil formation. PIV was formed from mica and PIV/Ch(l:l) in soils with a pH(KCl) of 3.5 to 4.0 and little organic matter. Vermiculite was formed from mica in surface soils with a low pH (pH(KCl) 3.5) and abundant organic matter. PIV would form directly from mica without an intermediary phase of vermiculite and form from PIV/Ch(l:l) by partial dissolution of interlayers in chlorite layers.     

Mechanisms and processes of crust formation on artificial aggregates

Crust formation process initiates from aggregate disinategration occurred through the combined effects of inherent soil properties and external actions. Rainfall plays a major role in determining the moisture conditions of the soil surface and in triggering aggregate disintegration mainly through slaking and the impact off raindrops. To analyze the relations between rainfall characteristics and crust formation, three artificially granulated aggregates were exposed to natural rainfall events. The rainfall characteristics were expressed in terms of the distribution pattern of rainfall intensity, I _10min; mean weighted drop diameters, D _n; raindrop concentration, N _Dn,; rainfall energy, Et;and kinetic energy, KE; as well as conventionally used parameters, such as total amount and mean intensity of rainfall. The degree of crusting was evaluated in terms of the water permeability and morphological observation just after the rainfall treatment. Based on a comparison of the rainfall characteristics and degree of crusting, we confirmed the following relations. Rainfall characteristics at the onset of the event considerably influence the degree of crusting, i.e. initial rainfall with high I _10min; and large drop size promotes aggregate disintegration by slaking due to rapid wetting, whereas initial gentle rainfall prevents slaking by slow wetting and subsequent crusting. When initial rainfall is not saaficient for slaking but Beads to a weakening sf the aggregates, subsequent disintegration and crusting are promoted by impact together with the residual influence of slaking. Impact is a dominant factor for aggregate disintegration under wet conditions after initial rainfall which is so gentle that slaking never occurs. Responses of the soils under different natural rainfall conditions were almost consistent with the results under simulated rainfall reported in the previous papers (Tanaka et $1.1997: Soil Sci. Plant Nutr., 43, 99-107, 109-115). Crusting was not observed in the Fujino 1–3 soil gender any rainfall events. In the Wokuei 1–3 soil, characterized by a susceptibility to slaking and stability to impact, crusting was only formed under the rainfall with high I _{10 min} at the onset. For the Shimazu 1 soil, impact played a dominant role in crusting under any rainfall events, and a larger degree of crusting was found when the effects of impact and slaking were combined. The characteristics of rainfall monitored in this study may be within a normal range as natural rainfall in terms of the amount and the mean intensity, suggesting that crusting likely to occur if the aggregate stability of soils is Bower than or similar to that sf the Shimaza 1 and the Hokuei 1–3 soils.     

Forms and buffering potential of aluminum in tropical and subtropical acid soils cultivated with Pinus taeda L

Purpose

Several interactions between Al and the solid phase of soil influence Al buffering in soil solution. This work evaluated soils cultivated with Pinus taeda L. to determine Al forms in organic and mineral horizons using various extraction methods and to relate acidity with clay mineralogy.

Materials and methods

Organic and mineral horizons of 10 soil profiles (up to 2.1 m deep) in southern Brazil were sampled. Organic horizons were separated into fresh, aged, and fermented/humified litter. The following Al extraction methods were utilized: 0.5 mol L^?1 pH 2.8 CuCl₂–Al complexed in organic matter; 1.0 mol L^?1 KCl–exchangeable Al; water–Al soluble in soil solution; HF concentrated?+?HNO₃ concentrated?+?H₂O₂ 30% (v/v)–total Al. Six sequential extractions were carried out to isolate different forms of amorphous minerals that can buffer Al on soil solution: 0.05 and 0.1 mol L^?1 sodium pyrophosphate; 0.1 and 0.2 mol L^?1 ammonium oxalate; 0.25 and 0.5 mol L^?1 NaOH. Samples of clay were also analyzed by XRD.

Results and discussion

There was a clear effect of litter age on increasing total Al concentration. In the aged litter and fermented and/or humified litter, levels of total Al were 1.4 to 3.8 and 1.5 to 7.8 times greater than in fresh litter, respectively. The CuCl₂ method had higher Al extraction capacity than the KCl method for litter. The lowest Al–pyrophosphate values were observed in the Oxisol, which also had a predominance of gibbsite and the lowest levels of Al–KCl and Al–CuCl₂. There was an inverse relationship between degree of soil weathering and soluble and exchangeable Al in soils. Available Al increased with higher Si proportion in minerals of the clay fraction (2:1?>?1:1?>?0:1).

Conclusions

The worst scenario was soils with the combination of high soluble and exchangeable Al levels and high concentrations of amorphous forms of Al minerals. The best predictors of Al accumulation in the youngest litter horizon were extractions of amorphous minerals with pyrophosphate and NaOH. These extractors are normally used to predict the level of Al buffering in soils. Organic matter had less influence on Al dynamics in soils.

     

Preheating Method for the Determination of Total Phosphorus Content in Andosols

Abstract

The identification of rhizobial strains is a major problem in studies for the evaluation of the symbiotic effectiveness of specific strains in soils containing native rhizobia. Bradyrhizobium japonicum which includes most strains of soybean-nodulating bacteria is known to display a wide range of genetic diversity (Miyashita 1987). It is, therefore, necessary to develop a reliable taxonomic system based on the genetic traits, which would enable to differentiate and identify of the strains.