Rhizobial Ecology of the Woody Legume Mesquite (Prosopis glandulosa) in the Sonoran Desert

Soil samples were collected from the surface (0 to 0.6 m) and phreatic (3.9 to 4.5 m) root systems of a Prosopis glandulosa woodland in the Sonoran Desert of southern California. P. glandulosa seedlings were inoculated with these soils, and rhizobia were isolated from nodules. The phreatic soil, characterized by constant moisture and temperature but low nutrient availability, favored slow-growing (SG) isolates as nodule occupants (85%). SG isolates from the surface and phreatic soil were distinct based on differences in colony morphology. Isolates from the surface soil, characterized by high nutrient availability and widely fluctuating water content and temperature, were equally represented by fast-growing and SG rhizobia. Most SG isolates (83%) had nodule relative efficiencies of <0.80, whereas 54% of the fast-growing isolates had relative efficiency values of >0.80.     

Bradyrhizobium Populations Occur in Deep Soil under the Leguminous Tree Acacia albida

Soil cores were drilled under the leguminous tree Acacia albida growing in two different ecoclimatic zones of West Africa: the Sahelian area (100 to 500 mm of annual rainfall) and the Sudano-Guinean area (1,000 to 1,500 mm of annual rainfall). Soil samples were collected at different depths from the surface down to the water table level and analyzed for the presence of rhizobia able to nodulate A. albida. In both areas, population densities of rhizobia were substantially greater near the water table than near the surface. In the Sahelian area, rhizobia were present as deep as 34 m at a concentration of 1.3 × 10³/g of soil. In the Sudano-Guinean area, population densities at 0.5 to 4.5 m depth were higher than in the Sahelian area and, at several depths, comparable to that of temperate soils supporting legume crops (10⁴ rhizobia per g of soil). Surface and deep soil isolates from all four sites were found to be slow-growing rhizobia (Bradyrhizobium sp.). The proportion of effective isolates was almost the same within surface and deep soils.     

Changes in aboveground primary production and carbon and nitrogen pools accompanying woody plant encroachment in a temperate savanna

When woody plant abundance increases in grasslands and savannas, a phenomenon widely observed worldwide, there is considerable uncertainty as to whether aboveground net primary productivity (ANPP) and ecosystem carbon (C) and nitrogen (N) pools increase, decrease, or remain the same. We estimated ANPP and C and N pools in aboveground vegetation and surface soils on shallow clay and clay loam soils undergoing encroachment by Prosopis glandulosa in the Southern Great Plains of the United States. Aboveground Prosopis C and N mass increased linearly, and ANPP increased logarithmically, with stand age on clay loam soils; on shallow clays, Prosopis C and N mass and ANPP all increased linearly with stand age. We found no evidence of an asymptote in trajectories of C and N accumulation or ANPP on either soil type even following 68 years of stand development. Production and accumulation rates were lower on shallow clay sites relative to clay loam sites, suggesting strong edaphic control of C and N accumulation associated with woody plant encroachment. Response of herbaceous C mass to Prosopis stand development also differed between soil types. Herbaceous C declined with increasing aboveground Prosopis C on clay loams, but increased with increasing Prosopis C on shallow clays. Total ANPP (Prosopis+herbaceous) of sites with the highest Prosopis basal area were 1.2 × and 4.0 × greater than those with the lowest Prosopis basal area on clay loam and shallow clay soils, respectively. Prosopis ANPP more than offset declines in herbaceous ANPP on clay loams and added to increased herbaceous ANPP on shallow clays. Although aboveground C and N pools increased substantially with Prosopis stand development, we found no corresponding change in surface soil C and N pools (0–10 cm). Overall, our findings indicate that Prosopis stand development significantly increases ecosystem C and N storage/cycling, and the magnitude of these impacts varied with stand age, soil type and functional plant traits     

Impact of Prosopis invasion on a keystone tree species in the Kalahari Desert

Several Prosopis species were introduced into South Africa in the last century. Since then two species, Prosopis glandulosa var. torreyana and Prosopis velutina have invaded large parts of arid southern Africa. Here, we examine the extent to which increased mortality of Acacia erioloba, a keystone species in the Kalahari Desert, can be attributed to competition for water with Prosopis. We do this for A. erioloba and Prosopis sp. at invaded, as well as cleared sites through a determination of species abundance, canopy vitality, plant water stress and plant water source. Our stable isotope results show that in the riparian zone both A. erioloba and Prosopis are using the same water source. Our results also show that there is a 50 % increase in canopy dieback of A. erioloba in the invaded river plots relative to the cleared river plots. This dieback cannot be related to changes in rainfall and temperature as there were no adverse fluctuations (drought) in the weather in the 10 years preceding our study. We speculate that because A. erioloba is more water stressed in the invaded river plot this increase in mortality and dieback is related to plant moisture stress that is not related to climate but to competition for water with Prosopis. Our study gives strong support for the eradication of Prosopis from rivers in arid parts of Southern Africa.     

Inoculation Response of Legumes in Relation to the Number and Effectiveness of Indigenous Rhizobium Populations

The response of legumes to inoculation with rhizobia can be affected by many factors. Little work has been undertaken to examine how indigenous populations or rhizobia affect this response. We conducted a series of inoculation trials in four Hawaiian soils with six legume species (Glycine max, Vigna unguiculata, Phaseolus lunatus, Leucaena leucocephala, Arachis hypogaea, and Phaseolus vulgaris) and characterized the native rhizobial populations for each species in terms of the number and effectiveness of the population for a particular host. Inoculated plants had, on average, 76% of the nodules formed by the inoculum strain, which effectively eliminated competition from native strains as a variable between soils. Rhizobia populations ranged from less than 6 × 10⁰/g of soil to 1 × 10⁴/g of soil. The concentration of nitrogen in shoots of inoculated plants was not higher than that in uninoculated controls when the most probable number MPN counts of rhizobia were at or above 2 × 10¹/g of soil unless the native population was completely ineffective. Tests of random isolates from nodules of uninoculated plants revealed that within most soil populations there was a wide range of effectiveness for N₂ fixation. All populations had isolates that were ineffective in fixing N₂. The inoculum strains generally did not fix more N₂ than the average isolate from the soil population in single-isolate tests. Even when the inoculum strain proved to be a better symbiont than the soil rhizobia, there was no response to inoculation. Enhanced N₂ fixation after inoculation was related to increased nodule dry weights. Although inoculation generally increased nodule number when there were less than 1 × 10² rhizobia per g of soil, there was no corresponding increase in nodule dry weight when native populations were effective. Most species compensated for reduced nodulation in soils with few rhizobia by increasing the size of nodules and therefore maintaining a nodule dry weight similar to that of inoculated plants with more nodules. Even when competition by native soil strains was overcome with a selected inoculum strain, it was not always possible to enhance N₂ fixation when soil populations were above a threshold number and had some effective strains.     

Diazotrophic diversity in the rhizosphere of two exotic weed plants, Prosopis juliflora and Parthenium hysterophorus

This study is aimed at assessing culturable diazotrophic bacterial diversity in the rhizosphere of Prosopis juliflora and Parthenium hysterophorus, which grow profusely in nutritionally-poor soils and environmentally-stress conditions so as to identify some novel strains for bioinoculant technology. Diazotrophic isolates from Prosopis and Parthenium rhizosphere were characterized for nitrogenase activity by Acetylene Reduction Assay (ARA) and 16S rRNA gene sequencing. Further, the culture-independent quantitative PCR (qPCR) was performed to compare the abundance of diazotrophs in rhizosphere with bulk soils. The proportion of diazotrophs in total heterotrophs was higher in rhizosphere than bulk soils and 32 putative diazotrophs from rhizosphere of two plants were identified by nifH gene amplification. The ARA activity of the isolates ranged from 40 to 95 nmol ethylene h⁻¹ mg protein⁻¹. The 16S rRNA gene analysis identified the isolates to be members of alpha, beta and gamma Proteobacteria and firmicutes. The qPCR assay also confirmed that abundance of nif gene in rhizosphere of these two plants was 10-fold higher than bulk soil.     

Soil moisture redistribution as a mechanism of facilitation in savanna tree–shrub clusters

Plant–soil water relations were examined in the context of a selective removal study conducted in tree–shrub communities occupying different but contiguous soil types (small discrete clusters on shallow, duplex soils versus larger, extensive groves on deep, sandy soils) in a subtropical savanna parkland. We (1) tested for the occurrence of soil moisture redistribution by hydraulic lift (HL), (2) determined the influence of edaphic factors on HL, and (3) evaluated the significance of HL for overstory tree–understory shrub interactions. Diel cycling and nocturnal increases in soil water potential (Ψ_soil), characteristic signatures of HL, occurred intermittently throughout an annual growth cycle in both communities over a range of moisture levels (Ψ_soil=−0.5 to −6.0 MPa) but only when soils were distinctly stratified with depth (dry surface/wet deep soil layers). The magnitude of mean (±SE) diel fluctuations in Ψ_soil (0.19±0.01 MPa) did not differ on the two community types, though HL occurred more frequently in groves (deep soils) than clusters (shallow soils). Selective removal of either Prosopis glandulosa overstory or mixed-species shrub understory reduced the frequency of HL, indicating that Prosopis and at least one other woody species was conducting HL. For Zanthoxylum fagara, a shallow-rooted understory shrub, Prosopis removal from clusters decreased leaf water potential (Ψ_leaf) and net CO₂ exchange (A) during periods of HL. In contrast, overstory removal had neutral to positive effects on more deeply-rooted shrub species (Berberis trifoliolata and Condalia hookeri). Removal of the shrub understory in groves increased A in the overstory Prosopis. Results indicate the following: (a) HL is common but temporally dynamic in these savanna tree–shrub communities; (b) edaphic factors influencing the degree of overstory/understory development, rooting patterns and soil moisture distribution influence HL; (c) net interactions between overstory and understory elements in these woody patches can be positive, negative and neutral over an annual cycle, and (d) Prosopis-mediated HL is an important mechanism of faciliation for some, but not all, understory shrubs.     

Influence of an overstorey tree (Prosopis glandulosa) on associated shrubs in a savanna parkland: implications for patch dynamics

The arborescent legume, honey mesquite (Prosopis glandulosa), appears to play a central role in patch dynamics of southern Texas savannas by modifying soils and microclimate and by facilitating the ingress, establishment and/or growth of shrubs in its understorey. As an indirect test for the occurrence and persistence of facilitation in mature shrub clusters (patches), we examined the gas exchange, water relations and production of associated shrubs growing in patches where a Prosopis overstorey was present and in patches where Prosopis had succumbed to natural mortality. Surface (0–10 cm) soils associated with shrub patches were enriched in total [N] and [C] compared to soils of neighboring herbaceous zones. However, there were no detectable differences in soil [N] or [C] in patches with and without Prosopis. Foliar [N] and biomass of various shrub species were also statistically comparable for patches with and without Prosopis. These results are in accordance with other studies that indicate the nutrient legacy associated with Prosopis occupation of a patch may persist for decades after its demise. In comparison to plants growing in the absence of Prosopis, leaf water potentials (predawn and midday), and net photosynthesis and water vapor conductance (morning and midday) of outer-canopy sunlit leaves over an annual growth cycle were comparable for two common evergreen shrubs, Zanthoxylum fagara and Berberis trifoliolata, growing in patches with a live Prosopis. These findings indicate that the presence of Prosopis was not enhancing the growth or activity of mature understorey shrubs; facilitation may, therefore, be important only during early stages of cluster development. In addition, we found no indication that the loss of Prosopis has initiated a downward phase in a cyclic succession of patch initiation, growth and death. Rather, the understorey shrubs appear to be able to maintain growth and productivity in the absence of a Prosopis overstorey, and may, therefore, represent persistent components of woody patches on these savanna landscapes.     

Genetic diversity and symbiotic compatibility among rhizobial strains and Desmodium incanum and Lotus spp. plants

This work aimed to evaluate the symbiotic compatibility and nodulation efficiency of rhizobia isolated from Desmodium incanum, Lotus corniculatus, L. subbiflorus, L. uliginosus and L. glaber plants by cross-inoculation. Twelve reference strains and 21 native isolates of rhizobia were genetically analyzed by the BOX-PCR technique, which showed a high genetic diversity among the rhizobia studied. The isolates were also characterized based on their production of indolic compounds and siderophores, as well as on their tolerance to salinity. Fifteen of the 33 rhizobia analyzed were able to produce indolic compounds, whereas 13 produced siderophores. All the tested rhizobia were sensitive to high salinity, although some were able to grow in solutions of up to 2% NaCl. Most of the native rhizobia isolated from L. uliginosus were able to induce nodulation in all plant species studied. In a greenhouse experiment using both D. incanum and L. corniculatus plants, the rhizobia isolate UFRGS Lu2 promoted the greatest plant growth. The results demonstrate that there are native rhizobia in the soils of southern Brazil that have low host specificity and are able to induce nodulation and form active nodules in several plant species.     

Growth of Indigenous Rhizobium leguminosarum and Rhizobium meliloti in Soils Amended with Organic Nutrients

The ability of indigenous Rhizobium leguminosarum and Rhizobium meliloti to use organic nutrients as growth substrates in soil was assessed by indirect bacteriophage analysis. A total of 17 organic compounds, including 9 carbohydrates, 3 organic acids, and 5 amino acids, were tested (1,000 μg g⁻¹) in three soils with different cropping histories. Four additional soils were screened with a glucose amendment. Nutrient amendments stimulated growth of indigenous rhizobia, allowing subsequent replication of indigenous bacteriophages. Phage populations were enumerated by plating soil extracts on 19 R. leguminosarum and 9 R. meliloti indicator strains, including root nodule isolates from the soils assayed. On the basis of indirect phage analysis, all soils contained native rhizobia similar to one or more of the indicator strains, although not all indicator strains were detected in soil. All organic compounds stimulated growth of indigenous rhizobia, but the growth response varied for each rhizobial strain depending on the nutrient, the nutrient concentration, and the soil. Indigenous rhizobia readily utilized most organic compounds except phenylalanine, glycine, and aspartic acid. The ability of indigenous rhizobia to utilize a wide range of organic compounds as growth substrates in situ indicates their ability to successfully compete with other soil bacteria for nutrients in these soils.     

Nitrogen fixation (acetylene reduction) and cross inoculation in 12 Prosopis (mesquite) species

The leguminous tree mesquite (Prosopis spp) exists on millions of hectares of semi-arid regions of the world. No whole plant acetylene reductions for mesquite have been reported in the literature and nodulation has only been reported for three of the forty-four species. We report greenhouse studies in which 12Prosopis species representing African and North and South American germplasm (1) became nodulated when inoculated with rhizobia strain isolated from a North American mesquite, (2) grew on a nitrogen free nutrient media, (3) reduced acetylene to ethylene, and (4) had a positive significant correlation between the acetylene reduction rates and above ground dry matter. The capability of mesquite to fix nitrogen must now be considered firmly established.     

Genotypic and symbiotic diversity of native rhizobia nodulating red pea (Lathyrus cicera L.) in Tunisia

Nodulation and genetic diversity of native rhizobia nodulating Lathyrus cicera plants grown in 24 cultivated and marginal soils collected from northern and central Tunisia were studied. L. cicera plants were nodulated and showed the presence of native rhizobia in 21 soils. A total of 196 bacterial strains were selected and three different ribotypes were revealed after PCR-RFLP analysis. The sequence analysis of the rrs and two housekeeping genes (recA and thrC) from 36 representative isolates identified Rhizobium laguerreae as the dominant (53%) rhizobia nodulating L. cicera. To the best of our knowledge, this is the first time that this species has been reported among wild populations of the rhizobia-nodulating Lathyrus genus. Twenty-five percent of the isolates were identified as R. leguminosarum and isolates LS11.5, LS11.7 and LS8.8 clustered with Ensifer meliloti. Interestingly, five isolates (LS20.3, LS18.3, LS19.10, LS1.2 and LS21.20) were segregated from R. laguerreae and clustered as a separate clade. These isolates possibly belong to new species. According to nodC and nodA phylogeny, strains of R. laguerreae and R. leguminosarum harbored the symbiotic genes of symbiovar viciae and clustered in three different clades showing heterogeneity within the symbiovar. Strains of E. meliloti harbored symbiotic genes of Clade V and induced inefficient nodules.     

The amino acid composition of gum exudates from Prosopis species

The amino acid compositions of the proteinaceous components of the gum exudates from Prosopis alba, P. chilensis, P. glandulosa, P. laevigata, P. torreyana and P. velutina, and for a sample of commercial gum mesquite, are presented. In agreement with data published previously for the polysaccharide components of their gums, only minor differences in composition are shown by these species. The amino acid compositions are characterized by very high proportions of hydroxyproline and by high proportions of proline and serine; these three amino acids account for 62.5% of those present in the gum from Prosopis velutina. The amino acid compositions of these Prosopis gums are remarkably similar to that established recently for the gum from Acacia senegal (gum arabic).     

Growth of Fast- and Slow-Growing Rhizobia on Ethanol

Free-living soybean rhizobia and Bradyrhizobium spp. (lupine) have the ability to catabolize ethanol. Of the 30 strains of rhizobia examined, only the fast- and slow-growing soybean rhizobia and the slow-growing Bradyrhizobium sp. (lupine) were capable of using ethanol as a sole source of carbon and energy for growth. Two strains from each of the other Rhizobium species examined (R. meliloti, R. loti, and R. leguminosarum biovars phaseoli, trifolii, and viceae) failed to grow on ethanol. One Rhizobium fredii (fast-growing) strain, USDA 191, and one (slow-growing) Bradyrhizobium japonicum strain, USDA 110, grew in ethanol up to concentrations of 3.0 and 1.0%, respectively. While three of the R. fredii strains examined (USDA 192, USDA 194, and USDA 205) utilized 0.2% acetate, only USDA 192 utilized 0.1% n-propanol. None of the three strains utilized 0.1% methanol, formate, or n-butanol as the sole carbon source.     

Direct measurement of denitrification in a Prosopis (Mesquite) dominated Sonoran Desert ecosystem

Summary Denitrification was directly measured using the acetylene inhibition technique in a Sonoran Desert ecosystem dominated by Prosopis glandulosa. Soil under Prosopis and from the unvegetated area between Prosopis was wetted with 50 mm of water and denitrification measured for 48 hours. The mean denitrification rate under Prosopis was 11.6 g N ha^-1h^-1 compared to only 0.2 g N ha^-1h^-1 away from Prosopis. The denitrification response to wetting was rapid and rates peaked about 24 h after water application.The much higher denitrification under Prosopis probably results from high available organic C under Prosopis, but other soil chemical and physical changes effected by Prosopis may influence denitrification rates. About 0.5 kg N ha^-1 of Prosopis cover may be lost from this ecosystem by denitrification after infrequent major rainfalls.     

Plant response to variations in nitrogen availability in a desert shrubland community

Spatial variations in nitrogen availability were studied in a desert community codominated byLarrea tridentata (DC.) Cov. andProsopis glandulosa Torr. Measurements of natural ¹⁵N values in tissues suggested thatProsopis obtains approximately half of its nitrogen through direct symbiotic fixation. Soils were collected under 1)Prosopis shrubs, 2)Larrea shrubs 2 m fromProsopis (LP), and 3)Larrea 2 m from otherLarrea but> 5 m from the nearestProsopis (LL).Prosopis soils showed significantly higher rates of nitrogen mineralization than LL soils in both A and B horizons. Rates of mineralization in LP soils were significantly higher than rates in LL soils only in the B horizon and were not significantly different from rates inProsopis soils. Leaf nitrogen concentrations were significantly higher in LP shrubs (2.06%) than in LL shrubs (1.78%), although ¹⁵N values did not differ between the two shrub types. Nitrogen concentrations inPerezia nana Gray, a perennial herb, were greater in plants underProsopis shrubs (2.09%) than under LP shrubs (1.93%) or LL shrubs (1.67%). Despite apparent differences in nitrogen availability, biomass ofLarrea and density ofPerezia did not differ significantly among these sites.     

Microsymbionts of Phaseolus vulgaris in acid and alkaline soils of Mexico

In order to investigate bean-nodulating rhizobia in different types of soil, 41 nodule isolates from acid and alkaline soils in Mexico were characterized. Based upon the phylogenetic studies of 16S rRNA, atpD, glnII, recA, rpoB, gyrB, nifH and nodC genes, the isolates originating from acid soils were identified as the phaseoli symbiovar of the Rhizobium leguminosarum-like group and Rhizobium grahamii, whereas the isolates from alkaline soils were defined as Ensifer americanum sv. mediterranense and Rhizobium radiobacter. The isolates of “R. leguminosarum” and E. americanum harbored nodC and nifH genes, but the symbiotic genes were not detected in the four isolates of the other two species. It was the first time that “R. leguminosarum” and E. americanum have been reported as bean-nodulating bacteria in Mexico. The high similarity of symbiotic genes in the Rhizobium and Ensifer populations showed that these genes had the same origin and have diversified recently in different rhizobial species. Phenotypic characterization revealed that the “R. leguminosarum” population was more adapted to the acid and low salinity conditions, while the E. americanum population preferred alkaline conditions. The findings of this study have improved the knowledge of the diversity, geographic distribution and evolution of bean-nodulating rhizobia in Mexico.     

Isolation and Characterization of Alfalfa-Nodulating Rhizobia Present in Acidic Soils of Central Argentina and Uruguay

We describe the isolation and characterization of alfalfa-nodulating rhizobia from acid soils of different locations in Central Argentina and Uruguay. A collection of 465 isolates was assembled, and the rhizobia were characterized for acid tolerance. Growth tests revealed the existence of 15 acid-tolerant (AT) isolates which were able to grow at pH 5.0 and formed nodules in alfalfa with a low rate of nitrogen fixation. Analysis of those isolates, including partial sequencing of the genes encoding 16S rRNA and genomic PCR-fingerprinting with MBOREP1 and BOXC1 primers, demonstrated that the new isolates share a genetic background closely related to that of the previously reported Rhizobium sp. Or191 recovered from an acid soil in Oregon (B. D. Eardly, J. P. Young, and R. K. Selander, Appl. Environ. Microbiol. 58:1809–1815, 1992). Growth curves, melanin production, temperature tolerance, and megaplasmid profiles of the AT isolates were all coincident with these characteristics in strain Or191. In addition to the ability of all of these strains to nodulate alfalfa (Medicago sativa) inefficiently, the AT isolates also nodulated the common bean and Leucaena leucocephala, showing an extended host range for nodulation of legumes. In alfalfa, the time course of nodule formation by the AT isolate LPU 83 showed a continued nodulation restricted to the emerging secondary roots, which was probably related to the low rate of nitrogen fixation by the largely ineffective nodules. Results demonstrate the complexity of the rhizobial populations present in the acidic soils represented by a main group of N₂-fixing rhizobia and a second group of ineffective and less-predominant isolates related to the AT strain Or191.     

Tree-shrub interactions in a subtropical savanna parkland: competition or facilitation?

Abstract. Prosopis glandulosa var. glandulosa has played a central role in the encroachment of woody plants in southern Texas, grasslands and savannas by acting as a nurse plant for various shrubs that establish in its understory. To test for continued facilitation of established understory shrubs by Prosopis and to determine if established shrubs compete with the Prosopis nucleus, selective removal experiments were conducted and monitored over a 2–5 yr period. Short-term (1–3 days) and long-term (2 yr) growth and physiological activities (midday net photosynthesis and leaf/shoot water potential) of two common understory shrubs, Zanthoxylum fagara and Berberis trifoliolata, growing with Prosopis, were generally comparable to those of individuals occurring in clusters where Prosopis was removed. Shrubs growing with an intact Prosopis occasionally showed significantly higher leaf-[N] and pre-dawn water potentials than those in clusters lacking a live Prosopis, especially under drought conditions; however, these differences did not translate into greater midday leaf gas exchange or shoot growth. By comparison, removal of understory shrubs elicited large increases in Prosopis net photosynthesis, annual trunk growth in each of the 5 yr monitored, and seed pod production in three of the four years monitored. Seven of 26 Prosopis plants in experimental clusters with an intact understory died over a 5-yr period, compared to only two of the 26 plants in clusters with the cleared understory. Results indicate that (1) the founding overstory Prosopis plant may continue to facilitate understory shrubs following their establishment, but these beneficial effects appear to be small and transitory, and (2) the understory shrubs have a pronounced negative effect on Prosopis, such that competition between overstory and understory woody plants is strongly asymmetrical. These findings suggest that understory shrubs will likely persist despite changes in microclimate and soils (potentially) that occur after the Prosopis plant, which facilitated their ingress or establishment, has died. Soil resource depletion by shallow-rooted understory shrubs appears to be a primary factor contributing to the demise of the deeply rooted, overstory Prosopis plants, especially on upland sites with duplex soils where below-ground competition is accentuated.     

Mesorhizobium jarvisii is a dominant and widespread species symbiotically efficient on Astragalus sinicus L. in the Southwest of China

In order to identify rhizobia of Astragalus sinicus L. and estimate their geographic distribution in the Southwest China, native rhizobia nodulating A. sinicus were isolated and their genetic diversity were studied at 13 sites cultivated in four Chinese provinces. A total of 451 rhizobial isolates were trapped with A. sinicus plants from soils and classified into 8 different genotypes defined by PCR-based restriction fragment length polymorphism (RFLP) of 16S–23S rRNA intergenic spacer (IGS). Twenty-one representative strains were further identified into three defined Mesorhizobium species by phylogenetic analyses of 16S rRNA genes and housekeeping genes (glnII and atpD). M. jarvisii was dominant accounting for 76.3% of the total isolates, 22.8% of the isolates were identified as M. huakuii and five strains belonged to M. qingshengii. All representatives were assigned to the symbiovar astragali by sharing high nodC sequence similarities of more than 99%. Furthermore, the biogeography distribution of these rhizobial genotypes and species was mainly affected by contents of available phosphorus, available potassium, total salts and pH in soils. The most remarkable point was the identification of M. jarvisii as a widespread and predominant species of A. sinicus in southwest of China. These results revealed a novel geographic pattern of rhizobia associated with A. sinicus in China.