The efficiency of 13 phosphate-solubilizing bacteria (PSB; four Burkholderia sp., five Enterobacter sp., and four Bradyrhizobium sp.) was assessed in a soil plate assay by evaluating soil phosphorus (P) availability. A commercial argentine strain, Pseudomonas fluorescens, was used for comparing solubilizing activity. Burkholderia sp. PER2F, Enterobacter sp. PER3G, and Bradyrhizobium sp. PER2H strains solubilized the largest quantities of P in the soil plate assay after 60 days as compared with the other strains, including the commercial one. The effect of PSB inoculation on growth and nutrient uptake of soybean plants was also studied under greenhouse conditions. Plants inoculated with Burkholderia sp. PER2F had the highest aerial height and showed an appropriate N/P ratio. However, none of the PSB increased P uptake by plants. This suggests that PSB inoculation does not necessarily improve P nutrition in soybean, nor was there any relationship between P availability in the soil plate assay and P content in the soybean shoot in the greenhouse. We concluded that the selection of efficient PSB strains as possible inoculation tools for P-deficient soils should focus on the integral interpretation of soil assays, greenhouse experiments, and field trials.

JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range of content in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new forms of scholarship. For more information about JSTOR, please contact support@jstor.org.. Ecological Society of America is collaborating with JSTOR to digitize, preserve and extend access to Ecology.Abstract. The effect of single-tree influence areas on the physicochemical properties of the soil surface mineral horizon (0-18 cm) was studied in three stands of Pinus radiata D. Don introduced into Sierra de la Ventana, Argentina, grasslands 50 yr ago. Soil samples were taken at distances of 0, 1, and 2 m in transects from the tree to the periphery of the crown. Adjacent grassland soils with mollic epipedons were used as controls. Soil alteration was found to be highest near the trunk, with clear evidence of acid hydrolysis of primary silicates; the epipedon close to the trunk was classified as umbric rather than mollic. Decreasing values of pH, Ca, and exchangeable Mg, and increasing values of exchangeable H and Al, and also of fulvic acid-complexed Al, were registered from the grassland toward the axes of the trees. The soil properties analyzed fall into a distinctive spatial pattern of radial symmetry around each individual tree, with systematic and predictable variations, thus confirming the validity of the concept of "single-tree influence circles" for the study area. Each stand of P. radiata generates a patch of soil alteration within the undisturbed habitat matrix; the internal structure of these patches shows a radial pattern of different polypedons spatially and genetically associated with the inner ring of bark litter and the outer ring of leaf and twig litter. The present work shows that the introduction of P. radiata triggered changes in the evolutionary trend of the soils of such magnitude as to be reflected at the highest taxonomic level in soil taxonomy.

Seven phosphate-mobilizing pseudomonads were isolated, identified, and characterized in terms of their biofertilizer potential and root-colonizing properties. Pseudomonas protegens (ex-fluorescens) CHA0 was used for comparative purposes. Four isolates (LF-MB1, LF-P1, LF-P2, and LF-P3) clustered with members of the "Pseudomonas fluorescens complex," whereas the other three (LF-MB2, LF-V1, and LF-V2) clustered with members of the "Pseudomonas putida/Pseudomonas aeruginosa complex." Assays in buffered liquid growth medium supplemented with tricalcium phosphate enabled the separation of the isolates into two groups: group A (LF-P1, LF-P2, LF-P3, and LF-V1) solubilized P from 151 up to 182 μg mL −1 , and group B (LF-MB1, LF-MB2, and LF-V2) solubilized less than 150 μg PmL −1 . All isolates displayed acid and alkaline phosphatase activities. With the exception of LF-MB2, all isolates were able to degrade phospholipids from lecithin. Additionally, all isolates exhibited extracellular protease activity, and four isolates produced hydrogen cyanide, two traits that are related to biocontrol of phytopathogens. To study root colonization in non-sterile soil, isolates were doubly tagged with gfp and a tetracycline resistance cassette. After 15 days of competition with the indigenous bacterial flora, all tagged isolates colonized soybean roots at counts ranging from 7.6×10 5 to 1.7×10 7 CFU g −1 . The results indicate that there are already efficient phosphate-mobilizing pseudomonads adapted to agricultural bulk soils under no-till management in Argentina and thus having excellent potential for use as biofertilizers.

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