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.

[Impact of fire on carbon dynamics of Larix gmelinii forest in Daxing'an Mountains of North-East China: a simulation with CENTURY model].

Fire is one of the important natural disturbances to forest ecosystem, giving strong impact on the ecosystem carbon dynamics. By using CENTURY model, this paper simulated the responses of the carbon budget of Larix gmelinii forest in Huzhong area of Daxing' an Mountains to different intensities of fire. The results indicated that after the fires happened, the soil total carbon pool of the forest had a slight increase in the first few years and then recovered gradually, while the stand biomass carbon pool increased after an initial decrease, with the recovery rate of carbon pool of the stand fine components being faster than that of the coarse components. The fluctuation of the carbon pools increased with the increase of fire intensity. After the fires, both the net primary productivity (NPP) of forest vegetation and the soil heterotrophic respiration increased after an initial decrease, but the recovery rate of the NPP was faster than that of soil heterotrophic respiration, resulting in the alternation of the stand functioned as a carbon source or sink. After light fire, the forest still functioned as a weak carbon sink, and quickly recovered as a carbon sink to the level before the fire happened. After other intensities fire, the forest functioned as a carbon source within 9-12 years, and then turned back to a carbon sink again. It was suggested that lower intensity forest fire could promote the regeneration of L. gmelinii forest, reduce the combustibles, and have no strong impact on the stand carbon budget, while higher intensity forest fire would lead to the serious loss of soil- and tree carbon sequestration, retard the recovery of the forest, and thereby, the forest would be a carbon source in a longer term.