Ligularia virgaurea, a toxic perennial weed, has become a dominant species in the heavilygrazed alpine meadows of the eastern Qinghai-Tibetan Plateau over recent decades. We investigated changes in soil biochemical properties associated with L. virgaurea spreading in grazed alpine meadows at three sites. Soil and root biomass samples were taken at depths of 0-8 cm, 8-16 cm and 16-24 cm from patches where L. virgaurea was dominant and from areas between L. virgaurea patches, with only other native species. Across sites, root biomass, acid-extracted carbohydrate C, organic C and N mineralization, and microbial biomass C and N concentrations in the top 8-cm layer were significantly higher while nitrate N concentration was significantly lower in L. virgaurea patches than in areas between L. virgaurea patches. The increased activities of dehydrogenase, β-glucosidase, urease and phosphatase in the top 8-cm layer under L. virgaurea were associated with enhanced soil microbial biomass. Our results indicate a close association between changes in soil biochemical properties in the top 8-cm layer and the spread of L. virgaurea in grazed alpine meadows of the eastern Qinghai-Tibetan Plateau.
The role of Pseudomonas tolaasii as an important pathogen of the common mushroom Agaricus bisporus is difficult to study in the microbially complex growth medium used for mushroom production. Two strains of P. tolaasii that had been marked with kanamycin resistance and xylE genes were introduced individually into casing soil over mushroom compost. Survival studies revealed that P. tolaasii numbers in casing soil over mushroom compost decreased a 1000-fold in the first 9 days and then remained relatively stable over the rest of the monitoring period. The presence of the pathogenic colony form and the nonpathogenic colony variant was monitored on mushroom caps and in mushroom compost to detect any phenotypic variation while incubated in these environments. Reversion from the nonpathogenic to pathogenic form was detected following isolation and culture from diseased mushroom caps. Inoculation of the marked strains directly onto the cap or into compost beds seeded with A. bisporus resulted in the appearance of brown blotch symptoms.Key words: Pseudomonas tolaasii, Agaricus bisporus, phenotypic variation, mushroom disease.
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