Distribution of ectomycorrhizal and pathogenic fungi in soil along a vegetational change from Japanese black pine (Pinus thunbergii) to black locust (Robinia pseudoacacia).

The nitrogen-fixing tree black locust (Robinia pseudoacacia L.) seems to affect ectomycorrhizal (ECM) colonization and disease severity of Japanese black pine (Pinus thunbergii Parl.) seedlings. We examined the effect of black locust on the distribution of ECM and pathogenic fungi in soil. DNA was extracted from soil at depths of 0-5 and 5-10 cm, collected from the border between a Japanese black pine- and a black locust-dominated forest, and the distribution of these fungi was investigated by denaturing gradient gel electrophoresis. The effect of soil nutrition and pH on fungal distribution was also examined. Tomentella sp. 1 and Tomentella sp. 2 were not detected from some subplots in the Japanese black pine-dominated forest. Ectomycorrhizas formed by Tomentella spp. were dominant in black locust-dominated subplots and very little in the Japanese black pine-dominated forest. Therefore, the distribution may be influenced by the distribution of inoculum potential, although we could not detect significant relationships between the distribution of Tomentella spp. on pine seedlings and in soils. The other ECM fungi were detected in soils in subplots where the ECM fungi was not detected on pine seedlings, and there was no significant correlation between the distribution of the ECM fungi on pine seedlings and in soils. Therefore, inoculum potential seemed to not always influence the ECM community on roots. The distribution of Lactarius quieticolor and Tomentella sp. 2 in soil at a depth of 0-5 cm positively correlated with soil phosphate (soil P) and that of Tomentella sp. 2 also positively correlated with soil nitrogen (soil N). These results suggest the possibility that the distribution of inoculum potential of the ECM fungi was affected by soil N and soil P. Although the mortality of the pine seedlings was higher in the black locust-dominated area than in the Japanese black pine-dominated area, a pathogenic fungus of pine seedlings, Cylindrocladium pacificum, was detected in soil at depths of 0-5 and 5-10 cm from both these areas. This indicates that the disease severity of pine seedlings in this study was influenced by environmental conditions rather than the distribution of inoculum potential.

Distribution of Bacterial Species in Soil with a Vegetational Change from Japanese Black Pine (Pinus thunbergii) to Black Locust (Robinia pseudoacacia).

The effects of a nitrogen-fixing tree, black locust (Robinia pseudoacacia), on the distribution of bacterial species were examined in a Japanese black pine (Pinus thunbergii) and black locust-dominated area. DNA was extracted from the soil at depths of 0-5 and 5-10 cm, collected at the border between a Japanese black pine-dominated forest and a black locust-dominated forest, and the distribution of bacterial species was investigated by denaturing gradient gel electrophoresis (DGGE). The bacterial communities did not differ between the two forests. The distribution of some bacterial species correlated significantly with soil pH, soil carbon [C], soil nitrogen [N], and soil N/phosphate [P], but not with soil C/N or soil P. The distributional relationships between ectomycorrhizal (ECM) fungal species and bacterial species were also analyzed. A positive correlation was observed between the distribution of some ECM fungi and bacterial species. These bacteria may have some interactions with ECM fungi in the field.

Does ectomycorrhizal fungal community structure vary along a Japanese black pine (Pinus thunbergii) to black locust (Robinia pseudoacacia) gradient?

In this study we examined the role of the nitrogen-fixing tree, Robinia pseudoacacia (black locust), in ectomycorrhizal (ECM) formation and ECM community of Pinus thunbergii (Japanese black pine) seedlings. Two 200 m(2) experimental plots were established at the border between a Japanese black pine- and a black locust-dominated area in a coastal forest. The ECM fungal community of pine seedlings was examined by PCR-RFLP and sequence analysis. We analyzed the relationship between ECM formation, ECM community, growth, and nutrient status of pine seedlings and environmental conditions using the Mantel test and structural equation model. Percentages of ECM root tips, the number of ECM fungal species and ECM diversity on pine seedlings decreased in the black locust-dominated area. Cenococcum geophilum and Russula spp. were dominant in the Japanese black pine-dominated area, whereas Tomentella spp. were dominant in the black locust-dominated area. Nitrogen (N) concentration in soils or pine seedlings strongly influenced the percentage of ECM root tips, the number of ECM fungal species and ECM fungal similarity. These results imply the long-term eutrophication caused by N-fixing trees can change ECM formation and ECM community structure.