Abstract: Seasonal variations of vesicular-arbuscular mycorrhizal infections and VAM fungal (endogonaceous) spore densities were systematically investigated in the rhizospheres of Welsh onion Allium fistulosum) and corn (Zea mays) in Beijing from 1986 to 1987. The relationships among the occurrence of VAM and VAM fungi, several soil factors (contents of N, P, and organic matters, pH) and climatic factors (soil and atmospheric temperature, accumulated temperature, precipitation, photoperiod and relative humidity) were tentatively evaluated with linear correlation and relational grade. The results obtained showed that mycorrhizal infection rate of the two plants was gradually increased with the extention of their growth period, especially in June and September during the whole year the infection rate was the highest. Spore densities in the rhizospheres of two plants varied greatly and two 'peaks appeared in June, July and October respectively throughout the year. In the rhizospheres of Welsh onion and corn in Beijing, the investigated soil factors showed little Change during the whole growth period of plants, so it was considered that climatic factors were more important to the seasonal variations of VAM occurrence in the:same.region. Analytical results of linear correlation and relational grade indicated that the occurrence of VA mycorrhizae was most closely related to the accumulated temperature during the whole year.

Key words: VA mycorrhizae, Environmental factors, Ecology