Abstract: Seasonal variation of soil microbial amounts and rhizospheric effect of two forest trees were studied systematically on Betula luminifera forest and Pleioblastus amarus forest in Hongya County,Sichuan Province.The effects of two models of conversion of farmland to forest on soil microbial amounts and activities were analyzed and discussed too.The results showed that soil microbial amounts were increasing from spring to autumn,and the most in autumn,and the least in winter in B.luminifera forest and P.amarus forest.Bacteria amounts were predominant,actinomyces amounts were the next,and fungi amounts were the least among composition of communities of soil microbes.The sequence of total amounts of soil microbes was as follows:P.amarus forest(1.29×10⁹ individual·g^-1 dry soil)>B.luminifera forest(0.29×10⁹ individual·g^-1 dry soil)in a year.Rhizospheric effects of the two forest trees under two models of conversion of farmland to forest were different in different seasons.And the R/S values of microbes were 1～2 in different seasons,and the R/S values were 1.37 and 1.24 respectively in B.luminifera forest and P.amarus forest in a year.The ecological benefit of model of P.amarus forest was better than B.luminifera forest by evaluation of total soil microbes.

Key words: conversion of farmland to forest, soil microbes, rhizospheric effect