Fir(Abies faxoniana) distributes widely in the alpine and subalpine region of western Sichuan.To understand effects of freeze-thaw events on soil microbial community,microbial number was investigated monthly in a fir forest,employing the method of lab freeze/thaw cycles in combination with field soil sampling.Bacterial and actinomycete number in soil decreased and fungal count increased after a seasonal freeze-thaw season cycle.Bacterial and fungal counts increased with the decreased altitude,while actinomycete count decreased with the decreased altitude.Different microbes had various change patterns with altitude in humus layer(AL), illuvial layer(BL) and parent material layer(CL). Compared with the 3 582 m altitude,bacterial count in the AL and CL at the 3 298 m altitude increased 4.2%and 7.0%,respectively,but that in the BL decreased 0.6%.Meanwhile,bacterial count increased 11.5%,3.5%and 2.1%in the AL,BL and CL at the 3 023 m altitude in comparison with the 3 298 m altitude,respectively.Fungi count in the AL,BL and CL increased by 10.5%,10.3%and 16.4%at the 3 298 m altitude in comparison with 3 582 m altitude,respectively,while it increased by 2.4%,1.7%and 3.5%at the 3 023 m altitude in comparison with 3 298 m altitude,respectively. Actinomycete count in the AL at the 3 298 m altitude increased by 0.7%than that at the 3 582 m altitude, but decreased 5.9%and 13.9%in the BL and CL.In the AL and BL,it increased 5.2%and 5.2% respectively at the 3 023 m altitude than that at the 3 298 m altitude,while in the CL it decreased 12.4%.These results were in agreement with the simulation freeze/thaw cycle experiment in lab.The results also revealed that the seasonal freeze-thaw cycle increased the ratio of soil fungi to bacteria. These results demonstrated that different microbial groups responded differentially to the seasonal freeze-thaw cycle,which may be an important ecological mechanism for maintaining the seasonal frozen ecosystem.
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