The Microbial Community During The Fine Root Decomposition As Affected By Seasonal Freeze-thaw Cycle In The Subalpine/Alpine Forests

Microbial community profoundly participates in the process of fine root decomposition, so understanding the microbial community dynamic during the fine root decomposition process is crucial to realize the mechanism of fine root decomposition. Till now, little research concern microbial community dynamic during the fine root decomposition process in winter has been reported. Western Sichuan subalpine/alpine forest effort crucial and indispensable effect on a great many ecological processes, while this forest is also sensitive to global change for its peculiar seasonal freeze-thaw event and snowpack pattern. In this statement, global change could lead to change in soil freeze-thaw regime and snowpack pattern through temperature raise in winter, result in impact on microbial community structure and function during the fine root decomposition process. To understand how change in microbial community can effect on fine root decomposition process in future climate warming situation, this research conducts an altitude gradient to assimilate warming, three sites are choose at3023m(A1)、3298m(A2、3582m(A3). Fine roots of three predominate species in this area, namely spruce (Picea asperata), birch (Betula albo-sinensis) and fir(Abies faxoniana), have been selected as research object. By using buried litterbags, this research measures and analyzes microbial biomass, cultivable bacteria and fungi, microbial community diversity and microbial community structure during the fine root decomposition process in both freeze-thaw season and growing season of one year.Seasonal freeze-thaw event exerts significant impact on the microbial biomass during the fine root decomposition process. Microbial biomass carbon (MBC) and nitrogen (MBN) of all fine roots significantly declines when soil gradually freezing, while microbial biomass phosphorus (MBP) rises in the same time. During freeze-thaw season, MBC increases until middle thawing stage and then decreases, MBN exhibits no clear trend, and MBP occupies distinct dynamics among three altitudes. During growing season, more MBC and MBN appears in spruce than other species, the peak value of all fine roots emerges in late growing season. MBN in growing season is higher than it in freeze-thaw season with highest in middle growing season, except in A3site which highest value appears in late growing season. The ratio of MBC to MBN is lowest in late growing season of all fine roots, only exclude fir fine root in A3site.At the initial stage of fine root decomposition process, the cultivable bacteria and fungi shows considerable amount. The peak value of cultivable bacteria and fungi exhibits in deep freezing stage of most fine roots, then a sharp fall follows the peak, after that cultivable bacteria and fungi maintain at comparable low level. Less cultivable bacteria and fungi lives in growing season than freeze-thaw season. The maximum ratio of cultivable bacteria to fungi of all fine roots appears in onset of freeze, and the minimum ratio appears in late thawing stage except in spruce at Al site and fir at A2site, which appears in early thawing stage.During freeze-thaw season, there are differences in microbial diversity exhibit between different stages. The number of bacterial bands declines greatly in deep freezing stage, and the reduced amount takes40percent of total bacterial bands number. With the change in bacterial bands number, Shannon-Wiener index falls, and Simpson index raises. Another peak of bacterial bands number appears in middle thawing stage to late thawing stage, at that time, Shannon-Wiener index rises, and Simpson index falls. Till early growing season, the number of bacterial bands decreases again, with both diversity indexes decline.The similarity of microbial community during the fine root decomposition process is close in deep freezing stage. There is an obvious shift of microbial community emerges at middle thawing stage of spruce and birch fine roots, especially to spruce fine roots, the similarity of two groups across this stage only account50percent. Besides, microbial community in early growing stage can be treated by an independent group, and the difference is greater in spruce and birch fine roots. During freeze-thaw season, the general microbial groups during the spruce fine root decomposition process include Proteobacteria, CFB-group, Acidobacteria and Verrucomicrobia, the dominated microbial groups are Proteobacteria and CFB-group. By analyzing the consist of general microbial groups, the most frequent group is γ-Proteobacteria which account34%～65%to total microbial quantity, α-Proteobacteria and β-Proteobacteria also can be found in every stage, which account6%～17%and3%～8%respect,δ-Proteobacteria only can be observed in thawing stage at A1and A2site and in onset of freezing at A3site, which account less than5percent, CFB-group is the second predominate group, which at most take32%in middle thawing stage at A2site and never less than12%, Acidobacteria account much less percentage, only reaches2～10%with highest in later thawing stage at A1site and lowest in early growing season at A3site, Verrucomicrobia is a comparative rare group, which only presents after middle thawing stage at A1and A2sites, but never can be found at A2site.According to this research, during freeze-thaw season there are considerable microbial communities acting in the process of fine root decomposition, and a great deal of them could contribute to fine root decomposition. When comparing microbial communities of freeze-thaw season with growing season, a significant difference can be observed. Different freeze-thaw regimes, which produced by simulating climate change along altitude, could exert impact on fine root decomposition, also lead differences exist in microbial communities changing in the process of fine root decomposition. Divergent decomposition substrates could affect the efficiency of microbial taking up nutrition, which could limit microbial activity; this phenomenon can be found in different species. In a word, these conclusions struggling contribute to understand the integrated process of fine root decomposition, especially this process during freeze-thaw season.