Soil Bacterial Diversity In The Subalpine/Alpine Forests Of Western Sichuan Affected By Seasonal Freeze-thaw Cycles

Seasonal freeze-thaw cycle, affected essentially by global climate change, is a common natural phenomenon in middle and high latitude area, which could change forest ecological processes background with the global warming. As one of the most important components in the forest ecosystem, soil microbe plays a key and irreplaceable role in material cycling and energy flows, but it is regulated by many biotic and abiotic factors and their interacted effects. However, more concentration has been focused on the changes in growing season, few have paid attention to the structure and functions of soil microbial community in freeze-thaw season, which limited the further understanding of soil microbial function in subalpine forest ecosystem processes, such as litter decomposition, nutrition released/transformed and humid formed, even the possible changes background of the ongoing global climate changes. Therefore, a field investigation was conducted to the soil bacterial diversity by PCR-DGGE in the primary fir (Abies faxoniana) forest, fir and birch (Betula albosinensis) mixed forest and secondary fir (A. faxoniana) forest in western Sichuan including freeze-thaw season and growing period. The results were expected to beneficial for deeply understanding the soil processes of subalpine/alpine forest.(1) During the stage of soil freezing, more quantities of bacterial bands in soil organic layers were observed compared with those in soil material layers of three sampled forests, and showed the order as:MF> SF> PF before soil freezing, but PF> MF> SF after soil freezing with decline of 18%,38%,40%, respectively. In the stage of soil thawing, the quantities of bacterial bands in soil organic layers were lower than that in soil material layers of three forest communities, and showed the order as:PF> SF>MF, but the total quantities of bacterial bands declined followed by increase in the stage of soil thawing, and lowest values were found in April. The increase rates of bands were 54%,41%,49%, respectively. The ranked order of soil bacterial groups were SF> MF> PF. However, the total quantities of soil bacterial bands in three forests showed significant seasonal dynamic pattern as affected by freeze-thaw cycles. The results indicated seasonal freeze-thaw cycles had significant effects on the numbers of bacterial population in soil organic layer and material layer. (2) Shannon-Wiener Index(H), Eveness (EH), Simpson Dominance Index shared the same seasonal dynamics patterns as affected by freeze-thaw cycle. Shannon-Wiener Index of bacterial population in different soil layer declined distinctly after soil frozen completely. The Eveness of soil bacterial population presented decline current in Mixed Forest after soil freezing but no distinct difference in Primary Forest and Secondary Forest before and after soil freezing. On the contrary, the Simpson Dominance of soil bacterial population in Primary Forest and Secondary Forest increased markedly after soil frozen. However, the Shannon-Wiener and Eveness of these samples had a common change that Indexes decreased followed by increase in the period of soil thawing except time they reached to the lowest point was different, and Simpson Index show a verse current, which was accorded with the meanings of Shannon-Wiener changes.(3) There was a lager variability in the constitution of bacterial population in different soil layers of three forests, and the similarity was from 48% to 92%, meanwhile presented a markedly seasonal changes, which suggested that seasonal freezing-thawing events had significant effects on the soil bacterial population.(4) An contrast analyze of sequences and homology revealed that these soil samples contained Firmicutes, Proteobacteria, Acidobacteria, Chloroflexi, and Sample7 had a high similarity with Cyanobacterium and Deferribacterales. Sample 5 and Sample 9 was as 100% similarity as Proteobacteria. There was one sample as 96% similarity as uncritical bacterial clone. These results indicated that there were still comparative abundances of cold-tolerance dominant species under the extreme environment in order to maintain the lasting winter soil ecosystem processes.In conclusion, seasonal freeze-thaw cycles in subalpine/alpine forests had significant impacts on soil microbial communities. After soil frozen, which significantly reduced the abundances and diversities of soil microbial community, a majority of bacterial populations disappeared, but increased the dominance of soil microbial community. Moreover, the processes of thawing caused drastic dynamic changes on soil and water environment. Therefore, the changes of soil bacterial communities drove by soil freezing/thawing could be beneficial to understanding winter soil process, although the detail of mechanisms were still unclear. In addition, although there are still some lacks in PCR-DGGE method, it is the currently effective and rapid method in investigating the structure and diversities of microbial community. The results here only obtained under the current condition.