Soil Fauna Community In The Subalpine/Alpine Forests Of Western Sichuan As Affected By Seasonal Freeze-thaw

Many focuses have been paid on the structure and function of soil fauna community all over the world, since the important role in material cycling and energy transferation in ecosystem. Seasonal freeze-thaw cycle is the significant climate change processes in the regions with relative higher altitude and latitude. However, there are four obvious periods (the growing period, early frozen period, continuously frozen period and the thawing period) in the ecological processes as affected by seasonal snow-pack and freeze-thaw cycles in these areas. The currently crucial scientific questions are:whether or how does the soil fauna community change among the periods, what are the relationships between growing period and non-growing period? These unresolved questions limit our understanding of soil ecological processes in the areas with seasonal soil frozen.The alpine/subalpine forest of Western Sichuan is the representative forest in southwest China, which is located in the upper reaches of Yangtze River and transitional area between the Qinghai-tibet plateau and Sichuan basin, and plays important roles in conserving water and biodiversity, balancing global carbon cycling, and regulating regional climate. As one of most representative forest ecosystems in middle latitude region with relative high altitude, the alpine/subalpine forest is an optimal ideal site to study soil ecological processes in wintertime since its vertical distribution characteristic of climate, and obviously seasonal freezing of soil with a length of 5-6 monthes. So far, researches on the structure and fuction of soil fauna community in the alpine/subalpine forests of western Sichuan as affected by seasonal freeze-thaw have not been well documented, far less to ecological processes of the four key periods in this region. A field experiment was therefore conducted to investigate the composition, abundance, and diversity of soil fauna by Hand, Baermann and Tullgren methods in primary fir (Abies faxoniana) forest (PF), fir and birch (Betula albosinensis) mixed forest (MF) and secondary fir forest (SF), which were three representative and widely distributed forests in the subalpine and alpine forest region in western Sichuan, during the seasonal freeze-thaw period and next growing period from November 2008 to October 2009. The results are helpful to deeply understand the effect of seasonal freezing-thawing events on the process of the subalpine and alpine forest ecosystem and its relationship between the ecological processes in winter and summer.There were obvious seasonal dynamic characters of soil temperature in the three sampling forests, and the maximum and minimum average soil temperature were respectively recorded in August and January. Seasonal freeze-thaw period began in November 2008 and ended in April 2009. Meanwhile, soil had frozen in the early December 2008, and thawed in the latter April 2009. Seasonal freeze-thaw period respectively started on November 8, November 9 and November 23 in the PF, MF and SF, which respectively ended on April 13, March 15, and March 22. Furthermore, a total of 157,127, and 122 days length of continuously frozen period were respectively recorded in the PF, MF and SF. Correspondly, a total of 20 cycles,18 cycles, and 11 cycles of freeze-thaw cycle had also been respectively recorded in the PF, MF and SF.By preliminary identification,159038 individuals were detected, which belonged to 7 phyla,156 classes and 30 orders in the three sampling forests.61313,55897 and 41828 individuals were detected in the PF, MF and SF, respectively. The highest and lowest detectable individuals and groups of soil fauna were both respectively observed in the growing period and continuously frozen period. Although there were still comparable individuals and groups of soil fauna during seasonal freeze-thaw period, frozen and repeated freezing-thawing cycle of soil imposed significant influence on the structure and function of detectable soil fauna communities. In comparison with the growing period, the maximum average individuals and groups of soil fauna had respectively decreased 3379 individuals and 18 groups.The dominant groups were consisted of Coleoptera, Dipter, Acarina, Nematoda, and Collembola in the three sampling forests, meanwhile, frozen and repeated freezing-thawing cycle of soil imposed significant influence on the structure and function of soil fauna, and groups of Araneae, Polydesmida, Spirostreptida, and Hymenoptera were sensitive to freezing-thawing events.In comparison with the mineral soil layer, the soil organic layer had higher density and groups of soil fauna, and the highest density and groups of soil fauna were observed in the growing period, but except density of the MF. The highest density of macrofauna, which belonged to organic soil layer, were observed on 5 August in the PF and SF, and on 25 March in the MF, but the lowest groups of that were observed on 15 December in all sampling forests. Similarly, the highest densities of mesofauna and microfauna, which belonged to organic soil layer, were observed on 3 November in the PF and MF, and on 20 October in the SF, but the lowest groups of that were observed on 15 December in the PF and MF, and on 19 November in the SF. Frozen and repeated freezing-thawing cycle of soil imposed distinct influence on density and groups of soil fauna, and following dynamics of seasonal freezing-thawing, density and groups of soil fauna exhibited a decreased then increased trend, and this trend in soil organic layer was more significant in the PF, owing to higher intense and more frequent freeze-thaw cycles in soil organic layer of the PF. In comparison with the growing period, the maximum average density and groups of macrofauna had respectively decreased 201±18 individuals and 14 groups, correspongdly, the maximum average density of mesofauna and microfauna had decreased 169±19 (103) individualsThe lowest proportion of dominant groups of Marcofauna was in growing period in the three sampling forests, and the lowest proportion of dominant groups of Mesofauna and Microfauna was in frozen period of the MF and SF, and in growing season of the PF. Frozen and repeated freezing-thawing cycle of soil imposed dramatically influence on the density of Acarina and Collembola, and the value of Acarina to Collembola (A/C), this phenomenon was more significant in the PF. In comparison with others, the maximum average density of Acarina and Collembola respectively had decreased 286±26 (102) individuals,134±22 (102) individuals. the maximum value of Acarina to Collembola had decreased 0.94±0.21, especially in early frozen period and thawing periods.Furthermore, following freezing-thawing event, diversity indices of Shannon-Wiener (H'),Pielou (J) and Simpson (C) also obviously significant changed. The highest macrofauna indices of H' and J was in growing season, and the lowest in frozen period, but the highest macrofauna index of C was in soil thawing period, and the lowest in growing season. Meanwhile, diversity indices of mesofauna and microfauna were no significant changed.Similarly, the dissolve organic carbon, dissolve nitrogen, microbial biomass carbon, and microbial biomass nitrogen obviously changed during this study in the three sampling forests too. Frozen and repeated freezing-thawing cycle of soil imposed dramatically influence on nutrient sequestration and mineralization and microbial structure, and this trend in soil organic layer was more significant in the PF, owing to higher intense and more frequent freeze-thaw cycles in soil organic layer of the PF, especially during early frozen and thawing period. Pearson's correlation shown that density of soil fauna communities had certain degree negative correlation with dissolve organic carbon, and had positive correlation with microbial biomass carbon.In conclusion, the results indicated that there were obvious freeze-thaw cycles during seasonal freeze-thaw period in the subalpine and alpine forest of western Sichuan. These freeze-thaw cycles showed significant effects on the structure and function of soil fauna communities, especially which in soil organic soil layer. Moreover, the results not only revealed that soil fauna communities were sensitive to environmental change, but also indicated that the length, intensity and frequence of freeze-thaw cycles had significant effects on the structure and diversity of soil fauna community. Therefore, the changs in soil fauna community driven by freeze-thaw cycles could be contributed to understand the wintertime ecological processes, and the interaction in freeze-thaw season and growing period.