The Shift Of Microbial Community During The Process Of Leaf/Needle Litter Decomposition In The High-frigid Forest Stream

The shift of microbial community during the process of litter decomposition in the high-frigid forest streams plays an essential role in understanding not only the key ecological content of the material cycle of the stream ecosystem and the flowing of energy, but also the ecological relationship between land and water of the alpine forest. However, little information has been available on. Therefore, this study was based on the hypothesis that "the structure of microbial community can be profoundly affected by the changes of seasonal climate of high-frigid forest streams in the pattern of snow cover and freezing-thawing, and then the key ecological process of litter decomposition and its related key ecological process will be regulated". Litterbags with leaf/needle litter of four representative species (S. saltuaria, L. mastersiana, S. paraplesia and R. lapponicum) were placed in the high-frigid forest streams in western Sichuan with snow cover lasts for five to six months every year. Samples were taken at the five key stages of snow forming, snow covering, snow melting, early and late stage of growing season. The dynamics of microbial community structure characteristics (the microbial community composition, abundance, biodiversity and so on) were analyzed. The relationships of these parameters among the five key stages were also explored in combining with the dynamics of temperature and water environment monitoring at different stages.The ubiquity of seasonal freeze-thaw cycle, snow forming, snow covering, snow melting and soil freezing-thawing processes profoundly affected microbial groups, especially in water bodies. In the study, the microbial abundances in leaf/needle litter were lower in water bodies, which had large temporal and spatial variation and low in nutrition, than in land-forest habitat while the litter decomposed faster in water bodies. In the same natural habitat, each microbial abundance was significant different in different litter due to the heterogeneity in different litter. The highest microbial abundance was found in the litter with high substrate, S. paraplesia. Meanwhile, the microbial abundances had seen very different levels of impact from climate and environmental factors, which these impacts even weakened the influence from litter heterogeneity. Type of species of some litters did not affect the microbial abundances. In addition, by the method of real-time fluorescent quantitative PCR, in the same natural habitat and in the same species type of litter, the bacterial abundance was richer than the fungal abundance and more deeply affected by species type of litter and environmental factors, which illustrated that the bacteria played a role in early litter decomposition.The AAPB (Aerobic Anoxygenic Phototrophic Bacteria) in the litter was studied and the results showed that the relative number of AAPB was very low in overall bacterial groups (AAPB pufM gene abundance/bacterial 16S rDNA abundance< 0.2%). In different natural habitat, the change of AAPB abundances showed obvious seasonal dynamic feature, which was higher in the growing season with high temperature and lower in winter with low temperature and strong freeze-thaw cycles. Moreover, there was significantly positive correlation between AAPB abundances and dissolved organic carbon (P< 0.01) in all three aquatic habitats. However, the correlation degree between AAPB abundance and temperature in different aquatic habitats were different, which was significantly positive in both the stream and the riparian zone (P< 0.01), while not significantly in the river (P> 0.05). In addition, the changes of AAPB abundances in different litter and the differences of AAPB abundances response to the different habitats attributed to the heterogeneity in both the litter and the habitat.There were marked changes in the composition of microbial groups with the litter decomposition and all the changes showed obvious seasonal characteristic. On the one hand, in the stream and the river, with big spatiotemporal difference and low nutrition, bacterial Shannon-Wiener diversity indexes of litter were lower than in the land-forest habitat, and the changes had similar laws. It meant bacterial communities were affected especially by heterogeneity from different habitat. On the other hand, just in the river, there were significantly differences in the changes of bacterial Shannon-Wiener diversity indexes in different species type of litter, which showed the extent of impact from heterogeneity in the litter on bacterial communities were to grow. The cluster analysis indicated that the seasonal climate changes were the major factor influenced the bacterial communities in the water and soil, while the bacterial communities in the litter were mainly affected by both seasonal climate changes and litter species. The main bacterial species composition included Gordonia sp., Rhodococcus sp., Terrabacter sp., Alcaligenes sp., Bacillus sp., Mycobacterium sp., Pseudomonas sp., Sphingomonas sp., Saccharothrix sp., Streptomyces sp., Frankineae, Burkholderiales and so on during litter decomposition in two years. In the water bodies and the litter of the stream, the dominant bacterial group was Rhodococcus sp. and in both the water bodies and the litter of the river, the dominant status of Rhodococcus sp. received more prominent. This is due to environmental differences, which had a rapid reduction in relative abundance of Rhodococcus sp. in the water bodies and the litter of the riparian zone. Meanwhile, Bacillus sp., Pseudomonas sp. and some actinomycetes became the main dominant and in the soil and the litter of the forest, Bacillus sp. exhibited higher abundance and dominant status was more prominent. The results illustrated that the seasonal changes and heterogeneity among the habitats were the major factors influenced bacterial structure and composition, although the abundance and Shannon-Wiener diversity indexes were deeply affected by heterogeneity in the litter.The seasonal climate change, the freeze-thaw cycle, the physical and chemical factors of environment and heterogeneity in the litter of high-frigid forest all had greater influences on fungal community structure and diversity in different litter. These influences behaved much differently in the same habitat or not. Fungal diversity, affected by the similar in the aquatic habitats and fungal specific property of resistance to lower temperature than bacteria, showed the similar characteristics of change in both the water body and litter of all aquatic habitats. Although the litter decomposed more quickly in the aquatic habitats, fungal diversity was lower than in the land-forest habitat and showed larger fluctuations with the decomposition of the litter. It is mainly because there were more strong temporal and spatial variation in the aquatic habitats that the litter were broken down and materials were leached and lost. The cluster analysis also indicated that the seasonal climate changes was the major factor influenced the fungal communities in the water and soil, while the fungal communities in the litter were mainly affected by both seasonal climate changes and differences of litter species. The main fungal species composition included Pleosporales sp., Aspergillus sp., Rhizopus sp., Umbelopsis sp., Hanseniaspora, Cenococcum sp. Humicola sp., Massaria sp., cladosporium sp., some other Ascomycota and Zygomycota and so on during litter decomposition of two years. In the water bodies and the litter of the stream, the dominant fungal groups were Cenococcum sp. and Aspergillus sp., and Cenococcum sp. was more prominent during each period. In the water bodies and the litter of the river, the dominant fungal groups changed to Aspergillus sp., cladosporium sp., Massaria sp. and some other Ascomycota, Pleosporales r sp. was not detected. In the water bodies and the litter of the riparian zone, fungal species were more abundant, Pleosporales sp. increased largely, Rhizopus sp. and some other Zygomycota were detected at the same time. Among these species, Rhizopus sp. occupied the certain proportion in the litter, even was dominant in some period. In comparison with the aquatic habitats, fungal groups had higher diversity in the land-forest habitat. Pleosporales sp., Umbelopsis sp. and Aspergillus sp. were dominant and maintained during each period. Rhizopus sp. substantially reduced and detected only in the growing season. Based on the testing results, differences of litter species had deeply effects on fungal community composition.Like its abundances, AAPB Shannon-Wiener diversity indexes showed obvious seasonal dynamic feature, which were higher in the growing season with high temperature and lower in winter with low temperature and strong freeze-thaw cycles, especially in the stream and AAPB diversity were significantly affected by dissolved organic carbon. Because of the heterogeneity in the litter and the differences of habitats, each environmental factor had different effect on AAPB diversity. Just as bacteria and fungi, seasonal climate change was the major factor influenced AAPB communities in the water and soil, while the AAPB communities in the litter were mainly affected by both seasonal climate changes and litter species. The main AAPB species composition included Methylobacterium sp., Sphingomonas sp., Burkholderiales, Erythrobacter sp., Rhodopseudomonas sp., Rhodospirillum sp., Rubrivivax sp., Rhodovulum sp., Arctic spring bacterium sp., Microbacterium sp., Roseobacter sp., Halorhodospira sp., and so on, during litter decomposition of two years. In the water bodies and the litter of the stream, the dominant AAPB groups were Rhodospirillum sp., Rubrivivax sp., Rhodopseudomonas sp. and Burkholderiales. In the water bodies and the litter of the river, the dominant AAPB groups changed to Sphingomonas sp., Rhodopseudomonas sp. and Rubrivivax sp. In the water bodies and the litter of the riparian zone, Sphingomonas sp., Rhodopseudomonas sp. and Rubrivivax sp. remained dominant, and Sphingomonas sp. had higher proportion. In the land-forest habitat, Rubrivivax sp. was not detected, and Sphingomonas sp., Rhodopseudomonas sp. and Methylobacterium sp. took advantages.During two years of litter decomposition, microbe had significant contribution to litter decomposition. In the stream, there were remarkable (P< 0.05) or extremely remarkable correlation (P< 0.01) between the mass remaining rate and bacterial abundance of each litter species and fungal abundance, respectively. While in the other three habitats, the intensity of these correlations were weakened or not significant. And only in the stream, the mass remaining rate of S. paraplesia, Rhododendron lapponicum and Larix mastersiana, respectively, had significant correlation with AAPB abundance. Meanwhile, the results showed that there were extremely remarkable correlation (P< 0.01) between the mass remaining rate and fungal diversity of each litter species and AAPB diversity, respectively. While in the other three habitats, weakened or not significant. All the results indicated that in the stream, the litter decomposed faster than in the other habitats for greater impact from microbe. On the other hand, based on the heterogeneity in the litter, microbial abundance and diversity were higher in S. paraplesia which had higher quality, and then Salix paraplesia litter decomposed faster in each habitat.Above all, rich diversity of microbial groups were in different heterogeneous habitats of high-frigid forest ecosystem. During the decomposition of the litter, microbial abundance and community structure changed happening either in different litter of the same habitat or in the same species litter in the different habitat, affected by the combined effects from the seasonal climate changes, freeze-thaw cycle, and soil freezing process, differences in the habitats and heterogeneity in the litter. As it turns out, litter decomposition was affected comprehensively. All the results will be helpful to a deeper appreciation of the litter decomposition in the high-frigid forest ecosystem and its related microbial driven process.