Microbial Activity During Litter Decomposition As Affected By Seasonal Freeze-thaw Cycle In The Subalpine/Alpine Forests Of Western Sichuan

Microorganism is one of the dominate factors during litter decomposition. Many previous studies have documented that low temperature in winter could cause microbial in dormancy and even death, and thought that the litter decomposition in winter could be negligible. However, recent results have showed that microorganisms still keep activity under snow cover in high-altitude or high elevations where are affected obviously by seasonal snow cover and freeze-thaw cycles. The existed microorganisms did important effects on soil organic matter and litter decomposition. Meanwhile, the effects of snow cover and freeze-thaw cycles on litter quality would influence the characteristics of microbial substrates in next phase, thus regulate the microbial activity during litter decomposition process in the growing season. However, the relevant research is very limited. Therefore, in order to characterize the dynamics of microbial activity in litters of the most widely distributed eastern edge of Qinghai-Tibet Plateau, microbial biomass carbon (MBC), microbial biomass nitrogen (MBN), microbial biomass phosphorus (MBP), microbial count and enzyme activity were investigated during the decomposition of fir (Abies faxoniana), birch (Betula albosinensis) and spruce (Picea asperata) litter by litterbag method. The results showed that:(1) Soil temperature of four monitoring forest communities in all represented significant dynamic change during monitoring, the soil experienced significant freeze-thaw alternate, freezes and melting process. During two years, the soil average temperature of four altitudes in August was the highest, and in Jan. was the minimum. The duration of the second seasonal freeze-thaw period was lower than the first seasonal freeze-thaw period at four altitudes. The second freeze-thaw period four elevations of the basic natural freeze-thaw cycle times for zero, but its melting freeze were higher than the natural freeze-thaw cycle times with altitude first melts freeze. Meanwhile, the characteristics of soil accumulated temperature in the four altitudes during two seasonal freeze-thaw period respectively showed as:A4 A3>> Al, A2>> A2> Al. A4 A3 (2)Throughout the study period, microbial biomass carbon (MBC) of the 3 kinds of litters in the growing season was significantly higher than non-growing season. Among the 4 critical periods, MBC in red birch litter was significantly higher than and spruce litter in the soil thawing period, the growing season and soil freezing and thawing period, but during the soil freezing period, MBC among different litters had no significant differences. Growing season, red birch and spruce litters MBN were significantly higher than non-growing season. Throughout the study, MBP in red birch litter were higher than the other litters. The MBP of the four elevations among 3 litters showed the same variation during the second seasonal freezing-thawing season.(3) The first seasonal freeze-thaw period, the count bacterial community during litter decomposition several more, and the count fungi community during litter decomposition was high in the second seasonal freeze-thaw. During the litter decomposition bacteria in three litters during second freeze-thaw season were reduced by an average:85.38%(AF), 58.82%(BF) and 70.08%(PF). The fungi are, respectively, year on year average increase: 670.65%(AF),18968.20%(BF) and 522.97%(PF).(4) Throughout the study, the invertase activity in red birch litter was significantly higher than that in fir and spruce, and there was no significant difference between fir and spruce. Changes of invertase activity during three kinds of litter decomposition were basically the same. The invertase activity during the second seasonal freezing-thawing period reduced by:-296.67%-50.96%(AF),-161.37%-50.59%(BF) and-962.53% 38.30%(PF) than first. Overall, during the second freeze-thaw urease activity in fir and spruce litters compared with the first high, and red birch litter was basically the first thawing period below. Urease activity during the second seasonal freezing-thawing period was increased by:-77.81%～368.59%(AF),-77.49%～144.58%(BF) and～66.29% 758.59%(PF). The activity of phosphatase during the second seasonal freezing-thawing period was increased by:-39.51%～82.89%(AF),-43.04%～92.45%(BF) and-13.73%～92.93%(PF).The results provided important data in understanding the natural decomposition of litter in field, exploring the ecological linkages between growing season and non-growing season, and revealing material cycling and energy flux in alpine and subalpine forest ecosystems.