| Seasonal freezing and thawing event is a common natural phenomenon in middle and high latitude area, while changes in seasonal freezing and thawing caused by global warming will be certain to act on forest ecosystem directly or indirectly and in turn give a strong effect on the process of the forest ecosystem. Meanwhile, as one of the most important forest processes, litter decomposition is controlled by integrated biotic and abiotic factors. However, litter decomposition in most references were carried out at 3-25℃temperature over the past decades, and mainly emphasized the litter decomposition in the growing season and the responses to litter quality and environmental factors, while little information was available on the effects of changes in seasonal freezing and thawing on the process and the related microbial activity of litter decomposition in the high-frigid subalpine forest, which was difficult to meet needs of understanding the processes of high-frigid forest ecosystem in the future. Therefore, a simple litter decomposition trial with one natural seasonal freeze-thaw treatment was carried out in the fir (Abies faxoniana Rehder & E. H. Wilson) (FF) and birch (Betula platyhylla Sukaczev) (BF) forests which distribute widely in the eastern Tibet in order to obtain an understanding of the effects of seasonal freeze-thaw events on litter decomposition in the high-frigid subalpine forest ecosystem. Correspondingly, the rates of mass loss and bioelement release, microbial biomass and the activities of enzymes in litters were measured monthly and simultaneously in the process of litter decomposition to well understand the effects of seasonal freeze-thaw events on the process of the forest ecosystem. The results are helpful to deeply understand the effect of seasonal freezing and thawing events on the process of the high-frigid forest ecosystem.Over one seasonal freeze-thaw period, litter decomposition rates of fir and birch forests were 18.9% and 20.2%, respectively. Correspondingly, the litter decomposition rates with seasonal freeze-thaw treatments were 8.0% and 9.3% in one growing season, respectively, which were significantly lower than the rates of mass loss in fir and birch litters (the control) with no seasonal freeze-thaw treatments, which were 23.6% and 25.5%, respectively. The results revealed that seasonal freeze-thaw event had significant effects on litter decomposition in the subalpine forest, resulting from the changes in litter quality such as the decreases of nutrient contents and the increases of lignin content and ratios of C/N, L/N and C/P in litters over one seasonal freeze-thaw period.During one freeze-thaw period, the release rates of C, N, P, K, Ca and Mg in the fir litter were 15.0%, 34.1%, 17.0%, 22.8%, 20.1% and 36.3%, and the corresponding values in the birch litter were 20.7%, 29.4%, 15.7%, 16.8%, 21.3% and 20.5%, respectively. Consequently, calculated by the leaf litter production in the corresponding forest, leaf litter released 10.17 kg·hm-2 and 5.61 kg·hm-2 of N, 0.68 kg·hm-2 and 0.34 kg·hm-2 of P, 4.08 kg·hm-2 and 1.21 kg·hm-2 of K, 0.461 kg·hm-2 and 0.300 kg·hm-2 of Ca, and 0.092 kg·hm-2 and 0.051 kg·hm-2 of Mg into soil in fir and birch forests over one freeze-thaw season, respectively, which were of ecological significance on the plant growth in the subalpine forest during the soil thawing period.The release rates of C, N, P, K, Ca and Mg in the fir litter after one seasonal freeze-thaw period were 16.3%, 22.3%, 11.2%, 27.1%, 28.1% and 2.3% in the growing season, and the corresponding values were 19.7%, 12.5%, 26.5%, 25.8%, 24.3% and 4.0% in the birch litter, all of which were significantly lower than the release rates of bioelments in the control litters with no seasonal freeze-thaw treatments, resulting from that seasonal freeze-thaw changed the litter quality such as the decreases of nutrient contents and the increases of lignin content and ratios of C/N, L/N and C/P in litters. In brief, seasonal freeze-thaw gave a significant effect on bioelment releases in the process of litter decomposition.Over one seasonal freeze-thaw period, lignin content increased 7.3% and 5.8%, and the lignin degraded 12.5% and 13.1% in fir and birch litters, respectively. Meanwhile, cellulose content deceased 5.9% and 8.3%, and the cellulose degraded 18.7% and 21.9% in the corresponding litters, respectively. Lignin content increased 7.9% and 7.1% in fir and birch litters, and cellulose content in the corresponding litter decreased 10.9% and 14.4% in comparison with the initial value over one growing season. Moreover, the lignin and cellulose degraded 7.9% and 14.9% in the fir litter and 8.5% and 15.7% in the birch litter, respectively. The results indicated that seasonal freeze-thaw event influenced greatly on litter degradation in the subalpine forest ecosystem.Seasonal freeze-thaw had significant effects on dynamics of microbial biomass carbon (MBC) and nitrogen (MBN) in the process of litter decomposition. MBC and MBN in the fir litter were more than 12.7% (ranging from 10.4% to 19.0%) and 107.8% (ranging from 60.1% to 374.0%) in comparison with the control with no seasonal freeze-thaw treatment, respectively. Similarly, MBC in the birch litter was more than 28.7% (ranging from 8.1 % to 63.6%), but MBN was less than 55.9%(ranging from 37.0% to 65.3%) in comparison with the control. Therefore, it was suggested that seasonal freeze-thaw influence greatly on micrbial acitivity in fir and birch litters of their decomposition processes.Seasonal freeze-thaw had influenced on the activities of enzymes in litters to a certain extent, depending on enzymes, litters and sampling times. The activities of peroxidase (POD) and dehydrogenase (DAH) in the fir litter with seasonal freeze-thaw treatment were lower than 5.7% and 9.0% in comparison with the control, respectively, while those of invertase (INV), polypehnol oxidase (PPO), urease (URE), and cellulose (CEL) were higher than 66.1%, 5.3%, 31.5% and 15.0%, respectively. Similarly, those of DAH, PPO and CEL in the birch litter with seasonal freeze-thaw treatment were lower than 9.0%, 7.6% and 28.9%, while those of POD, INV and URE were higher than 20.0%, 9.5% and 32.6% in comparison with the control, respectively.
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