| Seasonal freezing and thawing event is a common natural phenomenon in the middle and high latitude or high-elevation area,which will be certain to give a strong effect on the regional ecosystem process directly or indirectly.Meanwhile,fine root decomposition,as an important forest ecosystem process plays an important and irreplaceable role in mass cycle and energy flow of the forest ecosystem,and is controlled by integrated biotic and abiotic factors.However,fine root decomposition described in most references were carried out at 3～25℃temperature in the past decades,and few information are available on the effect of seaeonal freeze-thaw events on fine root decomposition and biochemical property in the process of fine root decomposition,which leads to neglect the most important processes in the forest ecosystem of the middle and high latitude or high-elevation region.A simple f'me root decomposition experiment with seasonal freeze-thaw treatments was therefore carried out in the fir(Abies faxoniana Rehder & E.H. Wilson)(FF) and birch(Betula platyhylla Sukaczev)(BF) which distribute widely in the eastern Tibet in order to obtain an understanding of the effects of seasonal freeze-thaw events on fine root decomposition and their biochemical process in the subalpine forest ecosystem.Correspondingly,the rates of mass loss and nutrient release,microbial count and enzyme activity in fine root of the decomposition process were measured monthly and simultaneously to well understand the effects of seasonal freeze-thaw events on the subalpine forest ecosystem.During one seasonal freeze-thaw period,mass loss of fir fine root with 0～1mm and 1～2mm diameters were 20.48%and 16.50%,and those of birch fine roots were 14.50% and 17.47%,respectively.Correspondingly,the decomposition rates of fir fine roots with 0～1mm and 1～2mm diameters and with seasonal freeze-thaw treatments were 22.76%and 23.19%in one growing season,respectively,which were lower than those with no seasonal freeze-thaw treatments.Similarly,mass loss of birch fine roots with 0～1mm and 1～2mm diameters and with seasonal freeze-thaw treatments were 25.91%and 21.30%,and the corresponding values with no seasonal freeze-thaw treatments were 22.44%and 23.60%, respectively.The results showed that seasonal freeze-thaw event influenced greatly on the decomposition of fine root in the subalpine forest.Seasonal freeze-thaw event had influenced greatly on the bioelement releases in the fine root of the subalpine forest.During one seasonal freeze-thaw period,the release rates of C,N,P and K in the fir fine root with 0-1 mm diameter were 27.30%,5.61%,14.36% and 42.73%,and the corresponding values with 1-2mm diameter were 21.94%,30.53%, 25.40%and 38.98%,respectively.The corresponding values in the birch fine root with 0-1mm diameter were 20.54%,36.03%,20.77%and 38.88%,and the corresponding values with 1-2 mm diameter were 21.88%,7.78%,22.21%and 36.42%,respectively. Consequently,calculated by the fine root biomass in the corresponding forest,over one freeze-thaw season the fir fine root with 0-1mm diameter released 0.416 kg·hm-2 of N, 0.196 kg·hm-2 of P and 3.656 kg·hm-2 of K into the soil,and the corresponding values with 1-2mm diameter were 0.89 kg·hm-2 of N,0.13 kg·hm-2 of P and 1.27 kg·hm-2 of K, respectively.Similarly,the corresponding values of birch fine root with 0-1mm diameter were 42.25 kg·hm-2 of C,1.81 kg·hm-2 of N,0.074 kg·hm-2 of P and 1.31 kg·hm-2 of K,and the corresponding values with 1-2mm diameter were 0.11 kg·hm-2 of N,0.031 kg·hm-2 of P and 0.58 kg·hm-2 of K,respectively.The results implied that nutrients released by fine root in the freeze-thaw season were of ecological significance on the plant growth in subalpine forest in spring.The release rates of C,N,P and K in 0-1mm fir fine root suffered from one seasonal freeze-thaw period were 42.54%,21.97%,52.59%and 55.57%during the growing season, and the corresponding value with 1-2mm diameter were 44.46%,-14.90%,47.49%and 63.72%,respectively.The release rates of C,N,P and K in 0-1mm birch fine root through one seasonal freeze-thaw period were 42.24%,21.22%,52.06%and 68.75%during the growing season,respectively,and the corresponding values with 1-2mm diameter were 38.15%,14.02%,42.75%and 62.69%,respectively.On the whole,the release rates of C,N, P and K in fine root went through seasonal freeze-thaw were lower than those with no seasonal freeze-thaw treatments regardless of the tree and fine root diameter,resulting from that the changes in fine root quality caused by seasonal freeze-thaw event.In brief, seasonal freeze-thaw gave a significant effect on the bioelment releases in fine root of the subalpine fortest. In the growing season,microbial counts of bacterial,fungal and actinomycete in fir and birch fine roots went through seasonal freeze-thaw event were higher than those not through seasonal freeze-thaw event,implying that seasonal freeze-thaw event influenced greatly on microbial growth in the fine root of the subalpine forest.Meawhile,the activities of cellulase and polyphenol oxidase in fine root with seasonal freeze-thaw treatment were higher than those with no freeze-thaw treatment,while the peroxidase activity varied with the fine root diameter.Similarly,seasonal freeze-thaw treatment also increased the activity of cellulase in birch fme root in comparison with the fine root with no seasonal freeze-thaw treatment,while the activities of polyphenol oxidase and peroxidase varied with the root diameter.Therefore,the biochemical property in fine root of the decomposition process was influenced to a certain extent by seasonal freeze-thaw event in the subalpine forest.Organic carbon content in the soil only with no litter input was higher 39.493 g·kg-1 and 14.912 g·kg-1 than that in the soil with no litter and free root inputs in the fir and birch forest,respectively.Fir and birch fine root turnover contributed 564.755g·m-2 and 195.337 g·m-2 to soil organic carbon pool over one growing season,respectively.
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