Effects Of Seasonal Freeze-Thaw On Soil Enzyme Activities In The Subalpine Forest

Seasonal freezing and thawing event is a common natural phenomenon in the middle-high latitudes and high elevation area. The ecosystem process has four critical periods including early freezing period, freezing period, soil thawing period, and growing period in these regions which are strongly subjected to seasonal snow cover and freeze-thaw cycles. However, more attentions have been given to the ecological process in the growing season. As an active participant in soil biochemical process and important component in soil ecosystem, soil enzymes play an important role in regulating ecosystem bioelement cycles to an extent. As yet, more attentions have concerned about the dynamics of the activities of enzymes in mineral soil during the growing season, and little information are. available on the changes in enzyme activities in both mineral soil layer and soil organic layer during other critical periods, which limits our understanding of the process in the high-frigid ecosystem. The subalpine and alpine forest in western Sichuan located in the eastern Tibet plateau and the upper reaches of Yangtze River is a middle-latitude and high-elevation region subjected to clear snow cover and frequent freeze-thaw cycles, and plays important and irreplaceable roles in nursing biodiversity, holding water, conserving soil, and indicating global climate change. In order to deeply understand the effects of season freezing and thawing on soil biochemical process in the high-frigid forest ecosystem, therefore, the activities of invertase, urease, acid phosphatase, neutral phosphatase, alkaline phosphatase, peroxidase, catalase and dehydrogenase, which are related with the cycles of carbon, nitrogen and phosphorus in both mineral soil layer and soil organic layer were simultaneously measured in primary fir (Abies faxoniana) forest, fir and birch (Betula albosinensis) mixed forest and secondary fir forest which are three representative forests in the study region, employing the method of field sampling in combination with laboratory analysis in four critical periods.The activities of the studied enzymes in soil organic layer were significantly higher than those in mineral soil layer regardless of the forest and sampling time. The changes of enzyme activities in both organic layer and mineral soil were more significant during early freezing period and soil thawing period which were frequently subject to freeze-thaw cycles in comparison with other periods. The activities of enzymes in soil organic layer were more sensitive to environmental change than those in mineral soil layer. However, little changes were found in deep soil freezing period.The invertase activity in the three sampling forests of soil organic layer shown significant decreased during early freezing period, late soil thawing period, early growing period and late growing period, and shown significant increased during early soil thawing period and middle growing period. Which of mineral soil layer shown significant decreased during late soil thawing period, and shown significant increased during early soil thawing and growing period. However, which shown little change during freezing period of both soil organic layer and mineral soil layer.The urease activity in the three sampling forests of soil organic layer shown significant increased during early freezing period, early soil thawing period and late soil thawing period, and shown significant decreased during middle soil thawing period. Which of mineral soil layer shown significant increased during early soil thawing period and late soil thawing period, and shown significant decreased during middle soil thawing and early growing period. However, which shown little change during freezing period of both soil organic layer and mineral soil layer.The acid phosphatase activity in the three sampling forests of soil organic layer shown significant increased during early freezing period, early soil thawing period, late soil thawing period and early growing period, and shown significant decreased during middle soil thawing period and late growing period. Which of mineral soil layer shown significant increased during early freezing period, early soil thawing period and early growing period, and shown significant decreased during middle soil thawing period. However, which shown little change during freezing period of both soil organic layer and mineral soil layer.The neutral phosphatase activity in the three sampling forests of soil organic layer shown significant increased during early freezing period, middle soil thawing period and late soil thawing period, and shown significant decreased during early soil thawing period and late soil thawing period. Which of mineral soil layer shown significant increased during early freezing period, and shown significant decreased during early soil thawing period and middle soil thawing period. However, which shown little change during freezing period of both soil organic layer and mineral soil layer.The alkaline phosphatase in the three sampling forests of both soil organic layer and mineral soil layer shown significant increased during early soil thawing period and early growing period, and shown significant decreased during middle soil thawing period and late growing period. However, which shown little change during freezing period.The peroxidase activity in the three sampling forests of soil organic layer shown significant decreased during early freezing period and growing period, and shown significant increased during early soil thawing period and late soil thawing period. Which of mineral soil layer shown significant decreased during early freezing period, middle soil thawing period and growing period, and shown significant increased during early soil thawing period and late soil thawing. However, which shown little change during freezing period of both soil organic layer and mineral soil layer.The catalase activity in the three sampling forests of soil organic layer shown significant increased during early freezing period, early growing period, late growing period and early growing period, and shown significant decreased during middle soil thawing period and late growing period. Which of mineral soil layer shown significant increased during early freezing period and early soil thawing period, and shown significant decreased during middle soil thawing period. However, which shown little change during freezing period of both soil organic layer and mineral soil layer.The dehydrogenase activity in the three sampling forests of soil organic layer shown significant decreased during early freezing period, middle soil thawing period, late soil thawing period and late growing period, and shown significant increased during early soil thawing period and early growing period. Which of mineral soil layer shown significant decreased during early freezing period, and shown significant increased during early soil thawing period. However, which shown little change during freezing period of both soil organic layer and mineral soil layer.