Effects Of Seasonal Freeze-thaw On Soil Microbial Activity In The Subalpine Fir Forest

Seasonal freezing-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. The soil microorganisms are compositions and main decomposers of ecosystem, and promote material cycle and energy flow in forest ecosystem. So the understanding of the effect of seasonal freezing-thawing events on soil microbiological activity is becoming a key scientific issue of global climate change on forest ecosystems in seasonal unsaturated frozen soil areas. So far, the studies of freezing and thawing events on soil microorganisms is focus on soil nutrient, soil particle composition, and the effect of freezing-thawing events on microbial diversity, the microbial activity in the key process of forest ecosystem, and the responses of microbial community to freezing-thawing events on the background of global climate's wanning were not well known, which made difficult to understand the climate change. Therefore, the selective inhibition techniques was carried out in the fir (Abies faxoniana) forests which distribute widely in subalpine region of Western Sichuan in order to obtain an understanding of the effects of freeze-thaw events on soil microbial activity. Correspondingly, the microbial biomass, major microorganism number and soil enzyme activity were measured monthly 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.Freezing-thawing events could significantly decreased the content of soil microbial biomass carbon (WSOC). Results showed that the content of WSOC decreased after one to three times of freezing and thawing cycles at -2℃, and the content of WSOC in the humus layer, illurial layer and meterial layer was decreased 18%, 26% and 16%, respectively. Meanwhile, the content of WSOC decreased after one to three times of freezing and thawing cycles at -5℃, and the content of WSOC in the humus layer, illurial layer and meterial layer was decreased 20%,35% and 24%, respectively. This is mainly due to short-term freezing-thawing events destroy microbial cells, as resulted to reduce the content of soil microbial biomass carbon. However, the results revealed that the soil microbial biomass nitrogen WSON was not affected by freezing-thawing events. The content of WSON in the humus layer and illurial layer increased 0.7% and 0.7% after one to three times of freezing and thawing cycles at -2℃, and the content of WSON in meterial layer was decreased 6%. Meanwhile, the content of WSON increased after one to three times of freezing and thawing cycles at -5℃, and the content of WSON in the humus layer, illurial layer and meterial layer was increased 6%,8% and 9%, respectively.Over one seasonal freeze-thaw period, the soil microbial biomass decreased with decreasing altitude. The number of bacteria and fungi increased with decreasing altitude. But the number of actinomyces in the humus layer increased with decreasing altitude, and the number of actinomyces in the illurial layer and meterial layer was decreased with decreasing altitude.Over one seasonal freeze-thaw period, the number of bacteria and actinomyces decreased significantly, and fungi increased. The number of bacteria in the humus layer, illurial layer and meterial layer was decreased 25%, 21% and 10%, respectively. Meanwhile the number of actinomyces in the humus layer, illurial layer and meterial layer was decreased 24%, 15% and14%,respectively. However, the number of fungi in the humus layer, illurial layer and meterial layer was decreased 24%,15% and 14%, respectively. The results indicated that seasonal freezing-thawing events influenced greatly on the structures of microbial community composition in the subalpine forest ecosystem.Over one seasonal freeze-thaw period, the enzyme activities of urease (URE) and invertase(INV) increased significantly, the activities of peroxidase (POD) and polypehnol oxidase (PPO) decreased significantly. Similarly, the laboratory experiments to test the effect of freezing-thawing events on soil enzyme activities revealed that nine freezing-thawing cycle increased the activities of peroxidase (POD), polypehnol oxidase (PPO), urease (URE) and invertase (INV) increased. In brief, seasonal freeze-thaw gave a significant effect on soil enzyme activities. The results are helpful to deeply understand the effect of seasonal freezing and thawing events on the process of the high-frigid forest ecosystem.