Abstract: Soil temperature is an important environmental factor in forest, which not only closely relates with plant growth, but also influences soil respiration. An optimal soil temperature can improve soil fertility via promoting soil microbial activity and litter decomposition. Therefore, the study on soil temperature will be helpful to understand the changes of carbon flux in forest ecosystem. Based on the data from 2003 to 2006 in a tropical seasonal rain forest in Xishuangbanna, the diurnal, seasonal, and inter-annual variations of soil temperature at different depths were analyzed, and the results indicated that the diurnal variation of soil temperature in near-surface soil layer demonstrated a sine trend, while that in deeper soil layers was relatively stable. With the increase of soil depth, the variation magnitude of soil temperature decreased, the peak of soil temperature lagged behind, and the phase change was obvious. The seasonal variation of soil temperature was obvious too, with the highest value in rainy season (May to October), lowest value in foggy-cool season (November to the next February), and intermediate value in dry-hot season (March to April). Different soil layers were different in the seasonal variation of soil temperature. The inter-annual variation of soil temperature was a single-peak curve, with the lowest temperature in January and the highest temperature in July. Generally speaking, the variation range of soil temperature was decreased in the order of yearly soil temperature>inter-annual soiltemperature>soil temperature among different soil layers>diurnal soil temperature.

Key words: Acid sulfate soil, Sulfur forms, Transformation