Abstract: Soil water potential was determined during April 2006 andMay 2007 at a limestone tropical seasonal moist rainforest site in Xishuangbanna, Southwest China. The results indicated that soil is drier in dry season ( from November to April ) than in rainy season (from May to October) , especially inMarch and April , with soil moisture content reaching the minimum value of - 0.055MPa at 75 cm depth. While in rainy season , soil humidity situation is much better than dry season, and soil water potential can reach to 0 . 010MPa . Hourly observation of soil volumetric water content on three days in
November 2006 , January 2007 and March 2007 showed that the difference between diurnal values and nocturnal values is not significant ( P > 0 . 05) , suggesting there is no obvious soil volumetric water content fluctuation in deferent soil depths . In other words , there is no hydraulic redistribution in limestone tropical seasonal moist rainforest . Intercepted fog drip and
throughfall by canopy, rainwater , soil water , groundwater and stem xylem water were collected during November 2005 and February 2007 for stable isotopic analysis . The stable hydrogen (δD) and oxygen isotope composition (δ18O) of fog drip ,throughfall, rainwater, soil water, groundwater and stem xylem water were determined from an isotope ratio mass spectrometer
. The results indicated that the rainfall is the main source of soil water and groundwater in rainy season . Through the canopy, throughfall is enriched than rainwater, because of evaporated fractionation. The fog drip was thought to contain water that has been evaporated and recycled terrestrial meteoric water (evaporation of nearby surface waters and canopy interception) , so the fog drip was more enriched than rain during both the dry season and rainy season . What′s more , although fog is heavy , plants mainly use groundwater and deep soil water in dry season in this region , hardly making use of fog drip. This result is related to the natural environment, limestone tropical seasonal moist rainforest is very vulnerable,
sothat , protect this pattern forest is necessary , further research will be able to provide a basis for this purpose .

Key words: Limestone tropical seasonal moist rainforest