Abstract: Based on the 1980〖KG-*2〗-〖KG-*7〗2005 observation data of ten-day soil humid ity, monthly air temperature, and monthly precipitation during crop growth perio d (from May to September) from 16 agro-meteorological stations on Songnen Plain , the spatiotemporal variation of surface soil (0〖KG-*2〗-〖KG-*7〗30 cm) humid ity and its responses to the air temperature and precipitation in study area wer e analyzed by using statistical method. The results showed that on Songnen Plain , the surface soil humidity during crop growth period in 1980〖KG-*2〗-〖KG-*7〗 2005 had a decreasing trend, and tended to be aridified, especially in the west and south parts of the Plain. Before the 1990s, the surface soil on the Plain was r elatively humid, but thereafter, the humidity kept decreasing, and partial dryne ss occurred. In the study period, the mean air temperature during crop growth pe riod had a periodical increase before 1992, with a small fluctuation in a cycle of 6 years, but increased obviously after then. The precipitation during crop gr owth period had a larger inter-annual variation, with a cycle of 4〖KG-*2〗-〖K G-*7〗5 years. The surface soil humidity during crop growth period was significa ntly negatively correlated to air temperature and positively correlated to preci pitation. Both air temperature and precipitation were the main climatic factors affecting the variation of surface soil humidity on Songnen Pl ain.

Key words: stable isotope, water source, water stress., degeneration, Populus simonii