grandis in mowing grassland tended to take up more water from the upper soil layers following precipitation events, but showed no sensitive change in water source from soil profile following the precipitation in the enclosed grassland, indicating a more sensitive change of soil water sources for the two species in mowing than enclosed grassland. grandis took up water from the middle to deep soil layers. chinensis took up relative more water from deep soil layer, and S. squarrosa took up water majorly from top soil layer due to its shaollow root system L. The results showed that (1) mowing had no significant effect on the soil moisture and its δ 18O, whereas precipitation significantly changed the soil moisture though no significant effect detected on its δ 18O. The oxygen stable isotope ratios (δ 18O) of soil water and stem water of these three species were determined, along with soil moisture, before and after precipitation events. We studied the water use sources of three coexisting dominant plant species Leymus chinensis, Stipa grandis and Cleistogenes squarrosa growing in both enclosed and mowing grassland in a typical steppe. Understanding of the dynamic patterns of plant water use in a changing environment is one of foci in plant ecology, and can provide basis for the development of best practice in restoration and protection of ecosystem.
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