Abstract: Hydrogen and oxygen isotope tracing is an important means in studying hydrological cycle in soilplantatmosphere continuum (SPAC), while the extraction of soil and plant water is the critical step to determine the stable isotope composition of the water. This paper compared the efficiency of azeotropic and vacuum distillation in extracting the water from three soil types (red clay soil, red sandy soil, and paddy soil) with three different water contents (35%, 25%, and 15%) and from the stems and leaves of two plants (orange tree and rice), and analyzed the stable hydrogen and oxygen isotopes in the extracted water. As compared with azeotropic distillation, vacuum distillation had a significantly higher water extraction efficiency (P<0.001). Soil type and soil water content had minor effects on the water extraction efficiency, but the extraction efficiency was significantly higher for rice than for orange tree, and for the leaves than for the stems of a given plant (P<0.001). No significant differences were observed in the δ¹⁸O and δD between the water samples from the soils extracted by vacuum distillation and the standard samples, but a significant difference was found in the δD when azeotropic distillation was applied (P<0.001). For the two plants, the δD and δ¹⁸O of the leaves were higher than that of the stems. It was suggested that as compared with azeotropic distillation, vacuum distillation could be more suitable to extract the water from soils and plants, and the extracted water could better reflect the stable isotope composition in the samples.

Key words: forest conversion, natural forest, plantation forest, soil CO₂ flux, water soluble organic carbon.