[Hydrogen and oxygen stable isotope characteristics of water in SPAC system of evergreen broadleaved forest in subtropical region]

Abstract

Forest soil-plant-atmosphere continuum (SPAC) is an important continuous process of water cycle. In this study, we analyzed hydrogen and oxygen stable isotope compositions of the precipitation, atmospheric water vapor, soil water, branch water, and leaf water to explain the characteristics of the continuous process and the associated controlling factors in a subtropical evergreen broadleaved forest. The results showed that the regression equations between hydrogen and oxygen stable isotopes were δD_P=7.97δ¹⁸O_P+12.68(R²=0.97) for precipitation, δD_S=4.29δ¹⁸O_S-18.62(R²=0.81) for soil water, δD_B=3.31δ¹⁸O_B-29.73(R²=0.49) for branch water and δD_L=1.49δ¹⁸O_L-10.09(R²=0.81) for leaf water of Podocarpus nagi, δD_V=3.89δ¹⁸O_V-51.29(R²=0.46) for atmospheric water vapor, respectively. In the process of water transport from precipitation to soil water to plant water, hydrogen and oxygen isotopes gradually enrich, while water vapor isotopes evaporated from soil and evaporated from plants were depleted. With the influence of precipitation and evaporation, soil water isotopes depleted with depth, and more enriched in the dry season than that in the rainy season. During the observation period, isotope content of branch water was slightly higher than that of soil water, indicating that water might be enriched by transpiration during the transportation process in plants. In the dry season, water isotope in branches of arbor plants was poorer than that of shrubs, indicating that arbor plants with deeper root distribution tended to use deep soil water. Because of the differences of leaf traits, transpiration rate and the response degree of environmental factors, the variation characteristics of water isotope composition in leaves of diffe-rent plants varied with the increases of leaf age. Environmental conditions in rainy season were more conducive to leaf transpiration, which made leaf water isotope enriched in rainy season than in dry season. The isotopic compositions in leaf water of Podocarpus nagi had a positive relationship with leaf water content (LWC), and a negative one with relative humidity, reflecting the water regulation function of plants in response to environmental changes.

Keywords: environmental factor; hydrogen and oxygen stable isotopes; leaf age; soil-plant-atmosphere continuum (SPAC); transpiration.