Distribution of hydrogen and oxygen stable isotope of water in soil-plant-atmosphere continuum (SPAC)system of a typical forest area

Abstract

Water cycle in the soil-plant-atmosphere continuum (SPAC) is an important research topic in hydrology and ecology. The differences in the composition of hydrogen and oxygen stable isotopes in different water bodies can indicate water cycle process. Based on the measurements of isotopic compositions in precipitation, soil water, and plant water, we analyzed water isotope evolution in a SPAC system located in a subtropical evergreen broad-leaved forest in Chengdu Plain. The different interface processes of regional water cycle was revealed. The results showed that the local meteoric water line (LMWL) equation was δD=7.13 δ18O+2.35 (R2=0.99), and the soil evaporation line (SEL) equation was δD=6.98δ18O-0.32 (R2=0.92). In the water transportation process of precipitation-soil water-plant water, hydrogen and oxygen isotopes were gradually enriched. The δ18O in water of the surface soil layer (0-35 cm) was sensitive to precipitation input, as it was directly affected by precipitation. In contrast, the δ18O in water of the middle and deep layers (35-100 cm) was relatively stable. The isotopes of plant xylem water were slightly more enriched than those of soil water, indicating the possibility of slight evaporation or transpiration through phloem or bark in plant water transportation. The estimation of plant water intake from different soil layers was performed by direct correlation method. Cinnamomum camphora mainly used water from the middle layer, Broussonetia papyrifera mainly used that from the surface layer, and Parathelypteris glanduligera tended to use surface soil water and precipitation intercepted by plants because of the shallow root system. Compared with P. glanduligera, C. camphora and B. papyrifera experienced stronger water evaporation and the isotopes were influenced by more intense dynamic fractionation.