Characteristics of stable isotopes in soil water and indication to soil water movement in a typical wetland of Poyang Lake

XU Xiu-li; LI Yun-liang; XING Zi-kang; CHEN Ting; CHU Xiao-dong

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China Environmental Science ›› 2025, Vol. 45 ›› Issue (5) : 2745-2756.

Characteristics of stable isotopes in soil water and indication to soil water movement in a typical wetland of Poyang Lake

XU Xiu-li^1,2, LI Yun-liang^3,4, XING Zi-kang^3,4, CHEN Ting⁵, CHU Xiao-dong⁵

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Abstract

Poyang Lake is characterized by significant water level fluctuations, leading to complex transformation processes among precipitation, soil water, and groundwater. Due to the limitations of intricate wetland conditions and traditional monitoring methods, it is challenging to conduct quantitative studies on soil water movement and its interaction with groundwater. In this study, three vegetation communities at different elevations in Poyang Lake were investigated to analyze the isotopic composition of precipitation, lake water, groundwater, and soil water (0~80cm). The characteristics of wetland soil water movement were examined across various hydrological periods. The results showed that the slope of the soil evaporation line (SEL) in the Artemisia capillaris community (5.91) was significantly lower than that of the local meteoric water line (LMWL, 7.60). The lc-excess values of soil water in 0~60cm layer were negative, indicating strong evaporation, with a maximum impact depth of 60 cm. The slopes of the SEL in the Phragmites australis and Carex cinerascens communities (6.70 and 6.75, respectively) were slightly lower than the LMWL, and the lc-excess values of soil water were close to 0, indicating minimal evaporation. Regarding soil water movement, the δ¹⁸O values of soil water in the A. capillaris community increased with depth during spring (May) and summer (June to August), indicating piston-flow dominated transport. During autumn (September and October), soil water δ¹⁸O values became enriched and decreased with depth, indicating the dominant influence of evaporation. Furthermore, the soil water δ¹⁸O values in the A. capillaria community were significantly enriched compared to groundwater isotopes. No depleted isotope signals or evidence of groundwater supply were detected in the soil water, even when the groundwater table was at its shallowest (1.92m). These results suggest that vertical hydrological connectivity between root-zone soil water and groundwater was blocked. In contrast, soil water movement in the P. australis and C. cinerascens communities was significantly influenced by groundwater level fluctuations. During the groundwater level rise period (April and May), shallow soil water (0~40cm) in these two communities primarily originated from atmospheric precipitation, while deep soil water (40~80cm) was replenished by capillary rise of groundwater. Groundwater contributed more than 50% to the replenishment of root-zone soil water. During the shallow groundwater table period (June and August), frequent exchanges occurred between soil water and groundwater in the P. australis community. In the groundwater table decline period (September and October), the P. australis and C. cinerascens communities exhibited non-uniform soil water flow processes, characterized by noticeable preferential flow.

Key words

floodplain environment / groundwater recharge / Poyang Lake wetland / preferential flow / soil water movement / stable isotope

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XU Xiu-li, LI Yun-liang, XING Zi-kang, CHEN Ting, CHU Xiao-dong. Characteristics of stable isotopes in soil water and indication to soil water movement in a typical wetland of Poyang Lake[J]. China Environmental Science. 2025, 45(5): 2745-2756

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