Variations of water sources for a typical mesophyte vegetation in the Lake Poyang wetland using stable isotopes

Abstract

水分是维持植物生长、决定种群分布的关键因子，研究植物水分利用来源是揭示水文过程与植被演替作用机制的基础，作为中国最大的淡水湖泊系统，鄱阳湖水文情势的显著改变已直接影响到湿地生态系统的水分补给来源.本文通过测定降水、土壤水、地下水、湖水和植物茎水中δ¹⁸O、δD同位素组成，识别鄱阳湖湿地典型中生植被——茵陈蒿（Artemisia capillaris）群落的土壤水分补给来源，并应用直接对比法和IsoSource多源混合模型估算优势种茵陈蒿的主要吸水区间及水源利用比例.结果发现：（1）与降水同位素相比，湖水和湿地土壤水同位素较为富集，地下水同位素较少发生分馏；（2）湿地地下水主要受历史长期降水和湖水共同补给，土壤水在雨季4—6月和秋季9—10月主要受降水补给，夏季7—8月深层土壤水受湖水侧向入渗和地下水的共同补给，并在蒸发作用下水分向浅层土壤传输；（3）茵陈蒿主要利用0~80 cm深度的土壤水，且能够在不同土层水源间灵活转换.当土壤含水量较高时（4—5月），主要利用0~40 cm浅层土壤水，利用率约49%~68%；当浅层土壤含水量较低时（6—8月），主要利用40~80 cm深层土壤水，利用率高达74%~95%；当植物进入生长后期（9—10月），主要利用0~15 cm表层土壤水，利用率介于41%~70%.总体发现，湖水是鄱阳湖湿地中生植物群落土壤水分的重要补给来源，优势种茵陈蒿能够响应土壤含水量的变化改变吸水深度，具有较强的干旱适应能力.研究结果可为鄱阳湖湿地植被生态系统演变和科学保护提供理论参考.;Water availability is a key factor that sustains plant growth and determines vegetation distribution pattern. Plant water uptake pattern is regarded as a foundation to reveal the interaction mechanism between hydrological changes and vegetation succession. As the largest freshwater lake in China, the hydrological regime of Lake Poyang has been encountering a dramatic change in terms of water level variations, which generated a large influence on the water recharge of the wetland ecosystem. In this study, the replenishment sources of soil water in the Artemisia capillaris community, a typical mesophyte vegetation in the Lake Poyang wetland, were explored by using the stable isotopes (δ¹⁸O and δD). The stable isotopes composition of rainfall, lake water, groundwater, soil water and the plant water were analyzed. The main water uptake depth and the proportions of water uptake by A. capillaris were further investigated, using a direct inference method and the IsoSource mixing model. The results showed that the lake water and soil water isotopes were enriched, while the groundwater isotopes were rarely fractionated, relative to local precipitation isotopes. The wetland groundwater is likely to be recharged by the long-term rainfall and the lake water. During the rainy season (April-June), soil water in the A. capillaris community was mainly recharged by the local rainfall. During the summer months (July-August), the deep soil water was most likely to be supplied by lateral lake water and the groundwater. Accordingly, the soil water moved upward from deep to shallow soil layers under strong evaporation. In the growing season, A. capillaris mainly absorbed soil water at 0-80 cm depths and showed plasticity in switching water sources from different soil layers. When the soil water content was high (April-May), A. capillaris mainly used water from shallow soil layers (0-40 cm), accounting for 49%-68% of the total water uptake. When the shallow soil becomes dry (June-August), A. capillaris mainly absorbed deep soil water from deep soil layers (40-80 cm). The proportions of soil water uptake at 40-80 cm were up to 74%-95%. However, the main water source of A. capillaris again switched to the soil water at top soil layer (0-15 cm) during the mature stage (September-October), and the proportions of water uptake were in the range of 41%-70%. Overall, these findings reveal that lake water is an important replenishment source of the soil water in mesophyte vegetation community in the Lake Poyang wetland. A. capillaris could adjust water uptake depth according to the soil water availability, suggesting a strong drought tolerance. This study is helpful to provide scientific reference for the ecosystem evolution and wetland protection in the Lake Poyang-floodplains.