Tracing Water Recharge and Transport in the Root-Zone Soil of Different Vegetation Types in the Poyang Lake Floodplain Wetland (China) Using Stable Isotopes

Abstract

Background: root-zone water transport is crucial in the water transformation from precipitation to groundwater, directly influencing soil moisture distribution and resource acquisition for wetland plants. Methods: This study investigated the movement mechanism of root-zone (0–80 cm) soil water in the Poyang Lake wetland, China, during a dry year. Hydrological observation and stable isotopes (δ18O and δD) were utilized. Results: The root-zone soil water content was low (2.9–12.6%) at the high site covered by Artemisia capillaris, while it remained high (25.2–30.2%) at the median and low sites covered by Phragmites australis and Carex cinerascens, respectively. The isotopic values of shallow soil water (0–40 cm) in the A. capillaris site followed the seasonal pattern of rainfall isotopes, indicating predominantly rainfall recharge. Rainfall was primarily transported by piston flow, with an infiltration depth of approximately 60 cm. Conversely, depleted water isotopes measured at certain depths in P. australis and C. cinerascens sites closely resembled those of rainfall, suggesting that preferential flow dominated. The average groundwater contribution proportions in root-zone soil water were 65.5% and 57.4% in P. australis and C. cinerascens sites, respectively, while no contribution was detected in A. capillaris site. Conclusions: Preferential flow and groundwater recharge occurred in the P. australis and C. cinerascens sites. They enhance the hydrological connection at the profile scale and are useful for maintaining a favorable root-zone moisture environment for wetland ecosystems in dry years. However, the hydrological connectivity between root-zone soil and groundwater was found to be obstructed in the A. capillaris site. This might be the main reason for vegetation degradation at high elevations in the Poyang Lake wetland.

Revealing the Hidden Consequences of Increased Soil Moisture Storage in Greening Drylands

“…This process can significantly enhance SM retention and support vegetation sustainability even during dry periods. This widespread occurrence is substantiated by research in regions such as the Pampas of Argentina [61], south-east Australia, and the Horqin sandy land of China [56,62,63], where similar interactions have been extensively documented.…”

“…The extent to which groundwater can recharge SM notably depends on regional characteristics. For instance, the existence of a shallow groundwater connection determines whether recharge can occur [55], soil properties such as texture and structure determine the soil's capacity to facilitate capillary rise [68], and the type of vegetation determines its ability to utilise deep water sources effectively [63,69]. These regional factors are crucial for understanding the variability in groundwater use by vegetation in greening drylands.…”