Response of plants water uptake patterns to tunnels excavation based on stable isotopes in a karst trough valley

Karstic aquifers are very sensitive and fragile to environmental changes. With more tunnels excavated in karst areas, groundwater levels are increasingly lowered and resulted in changed natural hydrogeological flow system and groundwater-dependent vegetation, soil and hydrology of surface water systems. Water, especially groundwater, is a limited and key factor and significant driving force for karst ecosystem processes. However, the effects of tunnels excavation on the ecosystems in karst areas remain largely unknown. This study aimed at investigating whether there were variations in soil water contents and shifts of plants water uptake patterns between rainy and dry seasons in a karst trough valley affected by tunnels excavation, and comparing their differences with those in a tunnel-free karst trough valley in southwest China. Monthly soil water contents at two soil layers of the upper 20 cm and lower 20–40 cm were measured, and soil water at two soil layers, subcutaneous water and plant (arbor and shrub) xylem water were sampled in February (mid dry season) and September (late rainy season) 2017, respectively, and the δ2H and δ18O of those waters were analyzed. The IsoSource model based on dual stable isotopes of δ2H and δ18O was used to estimate the contributions of different sources to the plant xylem water. Although the soil water contents at both valleys showed similar vertical and temporal variations resulted primarily from the evaporation and precipitation, the soil water contents of two soil layers in the tunneling affected valley were significantly lower than that of the tunnel-free valley at both seasons. Plants uptake water pattern changed from a dominant soil water source during the rainy season to a dominant subcutaneous water source during the dry season at both valleys. However, plants extracted more water from the subcutaneous zone in the tunneling affected karst trough valley than that of the tunnel-free karst trough valley at both seasons, of which the proportion of plants water uptake from the subcutaneous zone was 33% in rainy season and 76% in dry season, respectively, in the tunneling affected karst trough valley, while the proportion of plants water uptake from the subcutaneous zone was 24% in rainy season and 59% in dry season, respectively, in the tunnel-free karst trough valley. The above results indicated that tunnels excavation decreased the soil water contents and then changed plants water uptake patterns in karst areas.