Characteristics of stable hydrogen and oxygen isotope variations in soil water under different land covers in karst area.

Understanding the stable isotope changes of soil water, hydrogen, and oxygen under different land cover in karst areas is beneficial for revealing the infiltration and transport processes of soil water, as well as the impact of different land cover on hydrological processes, providing theoretical basis for regional water resource utilization and ecological environment construction. We measured hydrogen and oxygen isotope of soil water in 0-50 cm profiles under four different land covers (bare land, cultivated land, grassland, and shrubland) at the Puding Karst Ecological Station in Guizhou Province from May 2021 to April 2022 through regular field sampling and indoor experiments. The stable isotope changes of hydrogen and oxygen in precipitation and soil water at 10, 25, and 45 cm layers under four land covers were compared and analyzed. The results showed that: 1) Precipitation δ2H, δ18O, and waterline deuterium difference (lc-excess) values showed the same seasonal variations, with low values in summer and high values in winter. The equation for the atmospheric precipitation line in Puding was: δ2H=8.49δ18O+16.65 (r2=0.98), with a slope and intercept greater than the global atmospheric precipitation line, indicating a warm and humid monsoon climate of the area. 2) Soil water was mainly replenished by precipitation, and the δ18O of soil water was lower in summer and higher in winter, with the most obvious manifestation at 10 cm. 3) Soil water at a depth of 10 cm under four types of land cover was most affected by precipitation recharge and evaporation, with the largest range of stable isotope changes, the highest enrichment of δ18O heavy isotopes, and the lowest lc-excess value; following by that at 25 cm and 45 cm. 4) The lc-excess values for the four types of land cover were in an order of grassland (-13.27‰)>shrubland (-14.54‰)>cultivated land (-15.67‰)>bare land (-19.92‰). The corresponding soil water evaporation degree was bare land>cultivated land>shrubland>grassland. Our results indicated that land cover has a significant impact on the water cycle and that the lc-excess value can effectively reflect evaporation.