Characteristics of Stable Hydrogen and Oxygen Isotopes of Soil Moisture under Different Land Use in Dry Hot Valley of Yuanmou

Soil moisture, stable hydrogen, and oxygen isotopes were sampled and determined in a demonstration area of soil and moisture conservation at the Laocheng Town of Yuanmou County in Chuxiong Prefecture, Yunnan of three land use types: Leucaena Benth artificial forest, Heteropogon contortus grass field, and farmland. The characteristics of stable hydrogen and oxygen isotopes of soil moisture in these different land use types at different soil depths were analyzed to investigate the regularities in the quantitative formation of soil moisture balance. In terms of forest land, we found that the variable coefficient of hydrogen isotopes in the 0-20 cm soil layer was the smallest, but decreased with depth under 20 cm. While in grassland, the variable coefficient in 80-100 cm was the largest, and decreased with depth above 80 cm. As for farmland, the variable coefficient in the top 20 cm was the largest, followed by 40-60 cm, and the medium 20-40 cm was the smallest. The soil moisture hydrogen isotope values of three land use type were different at surface layer, but prone to be consistent in each type. Along the soil depth in forest land, the hydrogen isotope increased first and then decreased, while increased in the end, and the maximum appeared in 80-100 cm. In grassland, the hydrogen isotope increased initially as the forest land but then decreased continuously, so the maximum was found at 20-40 cm. And in grassland, the hydrogen isotope of all depths were higher than which of forest land and farmland. In same land use type, the hydrogen isotope of soil moisture changed significantly at the surface, and the variation of hydrogen isotopes was obviously decreased along the depth. Our findings could provide reference data which would contribute to the assessment of regional groundwater resources in the dry-hot valley of Yuanmou in this study.