Study of the water absorption depth of fruit trees based on stable isotope technology: Using the different fruit-grass complex systems in Luochuan Loess Tableland as an example

Water resource shortage and low water use efficiency are the limiting factors of sustainable development of the fruit industry in the Luochuan Loess Tableland. Therefore, it is important to accurately measure the water absorption depth of plants and analyze their water utilization at different soil depths to improve and promote soil management in orchards. In this study, the stable hydrogen and oxygen isotope technique was used to study the characteristics of δD values (hydrogen isotope ratio) of the xylem water of apple trees, stem water of white clover and ryegrass, and soil water under different depths in different apple trees–grass intercropping systems (apple–white clover and apple–ryegrass) in the Luochuan Loess Tableland and to explore the effects of apple trees–grass intercropping systems on the water sources of apple trees and soil water use in different depths. The results showed that in May, white clover and ryegrass increased the water content in the 0 − 50 cm and 50 − 100 cm soil depths, respectively, with water originating from the 10 − 20 cm soil depths, and there was no water competition with fruit trees. White clover and ryegrass increased the water use efficiency of fruit trees in the soil below 40 cm by 27% and 9%, respectively. In July, the main water of the white clover and ryegrass originated from shallow and deep soils, with water competition with fruit trees. However, white clover experienced less competition with fruit trees than with ryegrass. White clover and ryegrass improved soil water use below 40 cm and 0 − 20 cm by fruit trees, respectively. In August and October, white clover increased the water content of the 0 − 50 cm soil depths and the utilization rate of the 0 − 40 cm soil water by fruit trees. Therefore, planting white clover could better adjust the water utilization level of fruit trees and the utilization rate of each soil level than ryegrass. Thus, fruit trees could absorb water to a greater extent. Results from this study provide a basis for understanding the driving mechanism of the water cycle in soil–plant systems, and guidance for forage variety selection and optimized water management of orchards in the Luochuan Loess Tableland.