Impacts of deep-rooted fruit trees on recharge of deep soil water using stable and radioactive isotopes

Deep-rooted fruit trees mine more water from deep soils than their shallow-rooted counterparts. Understanding how deep soil water (DSW) is replenished and subsequently depleted by deep-rooted fruit trees, therefore, are important for informing sustainable water resources management particularly in arid regions. In this study, we collected soil samples from the surface down to 20 m under four land use types (farmland, 8-year apple orchard, 12-year peach orchard, and 25-year apple orchard) in China's Loess Plateau. We then measured the soil water content, stable (delta H-2 and delta O-18) and radioactive (H-3) isotopic compositions. The radioactive isotope was used to constrain the age of soil water while the stable isotopes were used to determine the types of storms that would have contributed to recharging the DSW. We then implemented a soil water balance model to identify the mechanisms underlying the changes in DSW. Mechanistically, our results show that water movement in these soils was predominantly via piston flow. The age of DSW below 8 m was determined to be older than 55 years. Altogether, these results support an interpretation that DSW may have only been recharged by high-intensity, low-frequency rainfall events during the wet season (July to September), but that the magnitude of DSW recharge was likely to be influenced by subsequent water mining by deep-rooted fruit trees. The deep-rooted fruit trees consumed more DSW than farmland vegetation, substantially limiting the magnitude of DSW recharge under the orchards. Our simple soil water balance model, informed by water stable isotopes and supplemented with information from tritium, provides a technique for partitioning soil water balance (SWB) and insights into the long-term effects of land use change on water resources in arid regions.