Partitioned Soil Water Balance and Its Link With Water Uptake Strategy Under Apple Trees in the Loess-Covered Region

It is important, but challenging, to partition soil water balance (SWB) to understand the impacts of afforestation on soil hydrological processes. This study will investigate the partitioning of SWB by combining stable and radioactive water isotopes to quantify the effects of afforestation, and analyze the mechanism by which SWB changes through identifying the water uptake strategies of apple trees of 18 and 26 years old (A18 and A26). Compared to the reference farmland, apple orchards significantly increased evapotranspiration (ET) by 5%-10%, which in turn decreased soil water storage and deep drainage by 5%-14% and 50%-95%, respectively. Further, the partitioning of ET showed that apple tree planting increased transpiration by 15%-28% but decreased evaporation by 17%-30%. The above change in SWB appeared to be closely related to plant water uptake strategies. The apple trees shifted their water source from shallow (0-2 m) to deep soils (below 2 m), utilizing approximately 62% of deep soil water in the late growing season. In particular, 23% of source water may come from soil water older than 50 years. The older apple trees tended to extract more water from deeper soils (45% for A26 vs. 38% for A18). Therefore, the soil water deficit was the cumulative effects of root water uptake. The methods for SWB partitioning provides technical support for similar studies, and the findings are helpful to better understand the hydrological processes in the thick loess deposition.