Applying stable water isotopes to determine root water uptake patterns in an urban forest land

The scarcity of water resources is an important issue in urbanization. Urban forest land water consumption accounts for a large part of urban water resources, the study of water uptake patterns in urban forest area is crucial for urban water saving and precision irrigation, but no identified research have investigated water uptake patterns in urban forest area until now. In this study, we measured the deuterium isotope ratio (δD) and the oxygen isotope ratio (δ¹⁸O) of precipitation, irrigation water, xylem water and soil water sources in a locust tree forest in Jinnan District of Tianjin City, China across 2019-2020. Water sources proportion in the root zone area of different growing seasons were obtained by IsoSource model, MixSIR model and SIAR model. Results show that there is a significant difference in soil moisture content between different stand age locust trees in time and depth variation. The trend of soil moisture of different stand ages of locust in time sequence intend to increase first and then decrease, the most significantly change of soil water content happened in shallow layer (0~40 cm). The change in vertical depth is about the same. The soil profile of 0-200 cm was discretized into three layers. The shallow layer (0~40 cm) soil water δD and δ¹⁸O fluctuated widely and decreased with the depth increased. This study revealed the dynamic replenishment of the root zone water in urban forest land, and provides insights into reforestation and water management in urban area.