Depth distribution of soil water sourced by plants at the global scale: a new direct inference approach

The depth distribution of soil water contributions to plant water uptake is poorly known. Here we evaluate the main water sources used by plants at the global scale and the effect of climate and plant groups on water uptake variability and depth distribution. We use meta‐analysis of published isotopic data (δ2H and δ18O) for soil water and xylem water from 65 peer‐reviewed papers published between 1990 and 2017. The isotopic composition of soil water was strongly related to climate. We applied a new direct inference method to quantify the overlap between xylem water and soil water sources used by plants. The median overlap between xylem water and soil water at different depths varied between 28 and 100%, but they were generally >50%. The shallow soil water overlap (0‐10 cm) was largest in cold regions (100% ± 0) and lowest at tropical sites (about 28%). Conversely, the median overlap between xylem water and deep soil water was largest in the arid and the tropical zones (>75%) and much smaller in the other two climates. Our results suggest that the isotopic composition of xylem water reflects mostly the signature of shallow soil water (<30 cm) in the cold and the temperate zones, while in the arid and the tropical zones, plants appear to exploit water in deeper soil layers. Our novel simple statistically‐based direct inference method performed well in determining these differences in water sources, and can be applied more widely to isotope‐based plant water uptake studies.