De-Coupled Water and Nitrogen Translocation From Subsoil to Canopy of Temperate Forest Trees

Water and nitrogen (N) transport from soil to canopy play a central role in tree functioning, yet direct evidence for their timing and coupling in mature forests remains scarce. We report results from a paired dual-isotope (²H, ¹⁵N) tracer experiment in a temperate forest, comparing water and nitrate uptake patterns across tree species (Douglas fir, European beech), soil textures (loamy, sandy) and rooting depths (surface, subsoil). Using simultaneous double tracer injections into the lower rooting zone, followed by canopy xylem sap and foliage sampling over 2 months, we quantified the appearance of water and nitrate in xylem sap and foliage, supported by sap flow measurements to estimate transit times. Water reached the canopy faster than nitrogen, revealing a marked asynchrony in resource translocation for both species and sites-the first such field observation. Douglas fir showed greater subsoil water and nitrate uptake on sandy than on loamy soil, and higher subsoil uptake compared to beech under identical conditions. Tree species, soil texture and soil depth jointly govern water and nitrate uptake dynamics, as well as their temporal decoupling. These findings provide novel field-based evidence of asynchronous water and nitrate transport, advancing our understanding of tree ecophysiology and nutrient cycling under field conditions.