Tree water uptake enhances nitrogen acquisition in a fertilized boreal forest – but not under nitrogen‐poor conditions

Summary Understanding how plant water uptake interacts with acquisition of soil nitrogen (N) and other nutrients is fundamental for predicting plant responses to a changing environment, but it is an area where models disagree. We present a novel isotopic labelling approach which reveals spatial patterns of water and N uptake, and their interaction, by trees. The stable isotopes 15 N and 2 H were applied to a small area of the forest floor in stands with high and low soil N availability. Uptake by surrounding trees was measured. The sensitivity of N acquisition to water uptake was quantified by statistical modelling. Trees in the high‐N stand acquired twice as much 15 N as in the low‐N stand and around half of their N uptake was dependent on water uptake ( 2 H enrichment). By contrast, in the low‐N stand there was no positive effect of water uptake on N uptake. We conclude that tree N acquisition was only marginally dependent on water flux toward the root surface under low‐N conditions whereas under high‐N conditions, the water‐associated N uptake was substantial. The results suggest a fundamental shift in N acquisition strategy under high‐N conditions.