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 ¹⁵N and ²H 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 ¹⁵N as in the low-N stand and around half of their N uptake was dependent on water uptake (²H enrichment). In 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 but 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. 

Isotopic labelling (deuterium and 15N) in boreal forests (one annually N-fertilized, and one N-poor reference stand). One square meter of forest floor was labelled (below the moss layer) using 5g ¹⁵N (potassium nitrate) and 2x200 ml D₂O. All trees growing within a 14 meter radius of the labelled square meter were sampled. In total, 6 plots were labelled and sampled (3 in the fertilized stand, and 3 in the reference stand).

15N uptake by trees detected via mass spectroscopy of needle samples.

2H uptake by trees detected via mass spectroscopy of current year stem wood.

Statistical modelling performed to test connection between water uptake and nitrogen uptake.

Hypotheses based on conclusions from McMurtrie & Näsholm (2018).