Seasonal dynamics of 2H2O passage and 15N accumulation in a dual-label pulse-chase experiment in a mature, irrigated Scots pine forest

Tree roots are responsible for the bulk of water and nitrogen (N) uptake by forests, but the detailed mechanism of resource uptake and their role in the spatial exploitation of water and nitrogen resources is incompletely understood. Theory and empirical evidence suggest that there are two processes delivering N to root surfaces for uptake: 1) convection of dissolved N carried in soil water as it is drawn toward the roots and 2) diffusion of dissolved N in still water toward the low concentrations at the root surface.  Quantifying these processes has, however, been difficult, particularly in forest trees. Recently paper by Henriksson et al. Our objective here was to make a similar comparison in an irrigation experiment.

We applied highly labelled ²H₂O and ¹⁵N on a small area at the beginning of the growing season a similar and tracked ²H and ¹⁵N in tissues of adjacent trees and shrubs, aiming to assess not only correlations between the water and N tracers at the end of the season, but also the dynamics of label passage and accumulation through repeated measurements. Synchronicity of the nitrogen accumulation with the water label passage would strengthen evidence for the role of water uptake in nitrogen uptake.

We found that the ²H-label in water, as measured in extracted water, passed through the system as a pulse, disappearing by late in the season. In contrast, the ¹⁵N label, as measured from leaf tissue, accumulated toward an asymptotic maximum. Among tree individuals, the rate of ¹⁵N increase was correlated with the plant-water ²H₂O labelling at any point in time, supporting the notion that the ¹⁵N uptake was predominantly driven by water uptake. No difference was detected between the irrigated and the non-irrigated plots, perhaps because high rainfall overrode any irrigation effects.

Next steps will be to also compare the cumulative δ²H of the wood in the new growth ring to the δ²H of the xylem water that passed through individual trees. If the correlation is strong, we will map the patterns of water and N uptake around the labelled plots to provide a detailed spatial description of the correlated water and N uptake processes. When combined with the current study, these results will show the spatial and temporal extent to which root water uptake facilitates nitrogen delivery to the roots.