THE LEGACY OF ENHANCED N AND S DEPOSITION AS REVEALED BY COMBINING δ13C, δ18O and δ15N IN TREE RINGS

Rossella Guerrieri (1) , Maurizio Mencuccini (1) , Lucy Sheppard (2) , Matthias Saurer (3) , Mike Perks (4) , Peter Levy (2) , Mark A. Sutton (2) , Marco Borghetti (5) , John Grace (1)

1 School of Geosciences [Edinburgh]
2 CEH - NERC Centre of Ecology and Hydrology
3 LAC - Laboratory of Atmospheric Chemistry [Paul Scherrer Institute]
4 Centre for Forestry & Climate Change
5 Department of Crop Systems, Forestry and Environmental Sciences

This study aimed to evaluate the effects of repeated aerial nitrogen (N) and sulphur (S) misting over tree canopies of a Sitka spruce plantation in Scotland. We combined δ13C and δ18O in tree rings to evaluate the changes in CO2 assimilation (A) and stomatal conductance (gs) and to assess their contribution to variations in the intrinsic water-use efficiency (WUEi). Measurements of δ15N enabled shifts in the ecosystem N cycling following misting to be assessed. We found that: i) N applications, with or without S, increased the ratio between A and gs in favour of A, thus supporting a fertiliser effect of added N. ii) After the treatments have ceased, the trees quickly adjusted to the reductions of N deposition, but not to the reduction in S deposition, which had a negative effect on WUEi by reducing A. This indicates that the beneficial role of N deposition may be negated in forests that previously received a high load of acid rain. iii) δ15N in tree rings seems to reflect the N dynamics caused by canopy retention, with the fingerprint also present in the litter, after the experiment stopped. iv) Both our results (obtained using canopy N applications) and a collection of published data (obtained using soil applications) showed that generally WUEi increased in response to an increase of N applications, with the magnitude of the changes related to soil conditions and the availability of other nutrients. The opposite directions of shifts in δ15N in tree rings suggest that both the quantity of the applied N and its quality, mediated by processes occurring during canopy N retention, are important determinants of the interactions between N and C cycles, which occur in forests. Stable isotopes are useful probes to understand these processes and to put in the results of short-term experiments into context.

Mots clés: Life Sciences

Rossella Guerrieri, Maurizio Mencuccini, Lucy Sheppard, Matthias Saurer, Mike Perks, et al.. THE LEGACY OF ENHANCED N AND S DEPOSITION AS REVEALED BY COMBINING δ13C, δ18O and δ15N IN TREE RINGS. Global Change Biology, 2010, 17 (5), pp.1946. ⟨10.1111/j.1365-2486.2010.02362.x⟩. ⟨hal-00603438⟩