c7.6.3 – Effects of nitrogen deposition on water-use efficiency as assessed by a triple isotope approach

Guerrieri R* (1-4), Borghetti M (2), Grace J (1), Mencuccini M (1), Perks M (3), Ripullone F (2), Saurer M (4), Sheppard LJ (5), Siegwolf RTW (4)

(1) School of Geosciences, Univesrity of Edinburgh, Edinburgh, UK; (2) DISCOFA, University of Basilicata, Potenza, IT; (3) NRS, Bush Estate, Roslin,EH25 9SY, UK; (4) LAC, Paul Scherrer Institute, CH-5232, Villigen PSI, CH; (5) CEH, Bush Estate, Penicuik,
Collocazione: c7.6.3 – Tipo Comunicazione: Presentazione orale
7° Congresso SISEF *
Sessione 6: “Ecosistemi forestali e fattori ambientali” *

Contatto: Rossella Guerrieri (rossellaguerrieri@gmail.com)

Abstract: The relevance of the increasing levels of nitrogen deposition (Ndep) on carbon (C) sequestration has lately been questioned by both experimental and modelling approaches. Widely different estimates of C sensitivity to Ndep have been reported in recent investigations, which highlight the need for a thorough re-assessment of all the physiological processes involved. For instance, how forests are responding, in terms of the balance between CO2 assimilation and water loss (i.e. water-use efficiency, WUE) to the increased atmospheric N input has been poorly investigated. Although the effect of N availability on tree WUE has been widely explored by previous studies, their relevance is limited because of: i) the young age of trees, making it difficult to up-scale to older trees; ii) the short duration of the N fertilization; iii) the method of fertilization, mostly as soil-N supply, which does not simulate completely Ndep at forest. In fact, part of the atmospheric N can be intercepted and retained by tree canopy, representing a direct addition to plant metabolism. Therefore, it could exert a stronger effect on WUE than would be predicted from studies that have excluded this pathway. We present a first attempt to investigate to what extent Ndep may increase the C sequestration of a Mediterranean forests, by improving the balance between CO2 assimilation and water loss under a scenario of marked reduction of precipitation. For this purpose, the impact of long-term exposure to NOx emission on the variation of intrinsic WUE (WUEi, i.e. the assimilation to stomatal conductance ratio) has been evaluated for two Quercus cerris. stands growing at different distances to an oil refinery established in 1996 in Southern Italy. Further, the relevance of canopy N uptake on changes in WUEi has been investigated for a Picea Sitchensis stand (Deepsyke forest, Scotland, UK) which is the unique site in Europe providing the opportunity to evaluate the long-term effect of the aerial N spraying onto a forest canopy, over 5 to 8 years. A simplifying approach has been adopted, based on the powerful combination of stable carbon (d13C), oxygen (d18O) and nitrogen (d15N) isotope composition in tree rings, which represent a valuable archive of climatic and human-induced changes. The site and species-specific response of trees to atmospheric N input will be discussed in term of WUEi and its coupling with tree growth. Further, the ecophysiological responses at the two different sites will be compared in order to understand the adaptation of different forest ecosystem types to current level of Ndep and future dynamics under increasing level of anthropogenic N. This study confirms the d13C in tree rings as a powerful tool to integrate long-term variations of WUEi, and it highlights the potential of combining d18O and d 15N in tree rings to disentangle the climatic or anthropogenic influence on the long-term changes of the ratio between assimilation and stomatal conductance.

Citazione: Guerrieri R , Borghetti M , Grace J , Mencuccini M , Perks M , Ripullone F , Saurer M , Sheppard LJ , Siegwolf RTW (2009). Effects of nitrogen deposition on water-use efficiency as assessed by a triple isotope approach . 7° Congresso Nazionale SISEF, Isernia – Pesche (IS), 29 Set – 03 Ott 2009, Contributo no. #c7.6.3