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TitreWhy tree ring d15N ratios can provide a retrospective look at anthropogenic nitrogen accumulations
AuteurSavard, M M; Bégin, C; Marion, J; Smirnoff, A
SourceCentral European Geology vol. 56, no. 2-3, 2013 p. 103-105, (Accès ouvert)
Séries alt.Secteur des sciences de la Terre, Contribution externe 20130038
ÉditeurHungary Science Academy
Réunion10th Applied Isotope Geochemistry symposium; Budapest; HU; Septembre 22-27, 2013
Documentpublication en série
Mediapapier; en ligne; numérique
Sujetsvégétation; dendrochronologie; rapports carbone-azote; isotopes; azote; substances polluantes; pollution; géochimie; géologie de l'environnement
ProgrammeDéveloppement durable des sables bitumineux, Géosciences environnementales
Résumé(disponible en anglais seulement)
In North America, the direct measurement of N-bearing air pollutants rarely goes be - yond the last 25 years in rural regions where stationary sources are active or in urban areas typified by diffuse industrial pollution. To palliate for the lack of long-term mea - surements of air quality and of depositional monitoring, tree-ring d15N values have started to be used in the early 2000s. Short-term tree-ring d15N variations have been interpreted to reflect climatic conditions [1], but most studies have interpreted the d15N trends in terms of contributions from various N sources [2], based on the premise that many atmospheric emissions have d15N values distinct from the natural N background (-10.0 to -5permil). However, the trends of exposed trees during the pollution periods generally show significant decreases even when industrial sources produce NOx with d15N values significantly higher than the soil background. Why? In order to closely look at the relationship between emission types or rates, and ring d15N responses in exposed trees, we investigated white spruce specimens (Picea glauca) growing in two distinct diffuse pollution contexts separated by 470 km in northeastern Alberta: the Lower Athabasca oil sands district (north of Fort McMurray), and the coal fired power plant (CFPP) area west of Edmonton. The NOx emissions started in 1967 and 1956, respectively, but the direct air quality monitoring has only been initiated in 1997 and 1985, in the same order. The signal of nitrate and ammonium PM emissions near the main oil extraction operations has been reported to be ~+2 and +11permil, respectively [3], whereas the one for NOx emitted by CFPP is gen- erally reported to vary between +8 and +26permil [4]. Tree-ring subsamples covering the 1880-2010 period were analyzed using a combustion-EA-CF-IRMS for their d 15 N values (precision = 0.3permil). Statistical analy- ses of the d15N values and climatic parameters for the pre-operation calibration periods (1910-1961 and 1900-1951) allowed to develop transfer functions for the two sites, and predict the natural d15N responses to climatic conditions for the operation periods. This approach [5] permits to set objective criteria for interpreting anthropogenic im - pacts on the local N cycle. The measured series depart negatively from the modeled natural trends, by an average of -1.5permil during the oil sands 1967-2009 period of oper - ations, and by -1.0permil during the 1956-2010 CFPP operations. Interestingly, the negative trends of the measured data in the two regions can be nicely reproduced by multi - ple-regression models combining local climatic parameters with acidifying emissions. We interpret the negative anomalies depicted here as anthropogenic perturba - tions of the nitrogen cycle in the two N-emission contexts. These anomalies agree with several negative trends documented in previous studies. However, they cannot be interpreted in terms of direct modifications of the overall soil d15N values, because such an effect would increase the overall soil ratios. Our hypothesis is that anthropogenic N favours micorhizal fungi activities associated to the root system of the two studied stands, which release light N for the host trees growing under non-saturated condi - tions. The overall increase of regional soil isotopic signal due to the positive d15N values of the emissions will only be detectable if soil N saturation is attained and fungi ac - tivities do not dominate the local N dynamics anymore. The perspective given by the tree-ring d15N series reflects similar changes in two contrasted N emission regions, and provides a retrospective recognition of changes in soil biogeochemistry. Most importantly, the hypothesis of micorhizal control on the tree-ring d15N trends has significant implications on the main interpretative frame for this type of results. If confirmed by further biogeochemical research, this model might open the possibility of semi-quantitative reconstruction of depositional N rates.
Résumé(Résumé en langage clair et simple, non publié)
L'accumulation sévère en forêt d'azote (N) émis par les activités humaines peut entraîner un excès d'azote, l'acidification et la détérioration des écosystèmes terrestres et aquatiques. Cet enjeux environnemental est une préoccupation en ce qui concerne le nord-est de l'Alberta où les émissions de contaminants azotés ont augmenté au cours des 15 dernières années. Pour deux importantes sources d'émissions dans cette région, l'extraction des sables bitumineux et la génération d'énergie par la combustion de charbon débutées respectivement en 1967 et 1956, les émissions ne sont mesurées que depuis 1997 et 1985. Le manque d'information pour la première partie de ces activités rend impossible l'estimation de l'impact des émissions actuelles ou des effets qui seraient associés à une intensification des activités industrielles. Cette étude a permis de: proposer les isotopes des cernes de croissance des arbres comme outil de surveillance pour pallier au manque de données historiques; de mieux comprendre les perturbations du cycle de l'azote dues aux activités humaines; et de décrire les changements chez les processus biogéochimiques des forêts exposées.