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TitleWhy tree ring d15N ratios can provide a retrospective look at anthropogenic nitrogen accumulations
AuthorSavard, M M; Bégin, C; Marion, J; Smirnoff, AORCID logo
SourceCentral European Geology vol. 56, no. 2-3, 2013 p. 103-105, Open Access logo Open Access
Alt SeriesEarth Sciences Sector, Contribution Series 20130038
PublisherAkademiai Kiado Zrt.
Meeting10th Applied Isotope Geochemistry symposium; Budapest; HU; September 22-27, 2013
Mediapaper; on-line; digital
File formatpdf
Subjectsgeochemistry; environmental geology; vegetation; dendrochronology; carbon nitrogen ratios; isotopes; nitrogen; pollutants; pollution; Air quality; Air pollution
ProgramEnvironmental Geoscience, Coal & Oil Resources Environmental Sustainability
AbstractIn 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.
Summary(Plain Language Summary, not published)
Major deposition in forests of nitrogen (N) emitted by human activities may lead to N excess, acidification and degradation of the terrestrial and aquatic ecosystems. This environmental issue is of concern in north-eastern Alberta where N-bearing emissions have been reported to increase for the last 15 years. For two of the major sources of emissions in this region, oil sands extraction and coal fired power generation that started in 1967 and 1956, respectively, the emissions have been monitored only since 1997 and 1985. The lack of information in the early period of these operations does not allow to estimate the impact of the current N emissions or the potential effects if the industrial activities were to intensify. This study has allowed to: propose tree-ring N isotope ratios as monitoring tool to palliate for the lack of historical data; better understand the man-made perturbations of the N cycle; and describe changes in the biogeochemical processes of exposed forests.

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