Environ. Sci. Technol. 2009, 43, 604–609
Tree-Ring Nitrogen Isotopes Reflect Anthropogenic NOX Emissions and Climatic Effects MARTINE M. SAVARD,* ´ GIN, AND CHRISTIAN BE ANNA SMIRNOFF Natural Resources Canada, Geological Survey of Canada, ´ ´ Division Quebec, 490 rue de la Couronne, Quebec (QC), G1K 9A9, Canada ¨ LLE MARION JOE Institut national de la recherche scientifique, Centre Eau, ´ Terre et Environnement, 490 rue de la Couronne, Quebec (QC), G1K 9A9, Canada ELISE RIOUX-PAQUETTE† Natural Resources Canada, Geological Survey of Canada, ´ ´ Division Quebec, 490 rue de la Couronne, Quebec (QC), G1K 9A9, Canada
Received August 29, 2008. Revised manuscript received December 05, 2008. Accepted December 10, 2008.
Anthropogenic emissions of atmospheric nitrogen have increased over the last century, but the monitoring of nitrous oxide concentrations is only recent. Can trees from temperate regions be used to infer past changes in nitrogen cycles? To consider this question, we investigate nitrogen isotope (δ15N) ring series from pine and beech trees near Montre´al, and beech specimens of Georgian Bay Islands. The δ15N values show coherent intertree and interspecies trends, independent of the sapwood-heartwood transition zones, implying that these results reflect local environmental conditions. At both sites, shortterm isotopic fluctuations correlate directly with precipitation and inversely with temperature. Long-term isotope decreases of 1.5 to 2‰ suggest progressive changes in soil nitrogen after 1951. In Georgian Bay, an additional important change is inferred on the basis of a 1.5‰ increase initiated after 1971. At both sites, long-term series correlate with a proxy for NOx emissions. We propose that the contrasted long-term δ15N changes of Montreal and Georgian Bay reflect deposition of NOx emissions from cars and coal-power plants, with higher proportions from coal burning in Georgian Bay. This research suggests that tree-ring δ15N series may record both, regional climatic conditions and anthropogenic perturbations of N cycles.
Introduction Globally, ammonia (NH3) and nitrous oxides (NOx) emitted by human activities and the related N deposition have the potential to impact water quality and the nutrient balance of ecosystems (1-3). Providing a historical view of human activities impacts on the N cycle constitutes an important * Corresponding author phone: 418 654-2634; fax: 418 654-2615; e-mail:
[email protected]. † Current address: De´partement de biologie, Faculte´ des sciences, Universite´ de Sherbrooke, 2500 boul. de l’Universite´, Sherbrooke (QC), J1K 2R1, Canada. 604
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issue, as there is no long-term measurement of atmospheric N emissions. Can natural archives provide such long-term records? Trees from temperate, northern regions with the high temporal resolution of their rings combined with their N characteristics possibly constitute natural records of changes in the N cycle. Ecosystems submitted to high rates of N deposition show increasing net nitrification (transformation of ammonium (NH4+) into nitrate (NO3-) by microbes). This bacterial mediation produces 15N-depleted nitrate and leaves behind higher isotopic ratios for NH4+, which is preferably taken up by plants (e.g., see refs 4 and 5). It is expected that the amount of precipitation flushing soils may influence the soil NH4+/NO3- ratio because NO3- is highly mobile in water relative to NH4+ (6, 7), and because a rise in temperature may increase the rate of nitrate uptake by trees (8). In other words, the overall isotope ratio of soil N at a given site should be influenced by local climatic conditions. The δ15N values and abundance of N available in soil are echoed in δ15N values of foliage in deciduous and coniferous trees (9). Monitoring of streamwater chemistry reveals that forest clear cutting locally increases the rate of NO3production due to change in proportions of heterotrophic and autotrophic nitrifiers (10). Such processes, combined with drainage modification after clearing, have been invoked to explain increases in δ15N values of trees (11, 12). Nitrogen is considered as an element of high radial mobility in tree stems (13), and N characteristics have been interpreted as unreliable for environmental monitoring. Some studies have documented inconsistent N variations for untreated wood in several tree species (14, 15). An important shift in concentration is shown to occur at the limit between heartwood and sapwood, the concentrations being much higher in sapwood than in heartwood (e.g. (4, 16),). Addition of 15N-labeled chemical fertilizers on field-growing specimens indicate that trees register highest peaks of labeled N in rings formed during years of fertilization, and lower labeled-N amounts in older rings (4, 17, 18). These studies show that translocation of N indeed takes place in stems but does not completely obliterate the effects of environmental changes on tree-N series. These considerations also suggest that N concentrations and isotope ratios in tree rings may record environmental perturbations. Several authors have suggested that tree-ring samples pretreated for removal of soluble compounds eliminate the influence of the sapwood/heartwood boundary on the temporal N patterns (19). The removal of secondary plant compounds (waxes, oils, resins), inorganic salts, and low molecular-weight polysaccharides from tree-ring wood can be done by using organic solvents and Soxhlet or sonic bath extractions (4, 19). A few studies using such methods suggested that δ15N values of tree rings register anthropogenic perturbations (18, 20-22). At this stage, the only link between tree-ring N characteristics and climate conditions has been suggested for nitrogen concentration in beech trees and August precipitation (18). This work presents N concentrations and δ15N values in rings of trees growing in regular field conditions of two periurban settings with different history of atmospheric pollution (Figure 1). All analyses were performed on wood samples treated for removal of soluble compounds. Our main objectives are to assess if tree-ring nitrogen characteristics can help us understand anthropogenic perturbations of forests, and if ring series can be considered as records of climatic conditions. 10.1021/es802437k CCC: $40.75
2009 American Chemical Society
Published on Web 01/09/2009
FIGURE 1. Locations of study sites, and of coal (black dots) and petroleum (gray dots) power plants of northeastern America, arrows indicate the main wind direction of July, modified from ref 23 (a); curves of atmospheric NOx adapted from refs 1 and 24, and domestic cars per province, compiled public data from Transportation Departments (b).
Materials, Methods and Approach Selected Sites. Centenary and older trees of large populations growing under field conditions were selected at the Arboretum Morgan near Montreal in southern Quebec, and at Georgian Bay Islands National Park (GBINP) in central Ontario. The Arboretum Morgan is a 2.42km2 protected research facility of McGill University and located 30 km west of downtown Montreal in a low density residential area (Figure 1a), but between two highways that were constructed in 1965 and 1966 to replace local roads. The Montreal area belongs to the St. Lawrence Lowlands physiographic region in which the Windsor-Que´bec city corridor is highly populated and industrialized (Figure 1 (25);). Climatic conditions have been compiled from five meteorological stations located in a radius of 25 km around Montreal to produce a continuous series covering the 1880-2002 period (Environment Canada stations: 701490, 7024400, 7025250, 7025280, and 7026839). The total precipitation amount in the region has been around 975 mm annually between 1905 and 2005, whereas average summer temperature has been of 20 °C and has increased by 0.9 °C. Old trees were found in a pure beech stand and in a mixed coniferous stand with dominant pine trees, both in the eastern part of the Arboretum Morgan facility. This specific area is characterized by brunisolic soils with a pH varying from 3.96 in L-H
horizons, to 40.49 in C horizon, and developed in sandy alluvial deposits. Overall, the American beech ([Fagus grandifolia Ehrh.] and White pine [Pinus strobus L.]) specimens selected for the study have a healthy appearance and do not show visual decline symptoms. These two species will help elucidate if tree-ring N characteristics reflect different physiological functions or site conditions. The protected GBINP covers 14 km2 and is located in the southern end of Georgian Bay. The total precipitation amount in the region has been around 1200 mm annually over the last century, whereas the average summer temperature has been of 17 °C and has increased by 0.4 °C. The site selected is in the southern part of Beausoleil Island, near the boundary between the Grenville and St. Lawrence Lowlands geological provinces. Adjusted precipitation and homogenized temperature from stations 6110606, 6163171, 6113490 (Environment Canada) are used to assess climatic conditions at GBINP over the 1895-2005 period. At the GBINP site, we selected healthy beech trees ([Fagus grandifolia Ehrh]), dominating a pure stand and growing on a brunisolic soil with a pH varying from 4.76 in H horizon, to 6.05 in C horizon. This pure-stand species and soil type are chosen for comparative purposes with Montreal beech series, i.e., to help assess if contrasted trends exist at the Arboretum Morgan and the GBINP. Air Quality at the Studied Sites. The dominant source of NOx in Que´bec Province is by far transportation (85%; Environment Canada data) as power is mainly provided by hydroelectric plants. From 1970 to 2005, the domestic car pool in the province passed from 1.58 to 3.78 millions (138% increase; Figure 1b), accentuating the emissions due to hydrocarbon combustion (26, 27). At the Que´bec City end of the Windsor-Que´bec corridor, 250 km downwind from Montre´al, studies have shown that up to 50% of some pollutants (e.g., Pb) were transported over long distances from southwestern sources located in the USA (28). In Montreal, pollutants from those distant sources mix with locally produced atmospheric contaminants. In consequence, the Arboretum Morgan soil content of nitrogen was likely influenced by significant levels of anthropogenic NOx during the life of the sampled pine and beech trees. Note that this site has been described as being influenced by phytotoxic pollutants on the basis of tree-ring, long-term, inverse δ13C and δ18O trends (29). The GBINP belongs to a sparsely populated area devoid of proximal point sources of pollutants, but it stands downwind from several major coal power plants of the Great Lakes region (Figure 1a), and it is located about 140 km north of the wide Metropolitan Toronto and of access highways to the Great Lakes region, the most densely populated region of Canada. According to the Ontarian Ministry of Environment, the two dominant sources of NOx in this region are transportation (the domestic car number has increased by over 50% between 1980 and 2000; Figure 1b), and coal power plants. Field measurements of atmospheric concentrations for most pollutants are only available locally for a maximum of 30 years in eastern Canada (Environment Canada data). For the two regions we have investigated, the data available is much shorter. In other words, the historical estimation of air quality cannot be based on direct local measurements but on pollution proxies for broad regions (Figure 1b). Nitrogen oxide emissions were modeled and presented as representative for Canada (30). The modeled NOx level is expressed as % (0 to 100) of the highest annual NOx emission over the last century (24). This previously modeled curve will be assumed here to reflect the historical evolution of NOx levels in the Montreal and Georgian Bay areas although it is clear that short-term changes in N emissions may vary locally. As mentioned above, the proportion of coal-power plant emisVOL. 43, NO. 3, 2009 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 1. Information Summary for Trees Selected for Geochemical Investigation variability tree species
number of trees dendrochronology/ geochemical analyses
beech pine
12/3 20/3
beech
11/3
sampled period
Montreal 1838-2003 1867-2003 GBINP 1864-2005
number of analyses
δ15N (‰)
181 177
0.3 0.4
184
0.6
sions in the annual N budget is more important in Ontario than in Quebec. In addition, the monitoring record since 2001 by Environment Canada shows that the average NOx atmospheric concentration in GBINP (station 65201) is higher than in western Montreal (station 50126) by about 80% (average of 22 and 12 parts per billion, respectively).
Analytical Procedures Sampling of Trees and Dendrochronology. Over twelve specimens of each population of trees were carefully chosen for dendrochronological analysis. Among those specimens, three trees were selected for isotope dendrogeochemistry (Table 1). Six cores of 5 mm diameter equally distributed around the stem were collected at 1.4m from soil surface on the three specimens selected for dendrogeochemistry. The cores selected for tree-ring measurements were handled using standard dendrochronological methods. Dating was subsequently verified using the COFECHA program (31). Mechanical separation of ring pairs used even numbers as initial years of each pairs. The same pair for all cores of a given specimen was pooled for geochemistry. Isotope Geochemistry. Analyses for N concentration and δ15N values were obtained from wood treated for removal of soluble compounds in order to minimize inconsistency caused by translocation (19). This procedure can be summarized as follows: ground wood is placed into filter bags and extracted in a 1:1 mixture of benzene and methanol, then acetone using an ultrasonic bath; the bags are thoroughly rinsed and soaked in deionized water and then boiled for one hour; finally, samples are dried and stored in vials. All treated wood samples (11-12 mg) were analyzed for δ15N values and N concentration using an Elemental Analyzer in Continuous Flow with an Isotope Ratio Mass Spectrometer. Calibration for δ15N analysis was performed using IAEA-N2, USGS-25 and verified with IAEA-N1. Over the period of investigation, the average precision for duplicates (n ) 73) was 0.2‰. The overall N average concentrations are 0.06% for pine and 0.09% for beech trees in Montre´al, and 0.13% for beech trees in GBINP. Note that N concentration shows poor intertree and interspecies coherence, implying that these results fail the test of environmental indicators as previously mentioned (4, 22). Therefore, N concentrations will no longer be discussed in this work. The absolute δ15N values vary between -2.2 and +1.6‰ (average ) -0.5‰) in beech, and -2.2 and +0.4‰ (average ) -0.9‰) in pine trees of Montre´al. Higher ranges are found for the GBINP trees (-1.5, +1.3, and average of -0.1‰). Isotopic results are normalized relative to the average values for each specimen so as to minimize differences in isotope ratios induced by species specific rates of NH4+ and NO3uptakes (30), potential metabolic effects, and to easily compare the interspecies isotopic behavior. The variation of results at each site and for each tree species is calculated as the mean deviation between the three trees for each pair of years (ring-pair sample). The final variability for the complete 606
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FIGURE 2. Nitrogen isotope ratios of the Montreal site (Arboretum Morgan): individual curves for the three studied specimens (a); solid line represents average values obtained through time, and gray band indicates variability of results for the three studied beech trees (b) and pine trees (c); rectangles indicate transition zone between heartwood and sapwood. series per species/site is obtained by averaging the mean deviation obtained for each pair of rings (Table 1).
Results and Interpretations For both beech and pine trees, the intraspecies comparison of individual curves show an excellent correspondence for δ15N values (beech-tree results shown as example; Figure 2a). In addition, these two different tree species in Montreal show similar long-term isotopic behavior (Figure 2b, c). In details, the average δ15N values of beech-tree rings show regular short-term oscillations (average of 0.3‰) superimposed over a long-term flat series from 1880-1881 to 1950-1951, and a decreasing trend occurring from 1952-53 to the end of the series (Figure 2b). The limit between these two trends precedes the heartwood-sapwood transition by 6 years. The decreasing δ15N trend is characterized by a drop of 2.0‰ over 50 years (from +0.5‰ in 1950-1951 to -1.5‰ in 2000-2001). The coherence of changes of the δ15N series observed for both beech and pine trees, which are not influenced by the heartwood-sapwood transition zone, suggests that the δ15N values can be used as environmental indicators (Figure 2b, c). This data set further suggests that local conditions at the Arboretum Morgan site progressively changed since the beginning of the 1950’s. If local changes in environmental conditions are responsible for the tree-ring δ15N trends, then one should expect to find GBINP isotopic time series different than the ones found at the Arboretum Morgan because the climate records and the history of NOx emissions of the two sites are significantly different. The GBINP average δ15N curve reveals second order variations (commonly more than 0.5‰; Figure 3) higher than the ones observed in Montreal (average 0.25‰;
FIGURE 3. Average nitrogen isotope ratios (dark line) and variability envelope gray band obtained for beech tree rings of the Georgian Bay Island National Park (Ontario). The rectangle indicates transition zone between heartwood and sapwood.
TABLE 2. Percentage of Parallel Agreement between Climate and Nitrogen Isotope Series (Gleichlaufigkeit test; 32). δ15N Montreal pine beech GBINP beech a
precipit.a(%)
temperatured(%)
65b 54c
62e 70f
65b
71g b
Total precipitation from: June, July, August, and July, August; d Mean temperature from: e August; f July, g and March, April and May. Underscored values indicate inverse relationships.
c
Figure 2). The GBINP long-term δ15N average trend also significantly varies from the one described in Montreal, particularly in its last part. The GBINP δ15N values increase by 1‰ from 1880-1881 until 1920-1921, stay around an average value of 0.2‰ until 1950, decrease by about 1.5‰ until 1970-1971, then continuously increase until present (by as much as 2‰; Figure 3). This latter shift postdates by six years the heartwood-sapwood transition of the GBINP beech trees. The differences between the Montreal and GBINP trends are also expressed in statistical relationships between treering δ15N values and the proxy for NOx emissions. The Montreal pine and beech values correlate negatively with the NOx index (Kendall’s Tau correlation coefficients of -0.56 and -0.53, respectively) whereas the GBINP data show weaker links (coefficient of -0.23, just above the significant threshold of 0.22). Therefore, the GBINP beech series seems to record conditions significantly different from the ones in Montreal. Note that visual comparison of the δ15N values from both sites with the proxy for NOx emissions indicate that only the long-term trends correlate with values of emissions. The parallel agreement (Gleichlaufigkeit test) between climate and δ15N series shows interesting results as well (Table 2). The δ15N values for the two sites and the two tree species correlate negatively with summer and spring temperatures, and positively with summer precipitation. These statistics reveal that a significant part of the short-term δ15N variations in the two species of trees can be interpreted as responses to climatic conditions.
Discussion One should realize that species specific physiological effects such as translocation would likely generate noncoinciding and different trends in beech and pine trees. Otherwise, changes in δ15N values in trees mainly depend on regional climatic conditions, soil temperature and pH, mycorrhizal activities, the relative rates of soil N transformations (immobilization, ammonification, and nitrification), amounts and δ15N values of soil N, and depth of N uptake by roots (5, 7-9, 32-,34). Some of these observations imply that under
FIGURE 4. Comparison of normalized average δ15N time series of: (a) the beech and pine trees from the Arboretum Morgan in Montreal; (b) beech trees of Georgian Bay Islands National Park and Arboretum Morgan. high rates of anthropogenic N deposition, the overall δ15N ratio of the available N pool may be echoed in the tree-stem δ15N values. The coinciding decreasing δ15N trends in pine and beech trees strongly support an interpretation in terms of modification in environmental conditions at the Arboretum Morgan of Montreal. No clear cutting or infectious diseases affected the studied stands. According to Gleichlaufigkeit tests (Table 2) and visual comparison of climatic trends with the isotopic series, local climatic conditions can only explain short-term changes. These facts indicate that the Montreal tree rings recorded changes in N soil dynamics after 1950-1951, i.e., N input to output ratio relatively higher and/ or NH4+/NO3- lower than before 1950 (Figure 2a). These changes are likely due to addition of N from an external source with an isotopic ratio lowering the soil N which is in equilibrium with the N fixed in tree stems after 1950 (Figure 4a). This addition is expressed by a decrease from an average value of 0.0 (1880-1949) to -1.1‰ (1950-2004) for beech trees, and from -0.6 to -1.4‰ for pine trees, and their strong correlation with the proxy for anthropogenic NOx emissions. Natural soil N and nitrous oxides (NOx) from car exhaust have previously been characterized with δ15N values ranging between +1 and +9‰, and -13 to +4‰, respectively (6, 35-37). It is expected that addition of car NOx to the soil-N pool will generate a decrease in δ15N values of N available to the root system. We therefore suggest that the pre-1950 short-term isotopic variations reflect natural conditions, whereas the post-1950 long-term isotopic change at the Arboretum Morgan reflects mixtures of natural N with increasing amounts of N derived from car emissions (Figure 1b). The long-term δ15N negative trend of the GBINP is similar to the Montreal curve in the period 1950-1972. However, the GBINP positive slope after 1972 departs from the Montreal isotopic curve (Figure 4b). We propose that the pre-1972 trend here also is mainly explained by incorporation Nderived from car NOx. Monitoring stations recently registered higher annual airborne N deposition at GBINP than at the VOL. 43, NO. 3, 2009 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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west end of Montreal Island. This being stated, why do the post-1971 δ15N trends of the GBINP site significantly depart from the ones at the Arboretum Morgan? It is known that trees increase their assimilation rate of nitrate with soil temperature (30), and that an increase in precipitation amount could increase the NH4+/NO3- ratio in soil solution (7). Annual mean temperatures have been systematically lower and total precipitation, higher at Georgian Bay than in Montreal for the last century. So difference in local climatic conditions cannot explain the long term, post-1972 departure of the Georgian Bay data from the Montreal trend. All other things being equal, we propose that the 1880 to 1971 δ15N results in Georgian Bay reflect natural conditions which after 1950 are modified by accumulation of car-exhaust NOx. The post-1971 departure of the GBINP series from the Montreal δ15N trend is generated by atmospheric mixture of car NOx with increasing proportions of emissions from the upwind fossil-fuel power plants of southern Ontario and eastern USA ((25); Figure 1a). In Montre´al, vehicle NOx constitutes the main anthropogenic source (Environment Canada data). Indeed, boilers of coal power plants likely emit NOx with positive δ15N values (+6 to +13‰ (36);), which will tend to increase the isotopic ratios of N available to trees in GBINP. The youngest part of the GBINP isotopic series shows values around 1.5‰ higher than at Montreal (Figure 4b). Recent analyses reveal that the δ15N values of total N in soil-H horizon are higher in Georgian Bay (5.4‰) than in Montre´al (4.5‰; A. Doucet; personal communication). This observation supports our hypothesis that the Montreal and Georgian Bay sites accumulate anthropogenic N with contrasted δ15N values. Interpretations of long δ15N series in terms of effects generated by airborne N-species have been previously advocated (20, 22, 38, 39). Here, we further propose that the contrasted δ15N trends obtained for wood samples from two regions reflect different regional anthropogenic N deposition combined with variations of climatic conditions. Tree-ring δ15N values may record both environmental perturbations and regional climate variations.
Acknowledgments A. Promaine and his team from the GBINP, B. Coˆte´ and C. Idziak from the Arboretum Morgan, and M. Luzincourt of the Delta-Laboratory are acknowledged for their support in this project. We would like to thank R.Vet of Environment Canada for providing data on air pollution, and D. Houle for a presubmission review of this article. This project has been supported by the Environment & Health Program of Natural Resources Canada, and the GBINP.
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