Environ. Sci. Technol. 2007, 41, 1331-1338
Carbon and Nitrogen Isotope Variations in Tree-Rings as Records of Perturbations in Regional Carbon and Nitrogen Cycles ANDREW R. BUKATA* AND T. KURTIS KYSER Queen’s Facility for Isotope Research, Department of Geological Sciences and Geological Engineering, Queen’s University, Kingston, ON, Canada, K7N 3N6
Increasing anthropogenic pollution from urban centers and fossil fuel combustion can impact the carbon and nitrogen cycles in forests. To assess the impact of twentieth century anthropogenic pollution on forested system carbon and nitrogen cycles, variations in the carbon and nitrogen isotopic compositions of tree-rings were measured. Individual annual growth rings in trees from six sites across Ontario and one in New Brunswick, Canada were used to develop site chronologies of tree-ring δ15N and δ13C values. Tree-ring δ15N values were ∼0.5‰ higher and correlated with contemporaneous foliar samples from the same tree, but not with δ15N values of soil samples. Temporal trends in carbon and nitrogen isotopic compositions of these tree-rings are consistent with increasing anthropogenic influence on both the carbon and nitrogen cycles since 1945. Tree-ring δ13C values and δ15N values are correlated at both remote and urban-proximal sites, with δ15N values decreasing since 1945 and converging on 1‰ at urban-proximal sites and decreasing but not converging on a single δ15N value in remote sites. These results indicate that temporal trends in tree-ring nitrogen and carbon isotopic compositions record the regional extent of pollution.
Introduction When atmospheric CO2 is photosynthetically incorporated into leaves, the carbon isotope ratio (δ13C value) of the photosynthate is affected by the photosynthetic pathway of the plant, the diffusion of 13CO2 into the leaf, and the partial pressure of CO2 inside the leaf relative to the atmosphere (1). For tree-rings, conversion of the photosynthate to cellulose is assumed to occur with minor isotopic fractionations not affected by environmental factors (2). Combustion of fossil fuels has increased atmospheric CO2 contents while lowering the δ13C value of atmospheric CO2 (3, 4) and this latter effect has been recorded in tree-rings, which generally show a decrease in δ13C values in cellulose since 1850 (5-7). Deviations from this trend occur when stress-induced stomatal closure or changes in water-use efficiency cause a change in carbon isotope fractionation between CO2 and the tree cellulose, with prolonged periods of stress appearing as positive deviations from the long-term trend toward lower δ13C values (8-10). Thus, both the changes in atmospheric * Corresponding author phone: (613) 533-2183; fax (613) 5336592; e-mail:
[email protected]. 10.1021/es061414g CCC: $37.00 Published on Web 01/13/2007
2007 American Chemical Society
CO2 and changes in stress in a tree can be evaluated from the temporal trends in the δ13C values of cellulose in treerings. Nitrogen has also been increasingly released into the atmosphere by various anthropogenic activities, particularly agriculture, industry, and automobile emissions (11-14). Ice core records indicate that the nitrogen isotope ratio (δ15N value) of nitrate has decreased from 12 to 18‰ prior to 1950 to -5 to +5‰ post-1950 (15). Increased deposition of reactive nitrogen (NOx and NHx) can stimulate forest growth, upset nutrient balances in the soil, and lead to soil acidification (11, 14), all of which can result in stress that could also affect δ13C values fixed in trees. The long-term impact of anthropogenic nitrogen deposition on ecosystems has received considerable attention recently, as reflected in programs such as GaNE (Global Nitrogen Enrichment; 16), NITREX (NITRogen saturation EXperiments; 17), and components of FACE (Free-Air CO2 Enrichment; 18). As the impact of anthropogenic nitrogen emissions on forest ecosystems is potentially significant (11, 14) and instrumental records extending back beyond 30 years are sparse, proxies that reflect the extent of perturbations to the nitrogen cycle from anthropogenic sources are required. Non-nitrogen fixing trees incorporate nitrogen as nitrate, ammonium, and organic nitrogen from soil reservoirs during growth, but prior to incorporation into living parenchyma cells, nitrate is reduced to ammonium in the leaves. At cell death (the transition from sapwood to heartwood), the nitrogen concentration decreases as nitrogen moves from the dying to growing parts of the tree (19). In conjunction with their longevity, large geographic range, and annual growth rings, the presence of non-mobile nitrogen in tree-rings make trees potential records of changes in the nitrogen cycle in forest ecosystems. The δ15N values of nitrogen pollution around cities, industrial areas, and roads are distinct from those of ambient nitrogen deposition in remote areas, and this is reflected in the δ15N value of vegetation samples from these areas (2024). If a relationship between tree-ring and foliar δ15N values exists, then tree-rings can potentially be used to develop chronologies of tree incorporation of pollutant nitrogen. Changes in the nitrogen isotopic composition of tree-rings have been used to infer perturbations to local nitrogen cycles (25) and changes in local nitrogen sources (26, 27), demonstrating that shifts in δ15N values of tree-rings correspond to the timing of nitrogen cycle perturbations. Poulson et al. (28) suggested that a decrease in δ15N values of tree-rings from two trees in their study might reflect anthropogenic nitrogen deposition beginning in 1950. Subsequent tree-ring based studies have also observed decreasing trends in δ15N values during the twentieth century, but these studies involved a single or a limited range of sites and were interpreted to represent local trends (21, 26, 27, 29). To examine whether the δ15N values of tree-rings were related to soil or foliar δ15N values, contemporaneous samples of each reservoir were analyzed. To examine whether longterm temporal trends in tree-ring δ13C values and δ15N values indicate an increasing anthropogenic contribution to the nitrogen cycle of forest ecosystems, we analyzed cores from red and white oak (Quercus rubra and Quercus alba) across central Ontario, Canada, at sites adjacent to and distant from point sources of anthropogenic pollution from Toronto, Hamilton, and Sudbury, and white and yellow birch (Betula papyrifera and Betula alleghaniesis) at a remote site in New Brunswick (Figure 1). These sites were chosen to cover a large geographic area, involve different species of trees, and include a range of bedrock types with soil pH ranging from VOL. 41, NO. 4, 2007 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 1. Map of Canada with inserts showing the location of the study sites in central Ontario and New Brunswick (solid circles). The predominant wind direction is indicated with arrows on the central Ontario insert. The location of monitoring stations NY52 and NY22, part of the National Atmospheric Deposition Monitoring Program, are indicated. Also included are the locations of Sudbury, Hamilton, and Toronto. acidic to neutral (Table 1). Tree species and number of trees sampled per site is given in Table 1. We have divided the sites into western Ontario, eastern Ontario, and New Brunswick. The western Ontario sites are close to urban centers and so should reflect greater amounts of anthropogenic pollution generated locally (Figure 1).
Experimental Section The stands sampled in this investigation represent remnant second-generation forest stands situated proximal to populated areas. Site management and use have mainly been recreational, with most sites containing maintained hiking paths. None of the stands have been subjected to stand-
clearing fire events in their recent past (30 years) as the areas have been subject to a policy of fire suppression. Coring. At each site, cores were extracted from dominant mature trees using a 5 mm diameter increment corer. At breast height, cores were taken at 90° intervals around each tree. For analysis, the four samples were grouped into a single suite for each tree. The increment corer was rinsed with deionized (DI) water between samples from the same tree and ethanol followed by copious amounts of DI water between trees. Even-numbered years from each tree sample suite were analyzed for carbon and nitrogen isotopic composition. Tree-Ring Dating. Ten to twenty separate trees were sampled at each site. Cores were mounted, sanded, and polished with increasing grit sandpaper until individual rings were visible. Cores from the same site were cross-dated according to the principles of Stokes and Smiley (30). Distinctive ring-width patterns were identified in the crossdated core and used to dissect the core for analysis first into decades, then into individual annual rings using a stainless steel blade. Samples for each year were placed into individual vials, with each vial containing the annual ring from each of the four orthogonal cores taken from the tree. When possible, the distinctive ring patterns were used to cross-date trees from multiple sites. Soil Sampling. At each site except New Brunswick, 3-10 soil samples were taken. The unconsolidated, recognizably organic detritus composing the O-horizon was removed with a trowel, and a sample of the top 5 cm of the A-horizon (the mineral horizon containing accumulated decomposed organic matter) was removed, placed in a plastic bag, and returned to the lab. Detailed soil horizon stratigraphies were not made. Rather, the top 5 cm of the A-horizon was taken as it represented the top of the rooting depth and potential source of bioavailable soil-derived nitrogen at each site. Samples were stored in a 4 °C refrigerator before and after processing. The samples were air-dried overnight and then sieved through a 2 mm mesh. Soil pH was determined on a 1:1 soil/water solution. A sub-sample was set aside for isotope measurement. Bulk soil was analyzed without carbonate extraction as none of the soil samples reacted with 20% HCl. This indicated that carbonate minerals were not present in the soil and carbon and nitrogen isotope analysis on a small selection (n ) 7) of soil samples indicated no significant difference between the isotopic composition of carbonate extracted and non-carbonate extracted soils.
TABLE 1. Summary of the Physical Characteristics of the Sampling Sites site
map ref
bedrock type
soil type
North Bay
46.35N 79.40W
granitic gneiss
Lake Opinicon
44.58N 76.32W
granitic gneiss
orthic humo-ferric podzol Monteagle sandy loam podzol
Burlington
43.28N 79.80W
shale/ mudstone
grey brown luvisol
Peterborough
44.37N 78.30W
carbonate
orthic melanic brunisol
Kingston -- North
44.33N 79.60W
carbonate
humic gleysol
Kingston -- East
44.26N 76.40W
carbonate
grey wooded podzol
Murray Brook, NB
47.50N 66.35W
granitic
humo-ferric podzol
a
na ) not analyzed
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soil pH (range)
soil δ15N (1 - σ) (‰; air)
soil δ13C (1 - σ) (‰; V-PDB)
4.62 (4.22-5.15) n ) 10 5.07 (4.78-5.92) n)7 6.43 (5.78-8.27) n ) 10 7.36 (6.62-7.93) n)9 6.87 (6.48-7.26) n)4 5.94 (5.69-6.65) n)3 naa
6.0 (0.9) n ) 14 4.3 (1.7) n ) 14 3.5 (0.8) n ) 16 3.8 (0.4) n)9 3.3 (0.6) n)4 4.1 (0.6) n)6 na
-25.6 (0.4) n ) 14 -25.7 (1.0) n ) 14 -25.9 (0.7) n ) 17 -25.6 (0.7) n)9 -25.7 (0.3) n)4 -26.4 (0.4) n)6 na
soil C:N (1 - σ) 15.89 (2.80) n ) 14 17.13 (1.76) n ) 14 13.06 (1.04) n ) 16 14.47 (0.85) n)9 15.10 (1.35) n)4 15.00 (1.08) n)6 na
trees analyzed 1 red oak 1 red oak 3 red oaks 1 red oak 2 white oaks 3 white oaks 2 white and 1 yellow birch
TABLE 2. Contemporaneous Tree-Ring, Foliar, and Soil Data Sampled in September 2006 site
tree-ring δ15N (‰)
Burlington
0.4 1.2 0.6 1.8 avg 1.0 (1σ) (0.7) L. Opinicon 0.1 -0.3 -1.7 -1.5 avg -0.8 (1σ) (0.9) North Bay -0.8 -1.2 -0.8 -1.9 avg -1.2 (1σ) (0.5) Kingston 0.9 East
leaf
δ13C (‰)
δ15N (‰)
δ13C (‰)
-25.3 -24.3 -24.4 lost -24.7 (0.6) -24.7 -25.0 -26.3 -26.3 -25.6 (0.9) -26.5 -26.5 -25.3 -25.3 -25.9 (0.7) -24.8
-0.3 1.0 0.2 0.6 0.4 (0.6) -1.2 -1.5 -1.9 -1.6 -1.6 (0.3) -1.1 -1.6 -1.3 -2.0 -1.5 (0.4) 1.4
-28.3 -27.5 -29.2 -28.2 -28.3 (0.7) -29.0 -29.0 -30.4 -29.9 -29.6 (0.7) -28.7 -28.5 -28.4 -28.3 -28.5 (0.2) -29.0
soil C/N
δ15N (‰)
δ13C (‰)
C/N
19.3 17.4 19.5 21.3 19.4 (1.6) 22.3 26.3 26.7 25.6 25.5 (2.0) 20.5 24.7 21.1 21.9 22.1 (1.9) 22.8
2.7 3.9 3.4 3.3 3.3 (0.5) 3.7 2.2 3.2 2.7 3.0 (0.7) 5.7 6.6 4.4 6.0 5.7 (1.0) 3.6
-26.7 -25.6 -26.4 -26.3 -26.2 (0.5) -23.7 -26.4 -26.1 -26.4 -25.7 (1.3) -25.3 -26.3 -25.4 -25.6 -25.6 (0.4) -26.7
11.5 12.7 12.8 13.2 12.6 (0.7) 16.8 19.2 17.6 17.8 17.9 (1.0) 17.2 15.7 21.1 18.9 18.3 (2.3) 16.0
Foliar Sampling. Contemporaneous leaf, tree-ring, and soil samples were taken at four sites (Kingston East, Lake Opinicon, North Bay, and Burlington) in September 2006. Leaves were taken from branches accessible with a 3 m long tree pruner from each tree sampled. Samples were removed from the branch and stored in plastic bags. In the lab, twenty healthy leaves were selected from each tree, rinsed with DI H2O, and oven dried at 40 °C overnight. The leaves were then grouped into a single sample and homogenized using an IKA A11 Basic Analytical Mill and set aside for stable isotope analysis. Four tree cores and two soil samples were taken per tree as per the described methods. Stable Isotope Measurement. Annual growth rings for isotope analysis were further separated into earlywood and latewood. From the latewood sample, a sub-sample of 7-25 mg was weighed into a tin capsule and nitrogen concentration and isotopic composition was analyzed by EA-CF-IRMS using either a Carlo Erba NCS 2500 Elemental Analyzer coupled to a Finnigan MAT 252 IRMS or a Costech ECS 4010 coupled to a Finnigan MAT Delta Plus XP in the Queen’s Facility for Isotope Research. Precision and accuracy of nitrogen concentration was determined using NBS-1547, NBS-1577b, and a white oak sapwood laboratory standard. Sample homogeneity was checked using replicated analyses of selected samples during each run. A relative error of 3% on weight percent nitrogen measurements was determined. The δ15N values, which are reported in units of permil (‰) relative to air (δ15N ) 0‰) (31), were calibrated using ammonium sulfate RM-8548 (δ15N ) 20.3 ( 0.2‰), RM-8550 (δ15N ) -30.4 ( 0.5‰), and the white oak laboratory standard. An uncertainty of 0.3‰ (2σ) was determined for δ15N value measurements. Carbon isotope measurement was performed on R-cellulose samples by EA-IRMS. The R-cellulose extraction was performed using a modification of the method of Loader et al. (32), based on the procedure of Green (33). The δ13C values are reported in units of permil (‰) relative to V-PDB and were calibrated using NBS-21 (δ13C ) -28.10 ( 0.20‰), a laboratory graphite standard (δ13C ) -26.65 ( 0.35‰) and a laboratory R-cellulose standard (δ13C ) -24.5 ( 0.20‰). Carbon and nitrogen isotopic composition of 10-30 mg soil samples and 5-20 mg foliar samples were measured simultaneously using the Costech ECS 4010 coupled to a Finnigan MAT Delta Plus XP. The same isotope standard reference materials listed were used to calibrate the mass spectrometer.
FIGURE 2. Nitrogen isotopic compositions of contemporaneous foliar and tree-ring samples. At sites where multiple trees were analyzed, δ15N values from each tree were individually analyzed and then the trees were averaged to give a site chronology. The same was done for δ13C values at all sites except New Brunswick where the three trees were grouped prior to R-cellulose extraction. Statistical Analyses. Carbon isotopic, nitrogen isotopic, and C:N of leaves, tree-rings, and soil from each site were compared using either ANOVA with post-hoc Bonferroni test or two-tailed t-tests. The long-term trends in the data were compared using Spearman’s Rank Correlation Analysis. All statistical analyses were performed using the software DataDesk (Ithaca, NY) except t-tests which were performed in Microsoft Excel. Correlation analysis was performed for both the raw data and the 5-point moving average smoothed data. The level of significance for all statistical tests was R ) 0.05.
Results and Discussion Comparison of Contemporaneous Foliar, Tree-Ring, and Soil Samples. The δ15N values of soil from each site cover a wide range (3.0-6.0‰) and are significantly different (p < 0.0025; Tables 1 and 2). There is no significant difference in soil δ13C values among sites. The C:N ratios, which range from 12.6 to 18.3, are significantly different among sites (p < 0.002; Tables 1 and 2). Soil %C and %N, and δ13C and δ15N, values are significantly correlated (p ) 0.0028 and p ) 0.0348, respectively). Likewise, foliar δ15N values and C:N ratios are significantly different among sites (p ) 0.0001 and p ) 0.0115 respectively) and there is no difference in δ13C values among sites (Table 2). There are no significant differences in the 2006 tree-ring δ13C values but the δ15N values are significantly different among sites (p ) 0.0065; Table 2). The 2006 tree-ring δ15N and δ13C values are significantly correlated (p ) 0.0148) and tree-ring δ15N values are correlated with foliar δ15N values (p < 0.002; Table 2 and Figure 2). Soil C:N data are significantly different at each site and all sites have average C:N ratios less than 20 indicating that nitrogen should be released to the soil (19). Burlington and Peterborough have the lowest C:N values, consistent with receiving higher relative amounts of nitrogen deposition. This is also seen in the foliar C:N value (Table 2) that is significantly lower at the Burlington site than North Bay and Lake Opinicon. VOL. 41, NO. 4, 2007 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 3. Nitrogen isotopic composition of whole-wood samples of tree-ring late-wood (a + b) and carbon isotope composition of the r-cellulose of late-wood (c + d) from urban-proximal sites in Ontario. Panels (a) and (c) are individual data points, (b) and (d) are the five-point moving averages plotted to indicate the long-term trend in the individual data from each site. The legend for all panels is the same as that in panel (a). The isotope data from each site were compared (Spearman Rank Correlation) and are summarized in the Supporting Information. Site details are given in Table 1. Nitrogen isotope data from the Burlington site from 1950 to 2002 have been previously published (25). The tree-ring and foliar δ15N values are significantly correlated (Figure 2) with tree-ring δ15N values ∼0.5‰ higher than leaves from the same tree. This difference likely results from isotopic fractionation between foliar nitrogen compounds and tree-rings during incorporation of nitrogen. No significant correlations between contemporaneous soil δ15N values and tree-ring or foliar δ15N values were observed in this investigation (Table 2), consistent with previous investigations that have noted a lack of relationship between soil and foliar δ15N values (34). Correlation between foliar and tree-ring δ15N values and convergence of long-term treering δ15N value trends suggest that pollution signal is overriding the soil organic matter nitrogen. Long-Term Trends in Tree-Ring δ15N Values. Western Ontario sites, which are closer to major sources of urban pollution, have δ15N values that decrease after 1945 and converge near 1‰. (Figure 3a and b). Trees from eastern Ontario have long-term trends toward lower δ15N values since 1945 that also approach values near 1‰ (Figures 4a and b). New Brunswick, the only site with a δ15N value below 1‰ in 1945, has a long-term trend in δ15N values that is negatively correlated with all sites, and also converges on 1‰. Treering δ15N values from western Ontario sites since 1860 are all correlated (p < 0.04; smoothed) but those from eastern 1334
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Ontario sites are not (Figure 3 and 4, a and b; correlation tables are presented in the Supporting Information). With the exception of the Kingston East site, trends after 1945 in δ15N values at all Ontario sites are highly correlated with one another and negatively correlated with the trend at the New Brunswick site (p e 0.008; smoothed). Foliar δ15N Values and their Relationship to Pollution. The δ15N values of vegetation near sources of nitrogen pollution have been shown to differ from those of vegetation samples at nonpolluted sites (20, 21, 23, 24, 35, 36). Needles from Norway spruce (Picea abies) sampled along a transect leading away from a highway in Switzerland (23), Pinus silvestris needles collected at 27 sites around Germany (20), and mosses collected near busy roads in the U.K. (24) all have δ15N values closer to the δ15N value of local anthropogenic pollution than do more remote or less polluted sites. Vegetation and soil sampled from coniferous forests across a nitrogen depositional gradient in Europe indicate a strong link between the rate of nitrogen cycling (including deposition) and 15N enrichment factor (22). At European sites where vegetation originally had lower δ15N values than the pollution δ15N value, pollution caused the δ15N value of vegetation to increase (22), as occurred in tree-rings at the New Brunswick site in this study. Sites in central Canada close to densely
FIGURE 4. Nitrogen isotopic composition of whole-wood samples of tree-ring late-wood (a + b) and carbon isotope composition of the r-cellulose of late-wood (c + d) from remote sites in Ontario and New Brunswick. Panels (a) and (c) are individual data points, (b) and (d) are the five-point moving averages plotted to indicate the long-term trend in the individual data from each site. The legend for all panels is the same as that in panel (a). The isotope data from each site were compared (Spearman Rank Correlation) and are summarized in the Supporting Information. Site details are given in Table 1. Nitrogen isotope data from the Kingston site from 1950 to 2002 have been previously published (25). populated areas were found to receive higher nitrogen deposition and had lower δ15N values in stems, leaves, and litter than more remote sites (35). Foliar samples from epiphytes and soil-grown plants around a very 15N-depleted nitrogen pollution point source in southeast Brazil have low δ15N values near the point source relative to more remote sites (36). Correlation between foliar and tree-ring δ15N values and similar trends in tree-ring δ15N values from sites across central and eastern Canada in the current study indicate that the regional nitrogen cycle has been altered since 1945 and that trees can be used to generate a history of regional nitrogen pollution. Trends in Historic δ15N Values. Long-term trends in δ15N values from vegetation in North America, Europe, and South America indicate that the δ15N value of reactive nitrogen may have changed on the global scale over the last century. Herbarium-stored samples of tree, shrub, and herb leaves from the Mediterranean have δ15N values that decreased from 2.1‰ in 1920-1930, to 1.4‰ between 1945 and 1955, to -0.6‰ in 1985-1990 (21), and herbarium-stored epiphyte samples from Brazil have pre-1960 δ15N values of -1.5 ( 3.9‰ compared to -10.9 ( 2.0‰ in 1997 (36). A shift in δ15N values of bulk sediment from an alpine lake in the Colorado front range (United States) from 5‰ prior to 1950 to 1‰ in one core and 2.5‰ in a second core was attributed to
increased nitrogen deposition (37). Samples from different regions converge on different δ15N values, most likely reflecting different δ15N values and amounts of pollution at each area, but the decrease in δ15N values over the past several decades appears to be global, consistent with the observed decrease in δ15N value of nitrate recorded in ice core (15). Long-Term Pollution Trends in the Study Area. All sites in Ontario have received between 20 and 25 kg/ha/yr total nitrate between 1990 and 1994 except Peterborough, which received 15-20 kg/ha/yr (38). Between 1996 and 2000, all sites received the same amount of nitrate deposition as 19901994 except North Bay, which decreased to 15-20 kg/ha/yr. The Ontario sites all received 20-25 kg/ha/yr sulfate between 1990 and 1994, which decreased to 15-20 kg/ha/yr between 1996 and 2000. During both time periods, the New Brunswick site received 5-10 kg/ha/yr nitrate and 10-15 kg/ha/yr sulfate (38). Detailed nitrate and sulfate deposition data from two sites in New York State were available from the National Atmospheric Deposition Program (Figure 1 and Supporting Information Table S2). Site NY52 received higher nitrate and sulfate deposition between 1980 and 1990, decreasing to values within the same range as seen in Ontario for the period 1990-2000. Site NY22, located further northeast and only operational since 1999 received less nitrate and sulfate deposition, consistent with the depositional gradient that VOL. 41, NO. 4, 2007 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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exists of higher values along the shore of Lake Ontario that decrease to the north and east (38). The Ontario and New York State nitrate and sulfate deposition data indicate that a depositional gradient exists and that it has existed for a considerable time. Mechanisms to Explain Nitrogen Isotope Trends. The observed post-1945 decrease in δ15N values and convergence on 1‰ at sites proximal to urban centers could be due to changes to the soil nitrogen cycle induced by increased soil temperature, changes in soil pH, shifts in the soil microbe communities or mycorrhizal associations, and changes in regional hydrology. However, the wide range in soil pH (4.228.27), soil δ15N values (3.0-6.0‰), temperatures, and precipitation represented by the sample sites (Table 1) are unlikely to combine to generate the same δ15N value in the most recent tree-rings at western Ontario sites proximal to urban centers (Figure 3). There are physiological and climatological factors that could cause inter-ring variability in δ15N values. Nitrogen translocation could homogenize a shift in δ15N value, decreasing the magnitude and spreading it over several growth rings. Efficient nitrogen recycling by trees, such as translocation of one-quarter to two-thirds of the nitrogen from leaves into stem prior to leaf abscission and removal of nitrogen from parenchyma cells after death (19) suggests that most of the translocation would be restricted to the sapwood. The nitrogen fraction remaining in heartwood would be the least labile, and extraction experiments have shown that there is no significant difference between whole wood and extracted heartwood δ15N values (25). Translocation would likely be outward from the pith. As a result, the timing of the signal onset would not be affected, although nitrogen recycling may increase the apparent length of the effect. The Ontario sites are located within a non-mountainous area extending ∼350 km north-south and ∼300 km eastwest and there is considerable overlap in the average monthly temperature and precipitation values (Supporting Information Figure S1). Average monthly temperatures range from -13.4 to 22.3 °C with total precipitation of 49.7 to 113.5 mm/ month. The New Brunswick site has temperature and precipitation ranges similar to those of the North Bay, ON site. While meteorological parameters may affect the nitrogen cycle in the soil and therefore the δ15N value of bioavailable nitrogen, the similarity of these parameters at the sites in this investigation suggests that the long-term trend in treering δ15N values is due to non-climatological effects. The simplest process to explain the long-term trend in tree-ring δ15N values involves the trees receiving varying amounts of nitrogen from sources with δ15N values near 1‰. Previous studies indicate that at low deposition rates from a point source of nitrogen, the δ15N value of bioavailable nitrogen shifts toward the value of the pollution source, whereas at higher deposition rates, nitrogen isotope fractionation occurs in the soil and the resulting bioavailable nitrogen has a higher δ15N value (39, 40). Although it may be argued that the convergence on a δ15N value of 1‰ for trees at western Ontario sites in this study may reflect a local nitrogen isotopic fractionation caused by excessively high nitrogen deposition, the trends from the eastern Ontario sites are similar to those at sites in western Ontario and New Brunswick, consistent with a shift in the δ15N value of bioavailable nitrogen toward a single value. Short-term changes in the soil nitrogen cycle or stress-related nitrogen isotope fractionations are likely causes of high-frequency variation in the δ15N values of tree-rings at each site (Figures 3a and 4a) via anthropologically produced acid deposition, mobilizing nutrients in the soil, nitrate mobility, or increased denitrification at these sites (38). Nitrogen contents in tree-rings of red and white oak range from 0.07 to 0.35%. They decreased slightly from the pith to 1336
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a fairly constant concentration between 0.07 and 0.11% in the heartwood, then increased sharply from the heartwoodsapwood boundary to between 0.20 and 0.35% in the outermost growth ring. This is consistent with previous investigations (25, 28, 41) and is taken to reflect tree physiology, not environmental condition. There was no significant correlation between ring width and δ15N value in individual trees analyzed in this investigation. Previous investigations have noted no relationship between ring width and tree-ring δ15N value (27, 28) even when tree-clearing and land-use change have been correlated with significant shifts in tree-ring δ15N values (25). As a result ring widths are not considered in this investigation. Contribution of Ammonium and Nitrate to the Tree. Saurer et al. (27) used tree-ring δ15N values, nitrogen content, and the existence of long-term nitrate deposition data at their study site to model the potential relative uptake of ammonium and nitrate by trees. Their modeling suggested a long-term shift from ammonium to mixed ammonium/ nitrate dominated uptake at their site. Such detailed isotopic data on historic nitrogen-pollution does not exist at any of the sites in this investigation. The nitrogen concentration of tree-rings in oak has a well-characterized trend from pith to the outermost growth ring and our sites would be similar to the non-roadside sites in Saurer et al. (27) that had no correlation between tree-ring nitrogen content and δ15N value. Without correlation between isotopic composition and nitrogen concentration in tree-rings, it is impossible to similarly assess the long-term shift in pollution δ15N values at our study sites. Oak preferentially utilize nitrate and so the observed post-1950 shifts in tree-ring δ15N values in this study may reflect the same shift from ammonium to mixed ammonium/nitrate dominated pollution proposed by Saurer et al. (27). Application of Carbon Isotopes to Evaluate Pollution Stress. The extent of stress at each site was assessed by examination of the temporal trend in the δ13C values of the tree-rings. Temporal variations in δ13C values since 1860 from trees at all the sites except New Brunswick are directly correlated (p < 0.05; smoothed), consistent with a regional change in the isotopic composition of CO2 (Figures 3 and 4, panels c and d). Generally, higher δ13C values in trees from western Ontario sites proximal to local sources of pollution relative to those at eastern Ontario sites (Figures 3 and 4, panels c and d) are consistent with greater stress in the trees of the former sites. The New Brunswick site has the lowest initial (pre-1945) δ13C values and a long-term trend toward less negative δ13C values (Figure 2d). The New Brunswick trend is negatively correlated (p < 0.05; smoothed) with sites in western Ontario, consistent with the New Brunswick site being increasingly affected by anthropogenic pollution. Pollution stresses that cause constriction of the stomata result in a decreased fractionation of 13C during photosynthetic incorporation of CO2 (1) and can result in an increase in tree-ring δ13C value (9). The long-term trends in tree-ring δ13C values, combined with the correlation among sites suggest that all sites are affected by pollution, with those closest to urban centers affected more. Correlation between Carbon and Nitrogen Isotopes in Tree-Rings. Long-term trends in δ13C values and δ15N values are significantly correlated (p < 0.05; smoothed) throughout the entire record at each site, with the New Brunswick site negatively correlated. Carbon isotopic compositions of treerings are affected primarily by factors associated with photosynthesis (1, 2) whereas the isotopic composition of bioavailable nitrogen is affected by perturbations to the soil nitrogen cycle (42, 43). Anthropogenic pollution affects both carbon and nitrogen cycles, has been increasing since 1945, and is consistent with the observed correlation between the δ15N and δ13C values of tree-rings.
Current research has noted a negative correlation between atmospheric CO2 levels and foliar δ15N values (34, 44). As the concentration of atmospheric CO2 has risen sharply since 1945 it is possible that some of the long-term trend toward lower δ15N values in tree-rings may be due to the effects of increased CO2. While this effect may be contributing to the trend toward lower values at some sites, it does not explain the increase in tree-ring δ15N values seen over the last 30years at the New Brunswick site or the oscillating trend seen at the Burlington site (Figures 3 and 4). Increasing atmospheric CO2 concentrations may be responsible for some of, but not the entire, trend seen in tree-ring δ15N values. The amount of nitrogen from pollution has increased dramatically in the last 50 years and is projected to continue (16). This nitrogen pollution will continue to affect the δ15N value of bioavailable nitrogen in forest ecosystems by shifting the δ15N value in proportion to the amount and δ15N value of the pollution, and by altering the nitrogen cycle of the forest. Studies of vegetation around point sources have shown the effect of nitrogen pollution with a distinct δ15N value, and this investigation has shown that long-term alteration of the regional nitrogen cycle can be recorded by changes in δ15N values of tree-rings. In conjunction with data from other studies, our results also indicate that forest nitrogen cycles are being impacted on a global scale as a result of pollution, and tree-ring nitrogen isotope analyses should be included in studies examining long-term nitrogen dynamics in forest ecosystems.
Acknowledgments We are grateful to April Vuletich and Kerry Klassen in the Queen’s Facility for Isotope Research for their analytical and technical assistance and we thank Tom Al for providing the New Brunswick samples. This project was supported by OGS and Queen’s University Scholarships to A.R.B., and funding and support from NSERC Discovery and MFA grants, Canadian Foundation for Innovation and Ontario Innovation Trust grants to T.K.K.
Supporting Information Available Spearman Rank Correlation Tables, meteorological information, and sulfate and nitrate deposition data. This material is available free of charge via the Internet at http:// pubs.acs.org.
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Received for review June 13, 2006. Revised manuscript received November 9, 2006. Accepted December 5, 2006. ES061414G
