Environ. Sci. Technol. 2005, 39, 7777-7783
Response of the Nitrogen Isotopic Composition of Tree-Rings Following Tree-Clearing and Land-Use Change 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
Clear-cutting of forests affects the nitrogen cycle and the nitrogen isotopic composition of bioavailable ammonium and nitrate in the soil. Here, we have used nitrogen isotopic variations of tree-rings in red oak (Quercus rubra) and white oak (Quercus alba) as indicators of changes in the nitrogen cycle on a local scale. The δ15N values of latewood from trees at two remnant forest stands in Ontario, Canada, that underwent large-scale tree-clearing and permanent land-use change at different times were measured. Trees from the perimeter of each stand record a marked 1.52.5‰ increase in the δ15N values of their tree-rings relative to the values in trees from the center of the stand, with the shift synchronous with the tree-clearing and landuse change. This shift was most likely due to increased rates of nitrification and nitrate leaching in the soil as a result of tree-clearing combined with permanent changes in hydrology and probable fertilizer use accompanying the change in land-use. Nitrogen concentration in tree-rings was not affected by tree-clearing and the associated change in land-use. These results indicate that changes in nitrogen cycling in forest ecosystems, whether due to climate change, land-use change, or other environmental changes (increased O3, other atmospheric pollutants, insects, etc.), can be faithfully monitored with nitrogen isotopic compositions of tree-rings and that dendrogeochemical analysis can be incorporated into studies of the effects of long-term anthropogenic effects on forest ecosystems.
Introduction The nitrogen cycle in forested ecosystems has been increasingly disturbed by human activities since the Industrial Revolution. Manufacturing processes, automobile exhaust, increased use of fertilizers, changing land-use, and forest clearing have all disrupted the nitrogen cycle. In forested ecosystems where nitrogen is most often a limiting nutrient, increased atmospheric nitrate and ammonium deposition and perturbations to the nitrogen cycle can have adverse effects on forest health (1, 2). Addition of nitrogen to a normally nitrogen-limited system disturbs biogeochemical cycles and causes the system to seek a new steady state. Annual growth rings combined with reasonable longevity make trees possible long-term sentinels of such nitrogen cycle disturbance in forested ecosystems. * Corresponding author phone: (613)533-2183; fax: (613)533-6592; e-mail:
[email protected]. 10.1021/es050733p CCC: $30.25 Published on Web 09/07/2005
2005 American Chemical Society
FIGURE 1. Schematic of the nitrogen cycle in forests modified from Nadelhoffer and Fry (1994) (21). Arrows indicate the direction of kinetic nitrogen transformations. The ∆ values (δ15Nproduct - δ15Nsource) next to the arrows indicate the range of expected changes in the δ15N value of the product as summarized in the literature (4, 21). The dashed box inside the plant reflects internal nitrogen cycling within the plant. Clear-cutting has been shown to increase the rates of nitrification, denitrification, and nitrate leaching. The δ15N value of nitrogen that is bioavailable to a tree will be affected by (1) the introduction of an isotopically different nitrogen source and (2) altering the processes that make nitrogen available for uptake (3, 4). The δ15N value of foliage has been used to examine the nitrogen cycle in undisturbed forests (5-7), in forests receiving fertilization (8-11), and to determine the effects of clear-cutting on the nitrogen cycle of forests (12, 13). Changes in the δ15N value of source nitrogen have been shown to affect the δ15N value of foliage in fertilization (8, 9) and atmospheric nitrogen pollution studies (14) and as a result of salmon carcass deposition by bears (15). In each study, the introduction of nitrogen from a source with a δ15N value distinct from the existing foliage at the site was recorded by a shift in the foliar δ15N value. Only a few studies (16-19) have attempted to use the δ15N values of tree-rings to infer temporal changes in the nitrogen cycle. Poulson et al. (1995) (16) reported a decrease in the δ15N values of tree-rings of two eastern hemlocks (Tsuga canadensis) after 1960 that they attributed to anthropogenic nitrogen emissions. Pen ˜ uelas and Estiarte (1997) (17) found a 0.14‰ decrease in δ15N values of downy oak (Quercus pubescens) over the last century. Hart and Classen (2003) (18) sampled ponderosa pine (Pinus ponderosa) that had been labeled with an 15N-enriched tracer 14-years earlier and found that the tree-rings recorded the event but that there had been some radial translocation of the tracer. Saurer et al. (2004) (19) reported that both the nitrogen concentration and the δ15N values of tree-rings from Norway spruce (Picea abies) increased proximal to and coincident with construction of an adjacent highway. They attributed the increases to vehicle emissions. This investigation attempts to determine whether changes in land-use associated with permanent tree-clearing can produce changes in the nitrogen cycle that are recorded in the δ15N values of tree-rings from red oak (Quercus rubra) and white oak (Quercus alba), two species that preferentially use nitrate rather than ammonium. Clear-cutting has been shown to cause increased rates of nitrate leaching, increased denitrification (1, 2), and increased productivity of nitrifying bacteria (20). Bacterial nitrification of ammonium to nitrate is accompanied by an isotopic fractionation that results in the remaining ammonium having a higher δ15N value than the initial value (4, 21) (Figure 1). As the microbially available soil ammonium pool is nitrified, the δ15N value of the nitrate VOL. 39, NO. 20, 2005 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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produced will increase because this nitrate is produced from an ammonium pool with an increasing δ15N value. As the nitrate lost by leaching is replaced with newly produced nitrate, the δ15N value of bioavailable nitrate in the soil will increase. Increased denitrification will also act to increase the δ15N value of bioavailable nitrate in the soil by preferentially removing 14N to the atmosphere. If the land-use change includes the application of fertilizers, a new source of potentially isotopically different nitrogen is introduced to the system after tree-clearing. By increasing the productivity of nitrifying bacteria, increasing nitrate leaching, and increasing denitrification, clear-cutting forests should result in an increase in the δ15N values of ammonium and nitrate available for uptake by trees. Application of fertilizer to the clear-cut area will affect the nitrogen cycle of the remnant forest. Both the amount of fertilizer used and the δ15N value of the fertilizer may affect the δ15N values of the tree-ring record if the fertilizer migrates into the forest. Clear-cutting also removes future litter-fall at the site, which has a lower δ15N value than most soil nitrogen (21). The long-term elimination of 15N-depleted litter inputs to the soil effectively increases the δ15N values of the residual soil organic matter available for mineralization. The soil nitrogen cycle is not a closed system, and perturbation caused by tree-clearing and land-use change can be expected to move the system toward a new steady-state equilibrium. In this study, we examine the shift in δ15N values of tree-rings at two geologically distinct sites where deforestation occurred at different times.
Experimental Section Study Areas. Trees examined in this investigation were sampled from sites located in Burlington and Kingston, Ontario, Canada, on the north shore of Lake Ontario, ∼400 km apart. The Burlington site is a remnant second-growth forest running along a stream that was left when the surrounding land was cleared to construct a housing subdivision in the early 1970s. The soil on the site is a welldeveloped mineral soil (Gray Brown Luvisol) above shale and carbonate bedrock. Three trees from the center of the stand were selected, one ∼150-years old, and two ∼70-years old red oaks (Quercus rubra) spaced 20 m apart. To minimize the effects of the construction, these samples were collected at least 10 m from the current perimeter of the forest. In addition to the trees from the center of the stand, three red oaks were sampled from the perimeter of the stand, immediately adjacent to a field cleared for the construction of a high school completed in 1976. All three perimeter trees were approximately 70-years old. The Kingston site was a small (∼300 × 500 m) remnant second-growth forest bound by Lake Ontario to the south, a two lane road to the north, and bordered by cottage style homes to the east and west. Three white oaks (Quercus alba) from the center of the stand were sampled. One was ∼150years old, and the other two were both approximately 120years old. The trees are less than 10 m apart and rooted in thin (
