Anthropogenic N deposition and the fate of 15NO3- in a northern hardwood ecosystem

被引:42
作者
Zak, DR [1 ]
Pregitzer, KS
Holmes, WE
Burton, AJ
Zogg, GP
机构
[1] Univ Michigan, Sch Nat Resources & Environm, Ann Arbor, MI 48109 USA
[2] Univ Michigan, Dept Ecol & Evolutionary Biol, Ann Arbor, MI 48109 USA
[3] Michigan Technol Univ, Sch Forest Resources & Environm Sci, Houghton, MI 49931 USA
基金
美国国家科学基金会;
关键词
atmospheric NO3-deposition; ecosystem N cycling; microbial N retention; N saturation; N-15; tracer; plant N retention;
D O I
10.1023/B:BIOG.0000031045.24377.99
中图分类号
X [环境科学、安全科学];
学科分类号
08 ; 0830 ;
摘要
Human activity has substantially increased atmospheric NO3- deposition in many regions of the Earth, which could lead to the N saturation of terrestrial ecosystems. Sugar maple (Acer saccharum Marsh.) dominated northern hardwood forests in the Upper Great Lakes region may be particularly sensitive to chronic NO3- deposition, because relatively moderate experimental increases (three times ambient) have resulted in substantial N leaching over a relatively short duration (5-7 years). Although microbial immobilization is an initial sink (i.e., within 1-2 days) for anthropogenic NO3- in this ecosystem, we have an incomplete understanding of the processes controlling the longer-term (i.e., after 1 year) retention and flow of anthropogenic N. Our objectives were to determine: (i) whether chronic NO3- additions have altered the N content of major ecosystem pools, and (ii) the longer-term fate of (NO3-)-N-15 in plots receiving chronic NO3- addition. We addressed these objectives using a field experiment in which three northern hardwood plots receive ambient atmospheric N deposition (ca. 0.9 g N m(-2) year(-1)) and three plots which receive ambient plus experimental N deposition (3.0 g NO3--N m(-2) year(-1)). Chronic NO3- deposition significantly increased the N concentration and content (g N/m(2)) of canopy leaves, which contained 72% more N than the control treatment. However, chronic NO3- deposition did not significantly alter the biomass, N concentration or N content of any other ecosystem pool. The largest portion of 15 N recovered after 1 year occurred in overstory leaves and branches (10%). In contrast, we recovered virtually none of the isotope in soil organic matter (SOM), indicating that SOM was not a sink for anthropogenic NO3- over a 1 year duration. Our results indicate that anthropogenic NO3- initially assimilated by the microbial community is released into soil solution where it is subsequently taken up by overstory trees and allocated to the canopy. Anthropogenic N appears to be incorporated into SOM only after it is returned to the forest floor and soil via leaf litter fall. Short- and long-term isotope tracing studies provided very different results and illustrate the need to understand the physiological processes controlling the flow of anthropogenic N in terrestrial ecosystems and the specific time steps over which they operate.
引用
收藏
页码:143 / 157
页数:15
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