From Missing Source to Missing Sink: Long-Term Changes in the Nitrogen Budget of a Northern Hardwood Forest

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• 作者：Ruth D. Yanai ; Matthew A. Vadeboncoeur ; Steven P. Hamburg ; Mary A. Arthur ; Colin B. Fuss ; Peter M. Groffman ; Thomas G. Siccama ; Charles T. Driscoll
• 刊名：Environmental Science & Technology
• 出版年：2013
• 出版时间：October 15, 2013
• 年：2013
• 卷：47
• 期：20
• 页码：11440-11448
• 全文大小：304K
• 年卷期：v.47,no.20(October 15, 2013)
• ISSN：1520-5851

文摘

Biogeochemical monitoring for 45 years at the Hubbard Brook Experimental Forest in New Hampshire has revealed multiple surprises, seeming contradictions, and unresolved questions in the long-term record of ecosystem nitrogen dynamics. From 1965 to 1977, more N was accumulating in living biomass than was deposited from the atmosphere; the 鈥渕issing鈥?N source was attributed to biological fixation. Since 1992, biomass accumulation has been negligible or even negative, and streamwater export of dissolved inorganic N has decreased from 4 to 1 kg of N ha^鈥? year^鈥?, despite chronically elevated atmospheric N deposition (7 kg of N ha^鈥? year^鈥?) and predictions of N saturation. Here we show that the ecosystem has shifted to a net N sink, either storing or denitrifying 8 kg of N ha^鈥? year^鈥?. Repeated sampling over 25 years shows that the forest floor is not detectably accumulating N, but the C:N ratio is increasing. Mineral soil N has decreased nonsignificantly in recent decades, but the variability of these measurements prevents detection of a change of <700 kg of N ha^鈥?. Whether the excess N is accumulating in the ecosystem or lost through denitrification will be difficult to determine, but the distinction has important implications for the local ecosystem and global climate.