Atmospheric N Deposition Increases Organic N Loss from Temperate Forests |
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Authors: | E N J Brookshire H M Valett S A Thomas J R Webster |
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Institution: | (1) Department of Biological Sciences, Virginia Polytechnic Institute and State University, Blacksburg, Virginia 24061, USA;(2) Department of Ecology and Evolutionary Biology, Cornell University, Ithaca, New York 14853, USA;(3) Present address: Department of Ecology and Evolutionary Biology, Princeton University, Princeton, New Jersey 08544, USA |
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Abstract: | Atmospheric deposition of nitrogen (N) resulting from fossil fuel combustion has increased N inputs to temperate forests worldwide
with large consequences for forest productivity and water quality. Recent work has illustrated that dissolved organic N (DON)
often dominates N loss from unpolluted forests and that the relative magnitude of dissolved inorganic N (DIN) loss increases
with atmospheric loading. In contrast to DIN, DON loss is thought to be controlled by soil dynamics that operate independently
of N supply and demand and thus should track dissolved organic carbon (DOC) following strict stoichiometric constraints. Conversely,
DON loss may shift with N supply if soil (SOM) or dissolved organic matter (DOM) is stoichiometrically altered. Here, we assess
these two explanations of DON loss, which we refer to as the Passive Carbon Vehicle and the Stoichiometric Enrichment hypotheses, by analyzing patterns in soil and stream C and N in forest watersheds spanning a broad gradient in atmospheric
N loading (5–45 kg N ha−1 y−1). We show that soil N and DON losses are not static but rather increase asymptotically with N loading whereas soil C and
DOC do not, resulting in enrichment of organic N expressed as decreased soil C:N and stream DOC:DON ratios. DON losses from
unpolluted sites are consistent with conservative dissolution and transport of refractory SOM. As N supply increases, however,
N enrichment of organic losses is greater than expected from simple dissolution of enriched soils, suggesting activation of
novel pathways of DON production or direct N enrichment of DOM. We suggest that our two hypotheses represent domains of control
over forest DON loss as N supply increases but also that stoichiometric enrichment of bulk soils alone cannot fully account
for large DON losses in the most N-polluted forests. |
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Keywords: | DON temperate forests atmospheric deposition stoichiometry watersheds stream chemistry |
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