Effects of Succession on Nitrogen Export in the West-Central Cascades, Oregon |
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Authors: | Michael A Cairns Kate Lajtha |
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Institution: | (1) Western Ecology Division, National Health and Environmental Effects Research Laboratory, Office of Research and Development, U.S. Environmental Protection Agency, 200 SW 35th Street, Corvallis, Oregon 97333, USA;(2) Department of Botany and Plant Pathology, Oregon State University, Corvallis, Oregon 97331, USA |
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Abstract: | This study examined impacts of succession on N export from 20 headwater stream systems in the west central Cascades of Oregon,
a region of low anthropogenic N inputs. The seasonal and successional patterns of nitrate (NO3−N) concentrations drove differences in total dissolved N concentrations because ammonium (NH4−N) concentrations were very low (usually < 0.005 mg L−1) and mean dissolved organic nitrogen (DON) concentrations were less variable than nitrate concentrations. In contrast to
studies suggesting that DON levels strongly dominate in pristine watersheds, DON accounted for 24, 52, and 51% of the overall
mean TDN concentration of our young (defined as predominantly in stand initiation and stem exclusion phases), middle-aged
(defined as mixes of mostly understory reinitiation and older phases) and old-growth watersheds, respectively. Although other
studies of cutting in unpolluted forests have suggested a harvest effect lasting 5 years or less, our young successional watersheds
that were all older than 10 years still lost significantly more N, primarily as NO3−N, than did watersheds containing more mature forests, even though all forest floor and mineral soil C:N ratios were well
above levels reported in the literature for leaching of dissolved inorganic nitrogen. The influence of alder may contribute
to these patterns, although hardwood cover was quite low in all watersheds; it is possible that in forested ecosystems with
very low anthropogenic N inputs, even very low alder cover in riparian zones can cause elevated N exports. Only the youngest
watersheds, with the highest nitrate losses, exhibited seasonal patterns of increased summer uptake by vegetation as well
as flushing at the onset of fall freshets. Older watersheds with lower N losses did not exhibit seasonal patterns for any
N species. The results, taken together, suggest a role for both vegetation and hydrology in N retention and loss, and add
to our understanding of N cycling by successional forest ecosystems influenced by disturbance at various spatial and temporal
scales in a region of relatively low anthropogenic N input. |
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Keywords: | Cascades nitrate dissolved organic nitrogen dissolved organic carbon logging watersheds forest succession nitrogen export |
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