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EFFECTS OF SUCCESSION ON NITROGEN EXPORT IN THE WEST-CENTRAL CASCADES, OREGON
Citation:
Cairns, M A. AND K. J. Lajtha. EFFECTS OF SUCCESSION ON NITROGEN EXPORT IN THE WEST-CENTRAL CASCADES, OREGON. ECOSYSTEMS. Springer, New York, NY, 8(5):583-601, (2005).
Impact/Purpose:
To examine the impact of successional and disturbance history on N export in the west central Cascades of Oregon
Description:
Conceptual models predict that unpolluted, aggrading forest ecosystems tightly retain available nitrogen (N) until declining productivity by mature trees and storage in detritus reduces the demand for essential nutrients, and N export increases to equal input. Short-term nitrate loss following disturbance is similarly predicted to reverse as vegetation and microbial demands for biologically available N increase. This study examined impacts of successional and disturbance history 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 DON concentrations were less than one-half those of NO3-N. In contrast to studies suggesting that DON levels dominate in pristine watersheds, DON accounted for 24, 52, and 51% of the overall mean TDN concentration of our young, middle-aged and old watersheds, respectively. Young successional watersheds lost significantly more N, primarily as NO3-N, than did watersheds containing more mature forests, and also exported more DOC and DON. Seasonal nitrate loss patterns in this study did not conform to models of increased summer uptake by vegetation and thus lower stream NO3-N concentrations which has been observed in watersheds with elevated N inputs, but rather conformed to a pattern of flushing at the onset of fall freshets when excess N, accumulated during the dry season, is lost to streamwater during the ascending arm of the hydrograph. A multiple linear regression model containing stream DOC:DON ratio, fire history, and successional status explained 85% of the variation in NO3-N export. The results, taken together, suggest a role for both soils and vegetation in N retention 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.