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NITROGEN FLUX FROM WEST CASCADE RANGE HEADWATER STREAMS DRAINING FORESTS ALONG A SUCCESSIONAL GRADIENT
Citation:
Cairns, M A. AND K. J. Lajtha. NITROGEN FLUX FROM WEST CASCADE RANGE HEADWATER STREAMS DRAINING FORESTS ALONG A SUCCESSIONAL GRADIENT. Presented at 4th North American Forest Ecology Workshop, Corvallis, OR, June 16-20, 2003.
Description:
Conceptual models predict that unpolluted, aggrading forest ecosystems tightly retain available nitrogen (N) until declining productivity by mature trees reduces the demand for essential nutrients and export increases to equal N inputs. 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 dominated 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. Young successional watersheds lost significantly more N, primarily as NO3-N, than did watersheds containing more mature forests. Young watersheds had N export rates averaging 2.45 kg ha-1yr-1 (range 0.78-3.57 kg ha-1yr-1) while older watersheds exported an average of 0.97 kg ha-1yr-1 (range 0.18-2.22 kg ha-1yr-1). These N exports compare with deposition inputs of approximately 0.93 kg ha-1yr-1. Proportion of young successional forest cover, particularly that of the stem exclusion seral stage, appears to be an important predictor of DOC:DON ratio and N export rates. 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.