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NITROGEN LOSS FROM SMALL WATERSHEDS IN THE OREGON CASCADES: A STUDY OF FOREST SUCCESSION INFLUENCE
Citation:
Cairns, M A. NITROGEN LOSS FROM SMALL WATERSHEDS IN THE OREGON CASCADES: A STUDY OF FOREST SUCCESSION INFLUENCE. Presented at Forested Catchments: Hydrological, Geochemical, and Biological Processes (Gordon Conference), Andover, NH, July 22-27, 2001.
Description:
Traditional biogeochemical theory suggests that biotic N limitation (N demand by plants and soil microorganisms) controls ecosystem nitrogen (N) losses, and that stream N export should increase with successional age. I am examining patterns of inorganic and organic N export from 20 headwater streams characterized by their wide range of forest seral stage distribution. First-year data showed a trend contradictory to the prevailing theory. Older watersheds exported less N than did younger watersheds. Values ranged from 0.1 to 5.4 kgN/ha. Mean total dissolved N concentrations were seasonal and ranged from 0.024-0.301 mg/L. This seasonality was driven by the dominance of nitrate, which ranged from a mean annual concentration of 0.000 to 0.244 mgN/L and exhibited a pronounced spring/summer minimum. Ammonium concentrations were consistently low (mean = 0.004 mgN/L). The pattern of annual dissolved organic N (DON) loss was marginally seasonal (spring and fall increases) and ranged from 0.02 to 1.62 kgN/ha. Contrary to other published results DON loss was not correlated with dissolved organic carbon (DOC), which in turn varied seasonally in a pattern similar to that of nitrate. Despite the late successional character of the forests dominating several of my study watersheds, N export was low and all forests retained most of their N inputs. The reasons for this high retention are unknown. In future plot-scale studies I will attempt to explain some of the mechanisms controlling the magnitude and forms of N loss from the four seral stages by measuring soil solution N and DOC as well as forest floor and mineral soil C:N ratios.