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Red alder increases nitrogen levels in Oregon Coast Range streams
WED scientists examining the connection between nitrogen-fixing
trees and loss of nutrients from watersheds have analyzed the
chemistry of 26 small streams within the Salmon River basin of the
Oregon Coast Range. Disturbed landscapes in the study area have been
colonized by red alder, a native tree that hosts in its roots
bacteria that convert atmospheric nitrogen into forms of nitrogen
that are nutrients for plants and other organisms. Alders can add 50
to 200 kg of nitrogen per hectare per year to the ecosystem, 20 to
100 times more than that added by precipitation. Once in the soil,
this biologically fixed nitrogen can behave like fertilizer and be
converted into nitrate. Once available nitrogen exceeds the needs of
the ecosystem, it leaches as nitrate, and to a lesser extent
dissolved organic nitrogen, into groundwater, lakes, and streams.
The scientists found that nitrogen losses varied from watershed to
watershed, but the amount of loss is strongly related to the amount
of alder trees within the watershed. The research indicates that
nitrogen saturation is occurring in many of the area's forested
watersheds. The work suggests that more research effort should be
focused on the collective effects of human disturbance and alder
colonization on watershed-scale nutrient cycling and aquatic
productivity in the Pacific Northwest coastal region. (Contact J.E.
Compton, 541-754-4620, compton.jana@epa.gov)