Examining Soil’s Role in Tracing Nutrients From Salmon into Riparian Trees

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Pacific salmon transport large amounts of marine nutrients to freshwater and forest ecosystems when they migrate from the ocean, spawn, and die. 

Marine nutrient subsidies from salmon have been traced into riparian forest trees using stable isotopes of nitrogen, but few studies consider how soils affect nitrogen isotope values and thus the final signature recorded in trees. Scientists from USGS and the University of Washington used a 20-year salmon manipulation experiment in Bristol Bay, Alaska to evaluate how marine nitrogen isotopes in salmon make their way through soils and into riparian forest trees. They found that nitrogen isotopes in soil often greatly exceeded salmon isotope values, indicating a previous methodological issue, which has probably caused overestimates in most prior studies of how salmon fertilize streamside trees. The study provides a basis for more careful analyses of mitigation programs that target salmon nutrient enhancement in areas where populations have declined.

Feddern, M.L., Holtgrieve, G.W., Perakis, S.S., Hart, J., Ro, H., Quinn, T.P., 2019, Riparian soil nitrogen cycling and isotopic enrichment in response to a long-term salmon carcass manipulation experiment: Ecosphere, v. 10, no. 11, p. e02958, https://doi.org/10.1002/ecs2.2958

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