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Symbiotic nitrogen-fixing trees enhance forest fertility by increasing nitrogen and organic matter in soil. 

These trees can also alter soil nutrients such as phosphorus and molybdenum that are essential for nitrogen-fixation, but little is known about these relationships regionally. USGS and university scientists, supported by the NSF-USGS Graduate Research Internship Program, studied forests across western Oregon, Washington, and British Columbia to examine how nitrogen-fixing red alder trees alter soils. After two-decades of tree growth, topsoil under red alder trees accumulated more carbon, nitrogen, and phosphorus than soil under non-fixing trees. Surprisingly, the trace nutrient molybdenum that is essential for nitrogen fixation was unaffected by red alder trees, and instead molybdenum varied with environmental factors such as temperature and rainfall. Given their rapid influence on soils, future increases in red alder trees by forest management and disturbance could alter soil fertility across many Pacific Northwest forests.

Dynarski, K.A., Pett-Ridge, J.C., Perakis, S.S., 2020, Decadal-scale decoupling of soil phosphorus and molybdenum cycles by temperate N-fixing trees: Biogeochemistry,

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