Mycorrhizal fungi take up and store salmon nitrogen

Pacific salmon deliver marine-derived nitrogen (MDN) to terrestrial ecosystems, influencing riparian soils, plants, and animals. However, the role of soil fungi in retaining MDN has received little attention despite their importance in nutrient cycling. We investigated MDN incorporation into fungi, trees, and soils along three salmon streams in southwest Alaska, including one with a 21-year carcass addition/removal experiment. We measured carbon and nitrogen concentrations and stable isotope ratios (δ¹⁵N, δ¹³C) across ecosystem pools. MDN was detected in soils, vegetation, and ectomycorrhizal fungal (EMF) sporocarps, with up to 27% of fungal nitrogen derived from MDN. MDN also increased total nitrogen concentrations in EMF, indicating preferential storage. These results for the first time identify EMF as a major MDN sink and highlight fungi as a critical, previously underappreciated link in salmon–forest nutrient dynamics.