Large predators and biogeochemical hotspots: brown bear (Ursus arctos) predation on salmon alters nitrogen cycling in riparian soils

Two important themes in ecology include the understanding of how interactions among species control ecosystem processes, and how habitats can be connected through transfers of nutrients and energy by mobile organisms. An impressive example of both is the large in flux of nutrients and organic matter that anadromous salmon supply to inland aquatic and terrestrial ecosystems and the role of predation by brown bears (Ursus arctos) in transferring these marine-derived nutrients (MDN) from freshwater to riparian habitats. In spite of the recognition that salmon-bear interactions likely play an important role in controlling the flux of MDN from aquatic to riparian habitats, few studies have linked bear predation on salmon to processes such as nitrogen (N) or carbon (C) cycling. We combine landscape-level survey data and a replicated bear-exclosure experiment to test how bear foraging on salmon affects nitrous oxide (N2O) flux, carbon dioxide (CO2) flux, and nutrient concentrations of riparian soils. Our results show that bears feeding on salmon increased soil ammonium (NH4+) concentrations three-fold and N2O flux by 32-fold. Soil CO2 flux, nitrate (NO3-), and N transformation differences were negligible in areas where bears fed on salmon. Reference areas without concentrated bear activity showed no detectable change in soil N cycling after the arrival of salmon to streams. Exclosure experiments showed that bear effects on soil nutrient cycles were transient, and soil N processing returned to background conditions within 1 year after bears were removed from the system. These results suggest that recipient ecosystems do not show uniform responses to MDN inputs and highlight the importance of large mobile consumers in generating landscape heterogeneity in nutrient cycles.