Early Goose Arrival Increases Soil Nitrogen Availability More Than an Advancing Spring in Coastal Western Alaska

Abstract

An understudied aspect of climate change-induced phenological mismatch is its effect on ecosystem functioning, such as nitrogen (N) cycling. Migratory herbivore arrival time may alter N inputs and plant–herbivore feedbacks, whereas earlier springs are predicted to increase N cycling rates through warmer temperatures. However, the relative importance of these shifts in timing and how they interact to affect N cycling are largely unknown. We conducted a 3-year factorial experiment in coastal western Alaska that simulated different timings of Pacific black brant (Branta bernicla nigricans) arrival (3 weeks early, typical, 3 weeks late, or no-grazing) and the growing season (ca. 3 weeks advanced and ambient) on adsorbed and mobile inorganic (NH4+–N, NO3−–N) and mobile organic N (amino acid) pools. Early grazing increased NH4+–N, NO3−–N, and amino acids by 103%, 119%, and 7%, respectively, whereas late grazing reduced adsorbed NH4+–N and NO3−–N by 16% and 17%, respectively. In comparison, the advanced growing season increased mobile NH4+–N by 26%. The arrival time by geese and the start of the season did not interact to influence soil N availability. While the onset of spring in our system is advancing at twice the rate of migratory goose arrival, earlier goose migration is likely to be more significant than the advances in springs in influencing soil N, although both early goose arrival and advanced springs are likely to increase N availability in the future. This increase in soil N resources can have a lasting impact on plant community composition and productivity in this N-limited ecosystem.