Goose herbivory alters soil microbial community functional potential in a sub‐Arctic wetland

Herbivory alters below‐ground resource dynamics via direct and indirect mechanisms, yet its effects on microbial functional potential remain poorly understood. This limits our understanding of the mechanisms that link animal, plant and microbial functional roles in zoogeochemical cycles.

We examined the impacts of simulated goose herbivory on soil microbial community function and assembly in a coastal, wet‐sedge meadow in sub‐Arctic Alaska. To do so, we applied a full‐factorial experiment over two growing seasons to simulate three different pathways by which herbivorous geese alter the soil environment: plant grazing (i.e. defoliation), faeces deposition and trampling.

Grazing, as opposed to faeces deposition or trampling, was the primary driver of changes in potential and expressed microbial function. Grazing resulted in greater predicted abundance of genes associated with nitrogen and phosphorus cycling, potentially leading to increased nutrient flux rates. Further, grazing increased the expressed activity of phosphorus‐acquiring enzymes relative to carbon‐ and nitrogen‐acquiring enzymes in the first growing season, indicating a relative increase in phosphorus cycling.

Grazing was also the primary driver of changes in microbial community composition and diversity, whereas the roles of faeces and trampling were mainly limited to interactive effects. Grazing influenced fungal and prokaryotic community composition, with the strongest effects occurring in the first growing season. In the first growing season, grazing increased the abundance of fungal saprotrophs and dimorphic yeasts and decreased alpha diversity of both fungal and prokaryotic communities.

Fungal communities were largely influenced by stochastic assembly processes, whereas prokaryotic communities were more influenced by deterministic assembly, as determined by null modelling approaches. The relative strengths of these processes were minorly affected by grazing.

Our results indicate that goose herbivory, primarily through grazing, in a sub‐Arctic wetland can rapidly shift microbial communities and alter their role in zoogeochemical cycles. Deterministic assembly was most important for shaping prokaryotic communities, suggesting that goose herbivore‐induced changes to prokaryotic function may be predictable. In contrast, the strong influence of stochasticity in shaping fungal communities suggests that changes to fungal community function may be more variable.