| Summary: | Plant productivity is generally increasing in the Arctic as a consequence of accelerated climate change. The change in plant communities may coincide with a loss of carbon from Arctic soils due to increased decomposition and respiration. Herbivores can mediate these changes through preferential foraging on highly productive plant species, trampling, and waste deposition. Soil fungi are also a major component in these interactions and are controlled by plant community and soil conditions. Soil fungi have large impacts on the cycling of carbon in soil and its subsequent release to the atmosphere. Understanding of the effects of large mammalian herbivores on carbon processes, such as respiration and decomposition, and fungal communities is important for understanding the context of future changes in carbon storage in tundra soils. I investigated the effect of herbivory on trace gas fluxes, decomposition and stabilization of organic matter, and soil fungal communities through the use of herbivore exclusion fences in tundra communities. Herbivory reduced ecosystem respiration in a meadow community, reduced stabilization under a deciduous shrub in a heath community, reduced arbuscular mycorrhizal fungi across the Arctic, and reduced ectomycorrhizal fungi locally in Swedish tundra. The presence of herbivores on the landscape can have complex effects on carbon in tundra habitats by reducing respiration rates and limiting fast cycling arbuscular mycorrhizal fungi, while simultaneously reducing the stability of organic matter as it decomposes and locally limiting slower cycling ectomycorrhizal fungi. The relative contribution of each of these processes to carbon cycling will determine the net effect of herbivores on tundra soils. Herbivory impacts are context dependent and the net effect on soil carbon is likely related to the proportion of different tundra community types on the landscape.
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