Regulation of Microbial Community Composition and Activity by Soil Nutrient Availability, Soil pH, and Herbivory in the Tundra

Soil nitrogen (N) availability and pH constitute major abiotic controls over microbial community composition and activity in tundra ecosystems. On the other hand, mammalian grazers form an important biotic factor influencing resource coupling between plants and soil microorganisms. To investigate individual effects and interactions among soil nutrients, pH, and grazing on tundra soils, we performed factorial treatments of fertilization, liming, and grazer exclusion in the field for 3 years at 2 contrasting tundra habitats, acidic (N-poor) and non-acidic (N-rich) tundra heaths. The effects of all treatments were small in the non-acidic tundra heaths. In the acidic tundra heaths, fertilization decreased the fungal:bacterial ratio as analyzed by soil PLFAs, but there were no effects of liming. Fertilization increased soil N concentrations more drastically in ungrazed than grazed plots, and in parallel, fertilization decreased the fungal:bacterial ratio to a greater extent in the ungrazed plots. Liming, on the other hand, partly negated the effects of fertilization on both soil N concentrations and PLFAs. Fertilization drastically increased the activity of phenol oxidase, a microbial enzyme synthesized for degradation of soil phenols, in grazed plots, but had no effect in ungrazed plots. Taken together, our results demonstrate that grazers have the potential to regulate the fungal:bacterial ratio in soils through influencing N availability for the soil microorganisms.