Soil Microbial Responses to Herbivory in an Arctic Tundra Heath at Two Levels of Nutrient Availability

We tested the hypothesis that herbivory accelerates nutrient cycling through nutrient-rich soils and decelerates nutrient cycling through nutrient-poor soils in a well-drained arctic tundra heath by measuring effects of herbivory on soil and plant properties in control areas and areas treated with NPK fertilizer. The impact of herbivores was studied with two types of exclosures, one excluding reindeer and the other excluding both rodents and reindeer. We predicted that herbivores would enhance soil microbial processes in the fertilized areas, but retard them in the unfertilized ones. Microbial respiration and microbial C were significantly lower in grazed areas than in herbivore exclosures in both unfertilized and fertilized treatments, indicating that herbivores limit the C available for the soil microbes. Microbial N was significantly increased in the exclosures in the fertilized treatment, but there were no effects in the unfertilized one. This reflects both an increase in resources by fertilization and the effect of herbivores on microbial N acquisition: there was surplus N in the fertilized areas to be immobilized by microbial biomass only when access by herbivores was prevented. Thus, mammalian grazers affect the resource coupling between the plant and microbial trophic levels. Fertilization did not affect the soil inorganic, organic, or microbial N contents, or litter decomposition rates, but it significantly increased soil and microbial P. Microbial C was significantly decreased by fertilization, contradicting some earlier fertilization studies in the arctic. One reason for deceleration of soil nutrient cycling in our study area may be that herbivory by both reindeer and rodents occurs mainly outside the growing season; hence, mammalian waste products do not contribute to soil nutrient availability at the times of highest nutrient demands by plants. In addition, grazing during reindeer migrations is likely to cause net resource output from the system.