Top-down control by nematodes over soil microorganisms – considered stronger over bacteria than fungi - may dampen microbial responses to global changes in tundra. To test whether large grazers alter the responses of belowground trophic networks to global changes, we employed factorial warming and nitrogen fertilization treatments in adjacent sites with different reindeer grazing intensities for the past 50 years. Lightly grazed tundra is dominated by dwarf shrubs and a more fungal-based microbial community, while in heavily grazed tundra, high reindeer densities during autumn migration have induced shift into graminoids and more bacterial-based microbial community. We analysed the soil micro-food web, i.e., the nematode density, trophic structure, and species composition as well as fungal, bacterial and total phospholipid fatty acids (PLFAs) after four growing seasons of warming and fertilization both before and during reindeer migration. We predicted that bacterivore densities are higher and fungivore densities lower under heavy than light grazing (i.e., nematode populations before migration reflect grazing effects via the base of food web), whereas reindeer migration induces negative impact on nematode densities under heavy grazing (i.e., disturbance by trampling is the driving factor). We further predicted that nematodes negate treatment effects on microbial biomass to a stronger extent in the bacterial-based heavily grazed than the fungal-based lightly grazed tundra. Fungivore densities were higher under light than heavy grazing, but nematodes did not respond to trampling. Warming increased fungivores and the fungal PLFAs irrespective of grazing and timing, but under heavy grazing, increased bacterivores while the bacterial PLFAs remained steady. Fertilization increased carnivores and influenced nematode species composition, diversity and maturity interactively with warming. Our data suggest that large grazers affect tundra soil nematodes via bottom-up effects through microbial community composition and biomass, which in turn may alter the strength of their top-down control soil bacteria under climate warming.

This file contains densities of nematode trophic groups and families together with soil phospholipids (PLFAs) after four growing seasons of factorial warming and fertilization at two 50-year reindeer grazing intensities. The study site is conducted in Raisduoddar (69°31’29 N, 21°19’16 E; altitude 430–570 m asl), northern Norway.