Data from: Long-term grazing intensity by reindeer alters the response of the soil micro-food web to simulated climate change in subarctic tundra

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.