Data from: Nutrient availability controls the impact of mammalian herbivores on soil carbon and nitrogen pools in grasslands

Grasslands have been subject to considerable alteration due to human
activities globally, including widespread changes in populations and
composition of large mammalian herbivores and elevated supply of
nutrients. Grassland soils remain important reservoirs of carbon (C) and
nitrogen (N). Herbivores may affect both C and N pools and these changes
likely interact with increases in soil nutrient availability. Given the
scale of grassland soil fluxes, such changes can have striking
consequences for atmospheric C concentrations and the
climate. Here, we use the Nutrient Network experiment to examine
the responses of soil C and N pools to mammalian herbivore exclusion
across 22 grasslands, under ambient and elevated nutrient availabilities
(fertilized with NPK + micronutrients). We show that the impact of
herbivore exclusion on soil C and N pools depends on
fertilization. Under ambient nutrient conditions, we observed no
effect of herbivore exclusion, but under elevated nutrient supply, pools
are smaller upon herbivore exclusion. The highest mean soil C and N pools
were found in grazed and fertilized plots. The decrease in soil C and N
upon herbivore exclusion in combination with fertilization correlated with
a decrease in aboveground plant biomass and microbial activity, indicating
a reduced storage of organic matter and microbial residues as soil C and
N. The response of soil C and N pools to herbivore exclusion was
contingent on temperature – herbivores likely cause losses of C and N in
colder sites and increases in warmer sites. Additionally, grasslands that
contain mammalian herbivores have the potential to sequester more N under
increased temperature variability and nutrient enrichment than ungrazed
grasslands. Our study highlights the importance of conserving
mammalian herbivore populations in grasslands worldwide. We need to
incorporate local-scale herbivory, and its interaction with
nutrient enrichment and climate, within global-scale models to better
predict land-atmosphere interactions under future climate change.