Data collected for: The contrasted impacts of grasshoppers on soil microbial activities in function of ecosystem productivity and herbivore diet

Herbivory can have contrasted impacts on soil microbes and nutrient
cycling, which has stimulated the development of conceptual frameworks
exploring the links between below- and aboveground processes. The
“productivity model” predicts that herbivores stimulate microbial
activities and accelerate nutrient mineralization in productive
ecosystems, while they have an opposite effect in less productive
ecosystems. In parallel, the “diet model” predicts that herbivores feeding
on conservative plants accelerate nutrient cycling while those feeding on
exploitative plants decelerate nutrient cycling, due to changes in litter
inputs. Since these two frameworks can lead to conflicting predictions in
some cases, experimental evidence combining herbivore diet and
productivity is required. During two consecutive years, we conducted an
experiment controlling the presence of three grasshopper species consuming
either grasses, forbs or both in twelve natural and managed alpine
grasslands of contrasted productivities. In order to assess the effects of
herbivory on soil microbes, we measured their enzymatic activities, their
biomass and the soil potential nitrogen mineralization (PNM). Soil and
vegetation characteristics were also determined in order to test if they
modulated the effects of herbivory on microbes. Contrary to the
predictions of the diet model, the effects of herbivory on microbial
characteristics did not depend on the herbivores diet but relied on
ecosystem productivity. The most productive sites were characterized by
exploitative plant species which depleted N resources in the soil, and by
microbes producing relatively few extracellular enzymes, leading to a
lower PNM. Herbivory increased microbial biomass and decreased the
production of extracellular enzymes in those sites, possibly through the
stimulation of root exudates produced by exploitative species. The least
productive sites were characterized by conservative plants, which led to
the sequestration of soil C, and by microbes having a resource acquisition
strategy (more extracellular enzymes, higher PNM). Herbivory decreased
microbial biomass and increased the production of extracellular enzymes in
those sites. This pattern can be explained by the loss of carbon
associated with insect respiration, which increases the need for microbes
to acquire resources and by a lower production of root exudates by
conservative species. Therefore, the effects of two years of herbivory on
soil microbes were at odds with the productivity model, which focuses
instead on longer term effects corresponding to herbivory-induced changes
in plant species composition. This highlights the multidimensional feature
of the impacts of herbivory on ecosystem functioning, both in space and
time.