The contrasted impacts of grasshoppers on soil microbial activities in function of primary production and herbivore diet
Abstract
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 plant 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 with contrasted productivities. In order to assess the effects of herbivory on soil microbes,
we measured their extracellular enzymatic activities, biomass and potential nitrogen mineralization
(PNM). Soil and vegetation were also characterized to test how much 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 were influenced by primary production,
though in a way that differed from the productivity model. The most productive sites were
constituted 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, high soil C content, 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 resource requirements
of microbes 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.
Domains
Biodiversity and EcologyOrigin | Publisher files allowed on an open archive |
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