To investigate the relationship between plant diversity and soil animal species diversity (focal groups are Earthworms, Collembola and Nematoda)
To investigate the importance of plant functional group diversity and primary productivity for soil animal community structure
To investigate the response of the above-ground community to experimentally reduced earthworm and soil insect populations
To investigate the response of soil processes such as nutrient cycling, decomposition and carbon storage, and soil microorganisms to experimentally reduced earthworm and soil insect populations
To investigate the role of soil invertebrates for plant competition and invasibility of plant communities and therefore for the stability of plant communities

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Specific goals

To investigate whether fungal grazers (collembolans) and soil processing invertebrates (earthworms) differentially affect ecosystem processes and plant performance
To investigate whether the effect of fungal grazing invertebrates and earthworms on
ecosystem processes are more pronounced in more diverse plant communities
To investigate whether the presence of legumes counteract the effect of earthworms on ecosystem processes and plant community structure
To investigate whether decomposer organisms modify the relative importance of plant diversity per se and plant functional group diversity for ecosystem processes
To investigate whether decomposer organisms differentially affect herbivore performance in low and high diversity communities
To investigate if modifications in plant performance and ecosystem functioning due to herbivores above the ground are affected by below-ground invertebrates

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Earthworms

Field work

Manipulation of earthworm densities to investigate the effect of altered densities on plant performance.

Exclusion of earthworms is done by using the octet method in earthworm reduction subplots. Furthermore, earthworm densities are increased in earthworm addition subplots.

Greenhouse experiment

Aim: Investigate the effect of earthworms on the invasibility of plant communities of different plant functional group diversity for three target plant species (legume, herb, grass).

The main questions are:

How do earthworms affect the invisibility of an established system?
How important is the plant functional group identity and diversity of the established system for the invasibility?
Does functional group identity of the invading plant species matter?

The microcosm experiment will run for about 4 months. In the first part of the experiment the plant community will be established. After 6 weeks the aboveground biomass will be harvested and seeds of invader plant species will be applied. Afterwards, the experiment will run for 8 weeks to investigate the performance of the invader plants.

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Collembolans

Field work

Collembolans are known to be very sensitive to insecticide treatments. To investigate the effect of these soil invertebrates on plant growth, Collembolan densities are manipulated in the insecticide subplots using a belowground insecticide. Applications occur monthly with regard to the high reproduction rate of this insect group. In addition to plant biomass, soil animals (including Collembolans) will be sampled in spring and autumn using Kempson and MacFadyen soil cores in order to gain information on the food web and the soil animal community on the field site.

Greenhouse experiment

In addition to the field work a microcosm greenhouse experiment will be conducted with the aim to investigate the role of Collembolans for plant performance and nutrient uptake. Of particular interest is

whether the effect of Collembolans on plant performance varies with animals of different functional groups
if Collembola species of different functional groups interactively affect plant performance
to which extent Collembolans of different functional groups are able to mobilise nutrients in the soil and which plant species mainly benefit from it.

To test for this, Collembolans of two different functional groups will be used. Epigeic species, living predominately on the surface of the soil and in the litter layer and endogeic ones which live directly in the soil, mainly occupying upper soil layers.

The effect on plant performance will be tested with nine plant species of three plant functional groups (grasses, legumes and herbs; three species per group) corresponding to the design of the Jena Experiment.

To test for nutrient mobilisation and nutrient uptake by the plants, ¹⁵N-labelled root material of Lolium perenne will mixed into the upper 5 cm of soil of every microcosm.

The experiment will run for approximately 4 months, followed by harvesting of plant material and counting of Collembolans.