Introduction

In 2002 a long-term experiment has been established in the Saale-valley nearby Jena (Germany) in order to study the interactions between plant diversity and ecosystem processes, focussing on element cycling and trophic interactions.

Experiment should allow the establishment of nested project-specific treatments, managements and manipulation experiments of several subprojects. A specific design was chosen as a consequence of taking samples for alternative projects. As result of sampling and experimental manipulations, the research site was divided into plots and subdivided into subplots in which particular experiments and measurements are carried out.

60 plant species, native and common to the Central European Arrhenatherum grasslands serve as species pool. Mixtures of one to 60 plant species and of one to four plant functional groups have been seeded as newly established communities on plots of 20 m x 20 m, in addition on specific small subplots (3.5 m x 3.5 m) to disentangle specific effects on species diversity, and scale effects.

The experiment is carried out on a former agricultural field with alluvial loam as substrate. The species assemblages serve as basis to study interactions not only among plant individuals and plant species, but also between different trophic levels. Emphasis will be on the ecosystem carbon balance and the turnover and losses of nutrients.

Different research groups are organized into subprojects that closely collaborating in the biodiversity project in order to assess ecosystem level effects on biodiversity that also provides feedback between project phases:

• In the first phase of the experiment (2002–2005), the experimental plots were established and the productivity as well as pools of carbon and nutrients were quantified as a function of diversity. For a number of processes, the relative importance of species richness, functional group diversity, and particular species could be established. In addition, a first quantification of trophic interactions was made and, as a measure of community stability, the invasion resistance of the different communities was quantified as a function of plant species diversity.

• The second phase (2005–2008) built upon the results of the first phase. The effects of diversity on nutrient cycling were examined in more detail and first nutrient balances were obtained for each diversity level. In addition, the analysis of biotic interactions, between plants, both intra- as well as inter-specifically, between plants and other organisms such as pollinators, herbivores, plant pathogens, microbes, vertebrate and invertebrate taxa was carried out in much more detail.

• The third phase (2008-2010) is on synthesis of the result. While in the first two phases this data has been analyzed mainly with respect to the response of particular variables to changes in plant diversity, the challenge is to integrate the data and to look at the multivariate response of ecosystem processes to diversity. A synthesis of data, which have all been obtained on the same set of experimental plots, for joint analysis goes well beyond all published meta-analyses of results and will greatly enhance our understanding of the role of biodiversity for ecosystem processes.