Plant-soil animal interactions in grassland ecosystems

Understanding the effects of plant functional groups on food chains within the soil subsystem and the density and diversity of soil organisms of different trophic levels is urgently required in current ecological research. To cover these largely unknown topics, nine grassy arable fallows of different age (three 2-, 7- and 12-year-old grasslands) will be investigated in a two-factorial design with the factors plant functional group (legume, grass, non-legume herb) and age of grassland (2, 7 and 12 years) arranged in three blocks. Collembola and Staphylinidae are selected as target groups because they fulfil key functions in terms of regulating litter decomposition and nutrient cycling (Collembola) or feeding on aboveground herbivores and belowground secondary decomposers (Staphylinidae). Moreover, both insect groups are rich in species in agricultural ecosystems and reveal a strong predator-prey relationship, which may enable to discover propagating effects of the plant functional groups through the soil food web (e. g. presence of legumes increased density of Collembola? increased density of Staphylinidae?). The study design enables to test the following hypotheses: 1) The densities of the different feeding guilds within the Collembola community are increased in legume plots. 2) The presence of legumes causes a strong bottom-up effect propagating through the food web leading to high microbial biomass and high densities of Collembola and their predators (Staphylinidae). 3) The density and diversity of Collembola and Staphylinidae increase with successional age of grassy arable fallows due to increasing diversity of habitat structure and food resources. Furthermore the differences in the density and diversity of the Collembola and Staphylinidae communities between the plant functional groups increase with successional age of the grassy arable fallows because of the decreasing influence of former disturbances. To test these hypotheses we will choose two innovative approaches: A) Digging out single plants with the associated soil enables to determine the root associated microbial biomass and the density and diversity of the root associated soil fauna of the different plant functional groups, thus covering a new "micro-scale approach" in soil ecology. B) To analyze the potential food chain "plant functional group Collembola Staphylinidae" we will additionally carry out analyses of d 15N and d13C values of the different plant functional groups and the dominant Collembola and Staphylinidae taxa which will allow us to identify links between prey and consumer species. Overall, we expect to provide new indication for the importance of aboveground components (different plant functional groups) to the soil food web, by focussing on the response of a special food chain of the soil food web (plant functional group Collembola Staphylinidae).

The new micro-scale approach of this project indicated that the soil fauna community of grassy fallows is strongly influenced by the presence of single plants species (especially the grass Bromus sterilis). Focusing on the study design the density of Collembola, gamasid and oribatid mites and several soil macrofauna groups in nine grassy arable fallows of different age were investigated in a factorial design with the factors plant species (legume: Medicago sativa, herb: Taraxacum officinale, grass: Bromus sterilis) and age class (A1: 2-3/3-4, A2: 6-8/7-9, A3: 12-15/13-16 years in 2008/2009). The densities of the saprophagous macrofauna ( Julidae, Isopoda and Lumbricidae), Collembola and Gamasida (predatory mites) were significantly higher in B. sterilis than in M. sativa and T. officinale samples indicating that Collembola and the sapropahgous macrofauna possibly benefited from the particularly high amount of fine roots in the B. sterilis samples leading to high densities of predatory mites feeding on Collembola. This Bromus-effect underlines the importance of ecological field studies focusing on the micro-scale of single plants. In contrast, the age of the fallows had no significant (Collembola, Gamasida) or just a moderate influence (soil macrofauna: Gastropoda, predatory beetles) on the densities of the investigated soil fauna groups presumably caused by the heterogeneity of the investigated sites. Measurements of ?13C and ?15N signatures of the investigated plant species and animal taxa gave us an insight in the soil food web of the investigated fallows which spanned 2-3 trophic levels starting with typical primary decomposers (e.g. Julidae). The other animal taxa formed a continuum with secondary decomposers (e.g. Collembola) and omnivorous ant taxa (e.g. Lasius sp.) to predators (e.g. several Staphylinidae taxa). The measured ?13C and ?15N values of the ant taxa Lasius sp., Myrmica sp. and Solenopsis sp. indicated a niche separation with different degrees of predatory behavior supporting the results of other studies. Moreover, due to the higher ?13C signatures of the shoot/leave material , the roots, the Julidae, the collembolan taxon Lepidocyrtus sp.and the ant taxa Myrmica sp., Lasius sp. and Solenopsis sp. in the young (A1) compared to the old (A3) fallows we were able to identify separated food chains within our experimental design. Altogether, it is ecologically desirable to maintain different age classes of grassy arable fallows that include the investigated plant species (Bromus sterilis, Medicago sativa and Taraxacum officinale) within the agricultural landscape of the Marchfeld region to protect the composition and diversity of the soil fauna communities and the structure of the soil food web (see analyses of the stable isotopes).