The macrofauna decomposer food web on alpine pastureland

Based on profound knowledge on the abundance and biomass of the soil macrofauna as well as on the humus forms on alpine pastureland in the Central Alps (Kaserstattalm, Stubai Valley) the proposed study aims (1) to point out the trophic relations of the decomposer communities under the impact of land-use changes using stable isotope analysis and (2) to elucidate interactions between saprotrophic soil animals regarding different litter qualities with mesocosm experiments. The widespread assumption is that due to unfavourable climatic conditions at the alpine treeline, poor quality of especially dwarf shrub litter on abandoned pastureland and the loss of keystone species, such as as earthworms, decomposition is slowed down. The absence of these "engineers" may be one of the most significant functional changes. This leads to an accumulation of organic material, an increase in soil acidity and the development of more differentiated humus forms consisting of several horizons (moders and mors). However, earlier studies on abandoned sites in the research area have shown that mull humus can still be found more than ten years after abandonment, the invasion of dwarf shrubs and afforestations. Also the loss of keystone species such as endogeic- anecic earthworms could not generally be confirmed . Therefore the trophic diversity and the interaction complexity of large saprotrophic soil animals mainly responsible for litter fragmentation and bioturbation on differently managed alpine pastureland are to be assessed. Based on previous studies the macrofauna decomposer food web will be established by analysing the natural 15N stable isotope variations of macrofauna groups/species from four differently managed sites. The results of this analysis will then be the basis for further experiments. Mesocosm experiments will be conducted, investigating the interactions between primary and secondary comminuters and the interactions among primary comminuters, their food preferences and their effects on decompostion and plant growth. For the last mesocosm experiment 15N-labelled grass and dwarf shrub litter will be used to trace the path of nitrogen. Additional factors to estimate overall decomposition and mineralization rates will be Substrate Induced Respiration, Basal Respiration, organic C-content, C/N ratio and nutrients (NH 4 , NO 3 , PO 4 ) in the leachate. The rationale of this study is to undertake research on the functioning of decomposition processes at the alpine treeline, leading to improvements in fundamental understanding on diversity-function relations under changing conditions in an ecologically sensitive system.

The soil macrofauna (earthworms, millipedes, woodlice, insect larvae) functions mainly as comminuters of litter material and ecosystem engineers by mixing and structuring the organic and mineral soil horizons. Interacting with the soil microflora (bacteria and fungi) they decompose the accumulating organic material. On managed alpine meadows and pastures decomposition processes are similar to those in lower regions. After abandonment dwarf shrubs immigrate, litter composition and quality changes and litter quantity increases due to lacking mowing and grazing activities. The accumulation of organic material on the soil surface has profound impacts on diversity and species composition of flora and fauna. Even human settlements are indirectly affected. Corresponding studies have shown that reducing or abandoning management enhance snow gliding and even soil erosion. In the completed project we investigated the structure of the macrofauna decomposer community and the changes in the decomposition processes on managed and abandoned alpine pastureland (Kaserstattalm, Stubaier Alpen, Tirol). In controlled field and lab experiments we could demonstrate that earthworms such as Lumbricus rubellus and Dendrobaena octaedra as well as millipedes such as Cylindroiulus fulviceps and C. meinerti are the key primary decomposers. Other millipedes such as Enantiulus nanus and earthworms such as the soil-dwelling Octolasion lacteum function as secondary decomposers. that abandonment of pasturing and mowing causes a shift in the composition of the original macrofauna, leading to a higher species diversity but a lower earthworm biomass. This implicates that the newly established decomposer community is hardly able to process and translocate the large amount of litter material on the abondoned sites. that contrary to the common assumption primary macro-decomposers accept dwarf shrub leaves as food source, at least those from deciduous dwarf shrubs (Vaccinium myrtillus, V. gaultheroides). Therefore, other factors than the quality of dwarf shrub litter may be responsible for the accumulation of organic material on abandoned sites.