Litter decomposition and humus formation in highalpine soils

The decomposition of dead organic matter is a key ecosystem process, returning the nutritional elements to the nutrient cycle, thus making them available to the organisms. Macro-decomposers such as earthworms, millipedes and insect larvae act at the beginning of the litter decomposition process. They fragment and mix the organic matter into more accessible units for microbial attack and incorporate it into the soil. Although it is assumed that macro- decomposers play an important role for litter decomposition in high alpine soils, where suboptimal conditions for litter decomposition occur, little is known on their actual identity and function. The present proposal directly addresses this gap of knowledge. Using a stable isotope approach the key macrofauna decomposers in the high alpine region will be identified and their decomposition efforts determined. Besides, the relationship between invertebrate decomposers diversity and litter decomposition will be determined to assess biodiversity-ecosystem functioning in high alpine habitats. Furthermore, a new molecular approach will be employed, allowing to assign the faecal pellets to their invertebrate producers at a species-specific level. Thereby, the main producers of organic matter can be identified in the field. The combination of stable isotope and molecular methodology will allow to gain new insights into the role of decomposers in high alpine areas and provide an important proof of concept for future studies looking into animal- related decomposition and soil formation processes.

We determined the main decomposer animals in high alpine soils, evaluated their decomposition efforts, assessed the gut microbiota of alpine dipteran larvae, and propose a new molecular approach to non-destructively study soil animals responsible for humus and soil formation. Litter decomposition is an essential ecosystem process, returning elements bound in organic matter to the nutrient cycle. The first actors in this process are macro-decomposers such as earthworms, millipedes and insect larvae. In high alpine soils little is known about these important soil animals, although these ecosystems are currently in a state of change due to climate warming and other human induced interferences. In this project we were able to close some gaps of knowledge: (1) We evaluated the food-web of high alpine soils: main decomposers are earthworms, larvae of black fungus gnats and nonbiting midges, and to a lesser extent millipedes, while the main predators are ground beetles and some fly larvae. In our case study area optimal living conditions prevail for earthworms to migrate as far as the top of the mountain in 2600 m a.s.l., mainly due to the presence of sheep dung, a favourable food source for these animals. (2) In a mesocosm experiment we tested whether decomposer diversity has positive impacts on litter decomposition. Our results show that functional diversity is highly important for the decomposition process, as litter mass loss is increased and the variability in litter decomposition is greatly reduced when three functionally different soil animals are present. (3) Soil dwelling dipteran larvae possess a characteristic microbial community which is independent and distinct of their ingested food type. Thus, they are resilient to substrate changes and are possibly more adaptable to effects of climate change and shifting land-use then taxa with diet-related, transient and substrate-dependent microorganisms.(4) To determine which soil animals are important in humus and soil formation, we propose a non-destructive molecular method, which allows to infer the biomass of soft-skinned animals such as earthworms from the number of their cells detected in a soil sample. The results of this project allow us to evaluate important ecosystem services in high alpine soils and to better predict future climate scenarios in these sensitive habitats.