Climate effects on woody debris on and in Alpine soils

The proposed project deals with coarse woody debris (CWD), decay processes in alpine forest soils, soil meso- and micro-organisms and humus forms and their relation to climate in an Alpine area (Trentino). Empirical investigations and field experiments are foreseen to investigate the involved decay mechanisms. To understand these different time-scales, the leitmotif of the planned research activities is grouped among 3 different "compartments": Coarse woody debris (Picea abies, climate) Humus forms SOM (soil organic matter). We have the following research questions: How does climate affect coarse woody decay above and especially in the soil of alpine sites? What time scales are involved in (CWD: Picea abies) decay as a function of climate? How quickly is CWD (Picea abies) integrated into soil organic matter fractions? In which way are the decay products of CWD stabilised? What are the links between decay mechanisms and the spatial distribution of humus forms? Can the humus form serve as a proxy for the soil biota (mesofauna and micro-biology) for the spatial extrapolation? Sites in Trentino (Val di Rabbi, Val di Sole) along altitudinal sequences, reflecting climate zones, will be investigated, using both an empirical and an experimental approach. The climosequence includes sites ranging from 1000 m to about 2200 m asl, both on north- and south facing slopes. All sites have a natural coniferous forest. The selected areas are in dry inner-Alpine valleys where the effect of climate on soil and vegetation properties is obvious. Furthermore, an already existing large dataset (soil profiles, chemical and mineralogical data, GIS datasets) serves as the basis for the planned investigations. The assessment of CWD quantity and decay mechanisms along the climosequence sites will be performed using dendroecology and -chronology, stable isotope (13C, 15N) and chemical (lignin components ratios) measurements. One main aim is to elaborate a decay model for coarse woody debris and its incorporation into the soil. Upscaling of deadwood to the whole area of interest (Val di Rabbi, Val di Sole) will be done using the k-Nearest Neighbours (k-NN) method together with remote sensing and GIS (geographical information systems) techniques. At each climosequence site, the content and quality of organic matter (using a chemical and physical fractionation; 13C, 15N and lignin component ratios), humus forms, activity and composition of faunal (microannelids, Enchytraeidae) and microbial communities (bacteria, archaea, fungi) will be assessed. The close relation between humus forms and the soil biota is based on the fact that humus forms result mainly from the activity of soil organisms and at the same time act as habitat for them. Using the knowledge about SOM evolution that finally gives rise to a limited number of humus forms, upscaling of CWD degradation processes (using GIS) and integration into the soil at regional and landscape level will be made possible. Three submodels will be created (a small scale site characteristic model, a CWD and a regional humus model) that will be the basis for the final model. The experimental approach foresees the establishment of 5 plots (at north and south-facing slopes; in total 10) along the altitudinal sequences. At each plot, coarse woody debris will be placed on a mesocosm site and analyses on the wood debris and soils are planned after 12, 25, 52 and 104 weeks. Furthermore, highly 13C labelled CWD will be added to specially reserved mesocosm sites along the climosequences and, similarly to the other investigations, analysed after 12, 25, 52 and 104 weeks. To trace the fate of the stable isotope added by CWD into the soil, the labelling of microbial biomass is measured. Soil microbial analyses (at the community level as well as at the level of functional key genes involved in the terrestrial C- and N-cycle) based on microbial DNA will be performed after each sampling period. The determination of SOM quality (density fractionation, stable and labile fraction, radiocarbon dating, 13C, 15N, lignin components) and soil chemical analyses are also foreseen. This approach guarantees a detailed and coherent insight into decay mechanisms, formation of humus and interaction with mesofauna and microorganisms. Sound knowledge on CWD decomposition, humus forms, pedofauna, microbiology and SOM can only be gained through an interdisciplinary approach of a joint project where complementary specific expertises from forest sciences (WSL), soil biology and ecology (Dept. of Microbiology, University of Innsbruck; Laboratory Soil and Vegetation, Univ. of Neuchâtel; Dept of Geography, University of Osnabrück), forestry (BOKU, Vienna) and soil chemistry (Dept. of Geography, Univ. of Zurich) are teamed together.

Mountain forests are complex ecosystems, very sensitive to environmental changes, such as global warming. Despite the crucial role of forest ecosystems as important sources and sinks of carbon, the complex mechanisms beyond the global C-cycle remain unclear and have still to be clearly understood. Deadwood plays an important role in the functioning of forest ecosystems and their structure, contributing to maintaining the biodiversity and natural regeneration in forests, as well as influencing nutrient cycling and overall carbon storage. Since microbial communities are the main drivers in deadwood decomposition, it is of paramount importance to evaluate their structure and activity, so as to unravel the mechanisms involved in its decay. Therefore, the aim of our work package was to gain an in-depth understanding of the structure and function of microbial communities (soil vs. wood) along the selected climosequence in Val di Rabbi (Trentino, Italy) addressing the following aspects:A) Multidisciplinary study site characterisation for evaluating climate-effects in terms of exposure (North vs. South)B) Impact of ground cover and slope exposure on micro- and mesobiota in forested subalpine soilsC) Deadwood decomposition dynamics as a function of exposure and degree of decayTen study sites along an altitudinal sequence, reflecting climate zones, were investigated, ranging from 1200 m to about 2400 m above sea level, at north- and south- facing slopes. All sites have a natural coniferous forest. Altitudinal-defined climate conditions provided specific vegetation types and soil environments thereby influencing the diversity, abundance and activity of soil microbial communities along the climosequence. Moreover, we obtained insights into the synergistic interplay of fungi, the primary wood decomposers, and bacteria, which so far have been ranked as subordinate during the deadwood decay dynamics. This opens new avenues for future explotation on fungal-bacterial interactions within the deadwood environment and its implication on the soil carbon balance and ultimately, forest productivity.