Carbon pools and microbiota along an alpine soil climosequence

High mountain ecosystems offer ideal sequential scenarios for monitoring carbon (C) dynamics due to their sensitivity to changing environmental conditions. In this context, slope aspect and altitude are considered important topographical features affecting the local microclimate (i.e., temperature and precipitation patterns) and in consequence soil weathering and biogeochemical processes with implications for both ecosystem regulation and C feedbacks. Within forest ecosystems, deadwood is an essential structural and functional component as a reservoir for biological diversity and nutrient stocks (C–store). Therefore, the present work focuses on the influence of climate on soil features and deadwood decomposition dynamics as a function of different thermal conditions due to different slope exposure (north- vs. south-facing slopes) and altitude (from 1200 m to 2400 m above sea level) in (sub)alpine ecosystems in the Italian Alps. This thesis offers a general introduction (Chapter I) and a compilation of papers in peer-reviewed journals. Following the intro, Chapter II deals with the main changes in composition, activity and diversity of the soil autochthonous microbiota in terms of slope exposure along the altitudinal climosequence in combination with a comprehensive overview of the soil physico-chemical properties. The findings show that the slope exposure largely influenced both edaphic properties and soil microbiota, even though such effects were altitude-dependent for most of the studied parameters. In particular, the three microbial domains (bacteria, fungi and archaea) responded differently to exposure in terms of abundance. Accordingly, enzyme type-specific reactions to slope exposure and altitude were also observed in this scenario. In Chapter III, the influence of slope exposure was determined on both the autochthonous soil microbiota and mesofauna, with a special focus on Enchytraeid community under different soil ground covers in subalpine forest ecosystems. The discriminatory assessment of the extracellular (eDNA) and intracellular (iDNA) fractions of the total soil DNA pool (soil metagenome) provided a new perspective on the exposure effects on soil microbiota, i.e., an index of soil microbial activity as well as information about the vertical distribution of eDNA through the soil layers. Overall, microannelids appeared to be sensitive, accurate and reliable biological indicators in these forested subalpine soils, showing a higher abundance at the north-facing slope on account of strong acidity indicator species. Furthermore, the exposure was found to be more determinant for shaping the composition of microannelid assemblages than the ground cover type. In Chapter IV, the exposure-effects on the abundance and activity of the wood-inhabiting microbiota (bacteria, fungi and archaea) and selected microbial groups related to the nitrogen cycle (ammonia-oxidising bacteria (AOB) and nitrogen fixers’ nifH gene) of Picea abies coarse woody debris were evaluated at different stages of natural decay (five-decay class study). All in all, higher microbial abundances were registered at the cooler, moister and more acidic north-facing slope and such exposure-effects (N>S) were in general more evident for the advanced decay stages. Accordingly, a more pronounced physical cell wood damage (by X-ray microtomography) together with a higher microbial activity (lower eDNA/iDNA ratio) was observed at the northern slope with respect to the southern slope. Furthermore, the impact of exposure was enzyme-specific and strongly dependent on the decay stage. Finally, in Chapters V and VI, the physico-chemical and microbiological changes in both P. abies wood blocks and the underlying forest soil were monitored – in a field mesocosm experiment – as a function of slope exposure and time. This experiment attributed shifts in the wood- and soil-inhabiting microbiota to different exposure-related thermal and moisture conditions. In particular, moisture conditions played a more prominent role at the subalpine sites, inducing shifts in the wood and soil microbial communities in terms of abundance and activity, while the influence of temperature was more dominant at the alpine sites. Summarized, the findings of this work provide insights into the interrelation between soil micro- and mesofauna in the Alpine topography, as well as into the shifts in microbial communities during the dynamic process of deadwood decomposition, thus contributing to unravel the complex picture of forest ecosystem functioning under future climate scenarios.