Abstract
Soil communities are extremely diverse and understanding the drivers of their distribution and structure is challenged by the multitude of abiotic and biotic factors that might influence them. However, studying soil organisms is essential because they are involved in many soil processes, such as organic matter decomposition and nutrient cycling. It is now established that soil macroinvertebrates are both influenced by their environment and can also affect it through their feeding and movement activities. However, our knowledge of the fine processes involved in the structuring of their communities and how they influence soil functioning is often limited. To fill these gaps, my PhD thesis first focuses on the abiotic and biotic factors shaping macroinvertebrate communities in temperate forest soils, before studying their role in litter decomposition and how they influence other soil organisms in the context of climate changes.The first part of the thesis (chapters 1 and 2) is an in situ approach, where macrofaunal communities were studied in forests of varying tree composition and specific diversity along a European latitudinal gradient (including sites in Italy, Romania, Poland and Finland). The first chapter examines the relative importance of tree diversity, tree functional type and physical environment for predatory and sapro-geophagous macrofaunal communities. In a second chapter, I focused on carabid beetle assemblages by applying a functional trait-based approach using individual measurements and data obtained from the literature. The objective was to describe how the forest environment and the characteristics of their food resource influence the functional structure of their communities.The second part of the thesis develops experimental approaches and focuses on the functional importance of the sapro-geophagous macrofauna, and in particular the interactions it maintains with plants on the one hand (chapter 3) and with other soil organisms such as nematodes on the other (chapter 4). The feeding preferences of six species of saprophagous macroarthropods commonly found in European forests are first described and discussed with regard to the physical, chemical and morphological trait-based characteristics of the litter. The consequences of these feeding preferences on the properties of the faecal pellets produced by these invertebrates on the one hand, and their implications for the decomposition of litter on the other, are discussed. Finally, in a controlled mesocosm experiment, we studied the response of nematode communities to a simulated drought and how the functional diversity of the sapro-geophagous macrofauna can limit the deleterious effects of drought.The importance of soil biodiversity, at different trophic levels and in its taxonomic and functional dimensions, is discussed from the point of view of soil functioning and temperate forest ecosystems. The results are then discussed in order to identify silvicultural management practices that preserve both soil biodiversity and the functions it performs at the ecosystem level, despite global changes.