In Mediterranean ecosystems, the projected rainfall reduction of up to 30% may alter plant–soil interactions, particularly litter decomposition and Home Field Advantage (HFA). We set up a litter transplant experiment in the three main forests encountered in the northern part of the Medi-terranean Basin (dominated by either Quercus ilex, Quercus pubescens, or Pinus halepensis) equipped with a rain exclusion device, allowing an increase in drought either throughout the year or concentrated in spring and summer. Senescent leaves and needles were collected under two precipitation treatments (natural and amplified drought plots) at their “home” forest and were left to decompose in the forest of origin and in other forests under both drought conditions. MS-based metabolomic analysis of litter extracts combined with multivariate data analysis enabled us to detect modifications in the composition of litter specialized metabolites, following amplified drought treatment. Amplified drought altered litter quality and metabolomes, directly slowed down litter decomposition, and induced a loss of home field (dis)advantage. No indirect effect mediated by a change in litter quality on decomposition was observed. These results may suggest major alterations of plant–soil interactions in Mediterranean forests under amplified drought conditions.

Soil organisms play a key role in carbon and nutrient cycling in forest ecosystems. While soil organisms are strongly influenced by litter chemistry and are highly sensitive to abiotic conditions, little is known about the interactive effects of these two factors. To address this gap in knowledge, we conducted a 10 week microcosm experiment in which we simulated the effects of climate change on soil ecology. More specifically, we studied relationships among litter nutrient concentration, microbial biomass, Collembola demographic parameters, and litter decomposition, exploring the potential impacts of increasing air temperature and decreasing soil moisture. To develop a gradient of nutrient concentrations, we created six tree litter mixtures with materials gathered from Quercus pubescens and its companion species. In contrast to microbes, we observed that Collembola abundance and litter decomposition were interactively affected by soil moisture and air temperature: the negative effect of increasing air temperature on Collembola abundance was amplified by reduced soil moisture, whereas the positive effect of increasing air temperature on litter decomposition disappeared under reduced soil moisture conditions. In contrast to fungi, the response of bacterial biomass and Collembola abundance to litter nutrient concentration was dependent on abiotic conditions. More specifically, the relationships between nutrients, especially calcium and magnesium, and bacterial biomass and Collembola abundance were less robust or disappeared under drier or warmer conditions. In conclusion, our findings underscore that ongoing climate change could affect soil organisms directly as well as indirectly, by altering their responses to litter nutrient concentrations. In addition, we found that nutrient‐rich habitats might be more affected than nutrient‐poor habitats by altered climatic conditions.

Agriculture is known to strongly influence soil functioning. Nevertheless, its long-term effects remain not well documented in the Mediterranean region, which has a long history of human land use. The “Parc Naturel Régional du Luberon” is a good illustration of these land use changes, as its territory is now covered with forests of differing ages and histories. This study investigates the effect of past land use (agricultural terraces) on microarthropods of current forest soils. In this way, Acari and Collembola of soils from ancient forests, recent forests (developed before 1958), and very recent forests (developed after 1958) were analysed. Different pedoclimatic conditions (Meso-Mediterranean vs. Supra-Mediterranean) and two contrasted seasons (winter and summer) were taken into account in mesofauna responses. A negative effect of past agricultural land use was observed on soil microarthropod abundance in very recent forests only, whatever the pedoclimatic conditions. After at least 60 years of reforestation, this negative effect was no longer observed, indicating a recovery of these communities. Land use legacy effect on oribatid mites in post-agricultural forests depended on the pedoclimatic conditions considered, suggesting that the recovery of microarthropod communities takes more time under more arid conditions. Microarthropods can be considered as good bioindicators of past land use effects depending on pedoclimatic conditions in forest soils.

Ce travail de thèse étudie les interactions entre les organismes du sol (microorganismes, détritivores et prédateurs) dans trois types de forêts du sud de la France (pinède, chênaies pubescentes et vertes) et comment le changement climatique attendu en région méditerranéenne (via une sécheresse amplifiée) pourrait affecter ces relations. Nous avons montré que les communautés d’organismes étaient fortement influencées par la nature biochimique de la litière et le type de forêt. La litière de chêne pubescent semble favoriser l’abondance de toutes les communautés du sol tandis que la forêt de chêne vert semble défavorable pour de nombreux groupes d’organismes. Dans un contexte de changement climatique, la sécheresse amplifiée affecte le réseau trophique du sol avec différentes réponses selon le groupe trophique et le type de forêt. Ces résultats suggèrent que dans une même région méditerranéenne, le changement climatique pourrait affecter différemment les organismes sol avec des conséquences sur les fonctions qu’ils conduisent dans le processus de décomposition des litières