Effects of tree species richness on topsoil carbon and fungal diversity in European planted mixed forests are modulated by environmental conditions

Mixed-species forests have emerged as a promising approach to mitigate climate change impacts through enhanced carbon (C) sequestration while maintaining productivity, biodiversity, and other ecosystem services. However, we still have a poor understanding of the context-dependency of soil C sequestration in tree mixtures, particularly how it is influenced by plant-soil-microbe interactions and environmental conditions.Using soil samples collected from nine European sites within the global network of tree diversity experiments, TreeDivNet, we examined how tree species richness is associated with topsoil C stocks, fungal community composition and diversity, and their interactions. We further investigated the influence of biotic, edaphic, and climatic factors on the relationship between tree richness and topsoil C stocks. We hypothesised that increased tree species richness leads to increased topsoil C stocks and fungal diversity, and that this effect is modulated by site-specific interactions between biotic and abiotic factors.Overall, we found topsoil C stocks in stands with high tree diversity to be greater than in monocultures across the study sites. Decreases in soil fertility, mean annual temperature, and interannual variability of temperature and precipitation were found to correlate with positive effects of tree diversity on soil C stocks. While tree diversity did not directly influence fungal diversity, topsoil C stocks were positively correlated to fungal species richness. In addition, fungal richness showed a positive correlation with the net diversity effect of tree mixtures on topsoil C, suggesting that fungal diversity may be one of several factors contributing to the context-dependency of tree diversity effects on soil C stocks.Our study shows that tree species diversity can increase topsoil C storage across Europe, influenced both directly and indirectly by fungal diversity and environmental conditions. The mediation of direct and indirect linkages between tree diversity, fungal diversity and topsoil C stocks by local abiotic context highlights the need to improve our mechanistic understanding for site-specific management of soil C sequestration in tree mixtures to promote climate change mitigation in European forests.