Despite their importance, how plant communities and soil microorganisms interact to determine the capacity of ecosystems to provide multiple functions simultaneously (multifunctionality) under climate change is poorly known. We conducted a common garden experiment using grassland species to evaluate how plant functional structure and soil microbial (bacteria and protists) diversity and abundance regulate soil multifunctionality responses to joint changes in plant species richness (one, three and six species) and simulated climate change (3°C warming and 35% rainfall reduction). The effects of species richness and climate on soil multifunctionality were indirectly driven via changes in plant functional structure and their relationships with the abundance and diversity of soil bacteria and protists. More specifically, warming selected for the larger and most productive plant species, increasing the average size within communities and leading to reductions in functional plant diversity. These changes increased the total abundance of bacteria that, in turn, increased that of protists, ultimately promoting soil multifunctionality. Our work suggests that cascading effects between plant functional traits and the abundance of multitrophic soil organisms largely regulate the response of soil multifunctionality to simulated climate change, and ultimately provides novel experimental insights into the mechanisms underlying the effects of biodiversity and climate change on ecosystem functioning.

Departamento de Biología y Geología Física y Química Inorgánica Escuela Superior de Ciencias Experimentales y Tecnología Universidad Rey Juan Carlos Móstoles Spain

Departamento de Biología y Geología Física y Química Inorgánica Escuela Superior de Ciencias Experimentales y Tecnología Universidad Rey Juan Carlos Móstoles Spain Department of Botany University of South Bohemia Ceske Budejovice Czech Republic

Departamento de Biología y Geología Física y Química Inorgánica Escuela Superior de Ciencias Experimentales y Tecnología Universidad Rey Juan Carlos Móstoles Spain Hawkesbury Institute for the Environment University of Western Sydney Penrith NSW Australia

Departamento de Biología y Geología Física y Química Inorgánica Escuela Superior de Ciencias Experimentales y Tecnología Universidad Rey Juan Carlos Móstoles Spain INRA USC1339 Chizé Villiers en Bois France

Departamento de Ingeniería Forestal Escuela Técnica Superior de Ingeniería Agronómica y de Montes Universidad de Córdoba Córdoba Spain

Hawkesbury Institute for the Environment University of Western Sydney Penrith NSW Australia

Hawkesbury Institute for the Environment University of Western Sydney Penrith NSW Australia Global Centre for Land Based Innovation Western Sydney University Penrith NSW Australia

Institute of Ecology and Earth Sciences University of Tartu Tartu Estonia

Zoologisches Institut Terrestrische Ökologie Universität zu Köln Köln Germany Cluster of Excellence on Plant Sciences University of Cologne Cologne Germany

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