An integrated perspective on the interactions between Quercus cerris fine roots and microbial community in top- and sub-layers of urban rhizosphere

The rhizosphere in urban environments is crucial to plant health and ecosystem sustainability. However, little is known about the spatial variability of the rhizosphere in urban forest ecosystems, especially for holobionts based on large organisms like trees. Thus, we aimed to identify the variations of the fine root functional traits of Quercus cerris and of the microbial community composition and their reciprocal dependencies analyzing the rhizosphere of the top- and sub-layer soil of non-urban, peri-urban and urban sites. Our findings showed that in non-urban forests, fine roots are mostly shallow and interact with diverse microbial phyla in the topsoil influencing necromass and, thus, carbon cycling. In peri-urban forests, fine root traits vary by depth, leading to higher specific root length in the top-layer and contributing to possible microhabitat fragmentation. In urban areas, more interactions occur at greater depths, mainly influencing root length and biomass, though without significant changes possibly indicating environmental disturbance. Although it is impossible to dissect every single interaction between roots and microbial phyla, underrepresented microbial phyla were demonstrated to play significant roles in shaping the root-microbiome interplay. These preliminary observations need further investigation to provide more insights into the reciprocal interplay spatially occurring in the rhizosphere.