Neighbours rewire plant-soil feedback patterns via reshaping root microbiomes

Soil microbiomes and neighbouring plants both greatly influence plant growth, thereby shaping community dynamics. Although these processes have been studied extensively, they are rarely integrated in a mechanistic framework. Consequently, how they interact mechanistically remains poorly understood. Here we address these questions using a plant-soil feedback approach with six grassland species grown alone or with five different neighbours in sterile soil and in soils with legacies of conspecific and heterospecific plants. Focal plants experienced stronger growth suppression by neighbours in sterile soil, but this was alleviated by microorganisms in soils with legacies, and particularly in soils with heterospecific legacies. Plants consequently often performed better with a heterospecific neighbour than with a conspecific one and in many cases reached equal or even bigger size than when growing alone. Neighbours restructured the root-associated bacterial community of the focal plant towards that of the neighbour, with stronger convergence in neighbour legacy soils, and these shifts mirrored the growth responses of the focal plants. Importantly, the degree of this microbial convergence predicted growth responses. An independent inoculation experiment with natural bacterial communities cultured from focal plants supported the idea that neighbour-induced bacterial community shifts contribute to these growth responses. Our results reveal that neighbour-induced bacterial reassembly, modulated by soil microbial legacies, mediates plant-plant interactions. This mechanism provides new insights into how soil microbiomes can integrate with direct plant interactions, which can ultimately influence coexistence and competition in grassland ecosystems.