The rhizosphere microbiome of seagrasses is central to their performance, but we know little about how essential microbes are acquired. Using a manipulative field experiment, we investigated how bulk sediment and neighbouring plants shape rhizosphere microbial community assembly in the seagrass Zostera muelleri. Focal plants with experimentally disrupted rhizosphere microbial communities were transplanted into sediments collected from within seagrass meadows or in sediments from unvegetated sediment and either grown alone, with neighbours with intact rhizosphere microbial communities or with neighbours with disrupted rhizosphere microbial communities.
Bulk sediment was the dominant bacterial source 7 days after transplantation (~57–70%), but neighbour plants became increasingly important over time, contributing ~50–65% of the focal rhizosphere bacterial community by one and two months. The sediment type in which focal plants were established further shaped rhizosphere assembly, with sediments from seagrass meadows consistently producing distinct focal plant rhizosphere bacterial communities compared to unvegetated sediment. These findings reveal that seagrass rhizosphere microbiome assembly is temporally dynamic and context-dependent, shifting from sediment-driven to neighbour-mediated bacterial recruitment over time. For seagrass restoration, this suggests that seagrass transplants placed in isolation, away from healthy seagrass, may struggle to acquire the microbial communities they need to thrive.