Sediment and rhizosphere microbes mediate seagrass growth responses to below‐ground nutrient limitation and enrichment

Abstract Soil nutrient availability fluctuates widely in the environment creating stressful conditions for plants. Below‐ground microbial communities can help plants cope with nutrient stress, yet the relative importance of rhizosphere and bulk soil microbial communities across a spectrum of nutrient conditions remains unclear, particularly for marine plants. We experimentally manipulated nutrient availability and the rhizosphere and sediment (analogous to soil) microbial communities of the seagrass Zostera muelleri in pots in the field to determine the effects of below‐ground microbial communities (disrupted or intact) on seagrass performance under low‐, medium‐ and high‐nutrient treatments. Disrupting rhizosphere or bulk sediment microbial communities had no effect on above‐ground biomass. However, disrupting rhizosphere or sediment microbial communities reduced root biomass under low‐ and high‐nutrient treatments compared to medium‐nutrient treatments, suggesting that below‐ground microbial communities mediate seagrass responses to nutrient stress. Under both low‐ and high‐nutrient treatments, the bacterial community composition of the rhizosphere and bulk sediment differed when microbial communities were intact compared to when they were disrupted. Under low‐nutrient treatments, we identified a higher relative abundance of potential nutrient‐cycling taxa, that is Flavobacteria, Methyloligellaceae and Desulfocapsaceae in the rhizosphere than under other nutrient treatments. No specific taxa in the rhizosphere changed in relative abundance under high‐nutrient conditions. Synthesis . This study suggests that below‐ground microbial communities are central to how seagrasses cope with nutrient variability. By providing field‐based, experimental evidence that both rhizosphere and bulk soil communities shape root growth, our work extends ecological understanding of plant–microbe interactions beyond terrestrial systems and into the marine realm. These findings underscore the importance of below‐ground interactions for predicting marine plant resilience to environmental change and highlight the need to incorporate below‐ground microbes into conservation and restoration strategies.