Data for: Sediment and rhizosphere microorganisms mediate seagrass growth responses to belowground nutrient limitation and enrichment

Soil nutrient availability fluctuates widely in the environment, with both low and high nutrient levels posing stress to plants. Belowground microbial communities can support plants under nutrient stress, but their role in plant performance across a nutrient spectrum, from deficiency to excess, remains unclear, particularly for marine plants in the field. In a fully factorial, field-based experiment, we manipulated nutrients, and the rhizosphere and sediment microbial communities of the seagrass Zostera muelleri to determine how microbes affect seagrass growth and survival under low, medium and high nutrient levels. Our results showed that where the rhizosphere and sediment microbes remained intact, root biomass remained high across all nutrient levels. However, root biomass declined where either or both microbial communities were disrupted, particularly under low and high nutrient levels, suggesting that microbial communities mediate seagrass responses to limiting and excess nutrient stress. Bacterial taxa involved in nutrient cycling, e.g. nitrogen fixers, were enriched in the rhizosphere under low nutrient levels. By shifting focus from aboveground to often-overlooked belowground processes and interactions, this study provides field-based, experimental evidence that root and sediment microbial communities are crucial for mediating Zostera muelleri's response to nutrient variability, highlighting their importance for conservation and restoration strategies.