Genotypic diversity increases plant performance and soil nitrogen while altering bacterial community composition in a salt-marsh mesocosm experiment

Species diversity strongly predicts productivity, yet the processes linking intraspecific diversity to ecosystem functioning remain poorly understood. Through a controlled marsh mesocosm experiment with the salt-marsh foundation species Scirpus mariqueter , we found that increasing genotypic diversity significantly enhanced plant performance (above-ground and total biomass) and reproductive output (corm number, corm biomass, inflorescence and seed production), with increased soil nitrogen and a significant overall effect on soil organic carbon. Intermediate genotypic diversity significantly altered bacterial community composition. Although overall bacterial community assembly remained predominantly stochastic, β-nearest taxon index (βNTI) analysis indicated a stronger deterministic assembly signal in four-genotype mixtures. Notably, trait expression transitioned from additive effects at four genotypes to nonadditive interactions at eight genotypes, indicating increasing complementarity at higher diversity levels. These findings collectively reveal that genotypic diversity enhances plant productivity and soil nitrogen pools while selectively altering bacterial community composition and assembly processes, providing mechanistic insights into how intraspecific variation stabilizes and promotes ecosystem functioning in salt marshes.