Our study determined the impacts of silicon (Si) supplementation on Medicago truncatula inoculated with Ensifer meliloti rhizobial strains that differed in their capacity for nitrogen fixation: Sm1021 (‘low-efficiency’) or Sm1022 (‘high-efficiency’). We then examined how Si and rhizobial efficacy influence plant resistance to a polyphagous insect, Helicoverpa armigera. These combinations were supplied with Si or untreated in a glasshouse experiment, where we quantified nodule flavonoids and foliar chemistry (free amino acids, soluble protein, elemental C, N and Si). Si supply increased nodule number per plant, specific nodule flavonoids, contents of foliar nitrogenous compounds and foliar C, but not foliar Si. We also demonstrated that rhizobial efficacy altered the magnitude of Si effects on various traits. For example, Si significantly promoted concentrations of foliar N and soluble protein in the plants associated with the ‘low-efficiency’ strain only and this was not the case with the ‘high-efficiency’ one. Additionally, increases in foliar free amino acids in response to Si addition did not increase susceptibility to H. armigera. Collectively, our study indicates that Si enrichment generates positive effects on M. truncatula, particularly when the association with rhizobia is relatively inefficient, and may play a more prominent role in rhizobial functionality than previously thought.

Elucidating mechanisms underlying community assembly and biodiversity patterns is central to ecology and evolution. Genome size (GS, i.e. nuclear DNA content) determines species’ capacity to tolerate environmental stress and therefore potentially drives community assembly. However, its role in driving β-diversity (i.e., spatial variability in species composition) remains unclear. We measured GS for 161 plant species and investigated their occurrences within plant communities across 52 sites spanning a 3200-km transect in the temperate grasslands of China. Using species distribution modelling, we found that environmental factors showed larger effects on β-diversity of large-GS than that of small-GS species and that communities with abundant resources had a greater representation of large-GS species. The latter finding was confirmed following analysis of data from a 10-yr resource (water, nitrogen, and phosphorus) manipulation experiment in which resource addition resulted in increased community weighted GS based on plant biomass estimates, suggesting that large-GS species are more sensitive to environmental resource limitation and explaining the greater environmental selection on β-diversity of large-GS species. These findings highlight the roles of GS in driving community assembly and predicting species responses to global change.

Elucidating mechanisms underlying community assembly and biodiversity patterns is central to ecology and evolution. Genome size (GS, i.e. nuclear DNA content) determines species’ capacity to tolerate environmental stress or to exploit new environments and therefore potentially drive community assembly. However, its role in driving β-diversity (i.e., the site-to-site variability in species composition) remains unclear. We measured GS for 169 plant species and investigated their occurrences within plant communities across 52 sites spanning a 3200-km transect in the temperate grasslands of China. We found environmental factors showed larger effects on β-diversity of large-GS than that of small-GS species. Community weighted mean GS increased with mean annual precipitation, soil total nitrogen and phosphorus concentrations, but decreased with mean annual temperature, suggesting a negative selection against species with large GS in resources-limited or warmer climates. These findings highlight the roles for GS in driving community assembly and predicting species responses to climate change.