| Summary: | The partitioning of belowground biomass (BGB) to aboveground biomass (AGB) is commonly described as the root-to-shoot ratio (R/S). Although a number of studies have shown that biodiversity can influence AGB and BGB in grasslands at the local and global scale, the global-scale patterns reflecting how plant diversity affects R/S and the factors controlling such effects remain unclear. In this study, we explored the global patterns and associated drivers of biomass partitioning responding to plant diversity by conducting a meta-analysis of 333 observations from 30 studies in grasslands worldwide. Overall, plant diversity significantly increased AGB, BGB, and total biomass, whereas significantly decreased R/S. The effects of plant diversity on biomass partitioning varied with experimental types. The effect size for AGB and BGB in the field was larger than in greenhouse experiments, but the effect size for R/S did not significantly differ between field and greenhouse experiments. Moreover, there was no significant relationship between R/S and species richness and experimental duration in greenhouse experiments. However, the effect size for AGB, BGB, and R/S increased logarithmically with species richness and experimental duration in the field experiments. Specifically, the effect size for R/S in the field experiments switched from negative to neutral as the species richness and experimental duration increased. Furthermore, the effect size for R/S was positively correlated with complementary effects of BGB, and it increased logarithmically with mean annual temperature (MAT) and precipitation. Structural equation models showed that species richness, experimental duration, and MAT impact R/S indirectly by changing the BGB. Overall, our findings suggest that plant mixtures invest less in BGB than monocultures, and highlight that low investment in BGB will disappear gradually over time as species richness increases.
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