Results

On average, root biomass was significantly greater in mixtures than the average of corresponding monocultures; however, R/S, WRD, RLD, SRL, MRD, and RN did not vary significantly (Fig. 1). The mixture effects on root biomass increased significantly with species richness in mixtures (95% confidence interval [CI], 8.6% - 16.1%, P < 0.001) and soil depth (CI = 1.9%-7.4%, P = 0.001), but not with stand age (CI = – 0.4% - 10.7%, P = 0.175) (Fig. 2). Plant mixture effects on RLD were not altered, while those on SRL decreased significantly with higher levels of species richness in mixtures (Fig. 2). Both the effects of plant mixtures on RLD and SRL increased with stand age and increased with soil depth for RLD, but decreased for SRL (Fig. 2). The mixture effects on R/S, WRD, MRD, and RN did not change significantly in response to the species richness in mixtures, stand age, or soil depth, as the null models (with intercept only) were those that were most parsimonious (Table S3).
The effect sizes of root biomass with the species richness in mixtures were more progressive for older stand ages and at deeper soil depths (both P < 0.001, Figs. 3a, 3b). The mixture effects on SRL decreased strongly with species richness in mixtures in young stands, but increased in stands older than 10 years (P< 0.001, Fig. 3c). The mixture effects on SRL decreased with species richness in mixtures in deep soils but increased in topsoil (P < 0.001; Fig. 3d).
The mixture effect on RLD decreased significantly, changing from positive to negative with MAT (P < 0.001, Fig. 4a), while, on average, mixture effects on root biomass increased with MAP (P = 0.008, Fig. 4b), with more pronounced increases in species-rich mixtures (Fig. 4b, P < 0.001). Moreover, plant mixture effect did not differ between different ecosystem types for root attributes except root biomass, with positive effects of species mixtures on root biomass in containers, grasslands, and planted forests, but not in natural forests (Fig. 5, Table S3). The mixture effects on WRD both marginally decreased with MAT and increased with MAP in natural forests, but not in other ecosystems (Figs. S2a, S2b). The negative mixture effects with MAT on RLD was found only in croplands (P = 0.001, Fig. S2c). The mixture effects on R/S, SRL, and RN did not change significantly with the interactive effects between ecosystem type and MAP or MAT (Tables S4, S5).