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).