Results
Soil microbial diversity . The dilution-to-extinction approach
was successful in creating a gradient of soil microbial (bacterial and
fungal) diversity; importantly, soil microbial biomass, as indicated by
respiration rates, was recovered after incubation (Fig. 2A, B and C;
Table S2). Compared with the
undiluted soil inocula, 57.91% of soil microbial taxa was lost through
the 10-3 dilution, and 78.18% was lost during the
following 10-6 dilution based on the number of soil
microbial amplicon sequence variants (ASVs) (Fig. 2A). Low-abundance
taxa, for instance, Chytridiomycota and Chlamydiae , were
first eliminated during dilution (Fig. 2B and
C). Although the microcosms were
open to microbial re-colonization from the air, which increased soil
bacterial diversity especially in the low soil biodiversity treatment,
differences of fungal diversity were still observed among soil dilution
treatments at the end of the experiment (Fig. S1 and Table S2). Besides,
the diversity of fungal mutualists in the phylum of Glomeromycotawas dramatically decreased at the 10-3 dilution and
was absent at the 10-6 dilution (Fig. S1A and Table
S2). The diversity of plant fungal pathogens was deceased by soil
dilution in the soil inocula, and was not altered at the last harvest
(Table S2). Because soil biodiversity is dominated by soil microbes,
soil microbial diversity was used as an indicator for soil biodiversity
in the present study.
Temporal stability of
plant community biomass production . Model selection with Akaike
information criterion (AIC) suggests that the additive models including
soil biodiversity (low, moderate, high) and plant diversity (both
species and functional richness) are better than the corresponding
models with the interaction term to explain all aspects of temporal
stability (i.e., temporal stability, mean, standard deviation,
asynchrony, and population variance) (Table S3 and S4). This indicates
that soil biodiversity and plant diversity independently influenced
stability-related indices (Fig. 3; Table S5 and S6). Soil biodiversity
loss had consistent negative effects on the temporal stability of
community biomass production and plant species asynchrony along a
gradient of plant species richness, and plant species richness was
positively associated with the temporal stability of community biomass
production and plant species asynchrony (Fig. 3A and B). Soil
biodiversity and plant species richness did not affect population
variance (Fig. 3D). The temporal mean of community biomass production
was positively related to plant species richness, while the standard
deviation was negatively correlated with plant species richness (Fig. 3C
and E). Soil biodiversity loss did not alter the temporal mean of
community biomass production, while statistically significantly
increasing the standard deviation (Fig. 3C and E). Asynchrony at both
plant species and functional group level was positively related to the
temporal stability of community biomass production, while soil
biodiversity loss did not exert a significant effect on the
asynchrony-stability relationship (Fig. S2). When monocultures were
excluded from the analysis, plant species richness did not exert a
significant effect on the temporal stability of community biomass
production, although the effect of soil biodiversity was still observed
(Table S5).
However, functional diversity in terms of functional richness increased
the temporal stability of plant communities excluding monocultures (Fig.
3F). Plant species asynchrony, population variance and the temporal mean
of community biomass production were increased by functional richness,
and decreased by soil biodiversity loss (Fig. 3G, H and I). Soil
biodiversity loss and functional richness did not alter the standard
deviation of community biomass production (Fig. 3J). Furthermore,
functional richness was closely related to plant species richness, and
78% of variance was explained by an additive model, which was much
higher than that of other functional diversity indices
(R 2 < 40%, Table S6). Plant species
richness did not explain functional evenness and the community-weighted
mean of all traits (Table S6). Functional divergence and dispersion were
increased by plant species richness, but were not correlated with the
temporal stability (Table S7).
Multitrophic
biodiversity was positively associated with
the temporal stability of
community biomass production, plant species asynchrony and the temporal
mean of community biomass production, and negatively correlated with the
standard deviation of community biomass production (Fig. 4A, B, C and
E). Multitrophic biodiversity was not related to population variance
(Fig. 4D). The linear relationships indicate that multitrophic
biodiversity can drive the
temporal stability of community biomass production through an increase
in either soil biodiversity or plant species richness.
Plant community composition . Plant species loss and community
evenness were affected by either soil biodiversity or plant species
richness, but were not related to the temporal stability of community
biomass production (Table S5). In the monocultures, soil biodiversity
did not affect the temporal stability of grasses, while increasing the
temporal stability of herbs and legumes (Fig. S3A, B and C). In
mixed-species communities, plant species richness increased the temporal
stability of grasses, but did not affect the temporal stability of herbs
and legumes (Fig. S3D, E and F). Moderate soil biodiversity increased
the temporal stability of herbs, while soil biodiversity did not
influence the temporal stability of grasses and legumes in mixed-species
communities (Fig. S3D, E and F). Soil biodiversity loss increased the
proportional abundance of grasses in mixed-species communities, but
decreased the proportional abundance of herbs and legumes (Fig. S3G, H
and I). The proportional abundance of grasses was positively associated
with the temporal stability of community biomass production, and the
proportional abundance of herbs and legumes was negatively associated
with temporal stability (Fig. S3J, K and L). However, soil biodiversity
loss still had negative effects on the temporal stability of community
biomass production (Fig. S3J, K and L).
Soil biodiversity loss did not alter the shoot biomass of grasses and
herbs during wet and dry periods in the monocultures (Table S8 and Fig.
5). Shoot biomass of legumes was increased by soil biodiversity loss
during wet periods, and was not altered during dry periods in the
monocultures. In mixed-species communities, the effects of soil
biodiversity loss on shoot biomass of plant functional groups were
similar along a gradient of plant species richness (Table S8 and Fig.
5). For instance, the loss of soil biodiversity increased the growth of
grasses, while strongly decreasing the growth of herbs and legumes,
independently of plant species richness. On average, there was at least
twice as much shoot biomass of grasses as legumes or herbs at low soil
biodiversity. Plant species richness did not affect shoot biomass of
plant functional groups during wet periods, but tended to increase their
shoot biomass during dry periods.