Results
Average changes of ecosystem functions in plant mixtures
In field experiments, we observed significant increases in mixtures
relative to monocultures for carbon processes including aboveground
biomass (AGB, 64.6%), belowground biomass (BGB, 45.0%), total biomass
(TB, 123.1%), soil carbon pool (SCP, 13.2%), soil respiration (Rs,
10.6%), heterotrophic respiration (Rh, 18.9%), microbial biomass (MB,
16.7%), fungal biomass (FB, 16.4%), and bacterial biomass (30.0%)
(BB, Fig. 2). We observed similar increases for nitrogen processes
including aboveground nitrogen pool (ANP, 26.5%), soil nitrogen pool
(SNP, 7.8%), and soil ammonium nitrogen (SAN, 27.0%). Decreases
occurred in soil nitrate nitrogen (SNN, -40.4%), soil nitrogen
mineralization (SNM, -52.3%), and soil nitrogen leaching (SNL, -76.0%)
(Fig. 2).
In greenhouse experiments, we similarly observed significant increases
in mixtures relative to monocultures for measures including AGB
(60.1%), BGB (53.1%), TB (49.6%), Rs (16.4%), and SNP (5.6%) (Fig.
2). The remaining attributes showed no significant differences.
The effects of plant diversity, experimental age, and
climate
Carbon and nitrogen attributes increased linearly with experimental age
in the field experiments, with the exceptions of FB and SNN (Fig. 3,
Table S2), and increased logarithmically with species richness except
for FB, SAN, SNN, and SNM (Fig. 4, Table S3). Importantly, significant
interactions between plant diversity and experimental age were found for
AGB, BGB, TB, SCP, Rh, MB, BB, ANP, SNP, SAN, and SNM (Table S4),
indicating stronger species diversity effects on those variables in
longer-term experiments (Fig. 4, Table S5). In greenhouse experiments,
AGB, BGB, FB, BB, and ANP increased logarithmically with species
richness (Fig. 5, Table S3). Few significant interactions between plant
diversity and climate (mean annual temperature and precipitation) were
observed, with only BGB, FB, and BB impacted (Fig. 6).
Predicted responses of soil carbon and nitrogen
pool
We utilized the differences in species richness and experimental age
effects in the field experiments to predict the impacts of long-term
diversity declines (Fig. 7). A 10% decrease in species richness (from
100 to 90%) over one year reduced SCP and SNP by 0.5 and 1.1%
respectively (Fig. 7). An 80% decrease in species richness (from 100 to
20%) over one year led to 2.3 and 4.8% reductions in SCP and SNP
respectively (Fig. 7). The declines in SCP and SNP in response to the
decrease in species richness became amplified with time. For example, a
10% decrease in species richness (from 100 to 90%) over five years led
to a 2.4 and 5.2% reduction in SCP and SNP, respectively (Fig. 7).