Microbial inoculation alters plant diversity-productivity
relationships.
There was a significant interaction between plant diversity and soil
inoculation effects on plant productivity (Table 1, F(1,
291) = 14.33, p < 0.001), and a marginally significant effect
of the interaction between plant diversity and leaf inoculation
(F(2, 291) = 2.63, p = 0.07), suggesting that soil
inoculation changed the relationships between plant diversity and
productivity. There was also a significant effect of the three-way
interaction of plant diversity, soil and leaf microbe inoculation on
plant productivity (Table 1, F(2, 291) = 3.99, p =
0.02), suggesting that the effect of one inoculum on
diversity-productivity relationships depended on the other inoculum.
Specifically, inoculation of leaf inoculum decreased plant productivity
at low diversity which increased the slope of positive plant
diversity-productivity relationships, however, the effect of leaf
inoculation disappeared when soil microbes were inoculated. In contrast,
soil inoculation alone had no significant effect on plant
diversity-productivity relationships, while on the condition of leaf
microbes inoculated (i.e. comparing ‘both soil and leaf inoculation’
versus ‘leaf-inoculation’), soil inoculation increased plant
productivity at low diversity and thus weakened the positive
diversity-productivity relationship (Table 2, Fig. 2).
The net biodiversity effect of diversity on plant biomass significantly
differed among microbial inoculation treatments. Compared to
non-microbial inoculation, the net biodiversity effect increased with
leaf microbes inoculated and decreased with soil microbes inoculated,
and inoculation of both leaf and soil microbes lead to a reduced
biodiversity effect on plant biomass. The contribution of leaf microbial
inoculation to net biodiversity effect was through increasing
complementarity among species rather than a selection effect (Fig. 3).