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).