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1.?Although a major part of plant biomass is underground, we know little about the contribution of different species to root biomass in multispecies communities. We summarize studies on root distributions and plant responses to species-specific soil biota and formulate three hypotheses to explain how root responses may drive species coexistence and ecosystem productivity.

2.?Recent studies suggest that root growth of some species may be stimulated in species mixtures compared with monocultures without hampering the growth of other species, leading to below-ground overyielding. Further studies suggest that these responses are the result of reduced impairment of growth by species-specific plant pathogens that accumulate in monocultures.

3.?First, we hypothesize that due to pathogen-constrained growth, monocultures are ‘under-rooted’, i.e. they do not have enough roots for optimal acquisition of nutrients. Although elevated root production in mixtures represents a cost to the plant, improved nutrition will eventually result in improved plant performance.

4.?Second, due to the plant species specificity of the soil biotic communities, we suggest that plant species in mixtures develop an intransitive competitive network in which none of the species is competitively superior to all other species. Competitive intransitivity is proposed as a mechanism of species coexistence.

5.?As a final hypothesis, we suggest that pathogen-mediated root overproduction in species mixtures determines the patterns of community productivity and overyielding, both directly, by improving plant performance, and indirectly, by releasing more carbon into the soil, resulting in enhanced availability of nutrients.

6.?Synthesis.Recent evidence suggests that species coexistence and ecosystem productivity may be the result of an interplay between pathogen-driven plant responses and nutritional consequences. We suggest that responses of the roots are an important yet mostly overlooked intermediary between soil biota and plant community responses to biodiversity.