(Table presented.). Summary: Trait-based approaches have led to significant advances in plant ecology, but are currently biased toward above-ground traits. It is becoming clear that a stronger emphasis on below-ground traits is needed to better predict future changes in plant biodiversity and their consequences for ecosystem functioning. Here I propose six ‘below-ground frontiers’ in trait-based plant ecology, with an emphasis on traits governing soil nutrient acquisition: redefining fine roots; quantifying root trait dimensionality; integrating mycorrhizas; broadening the suite of root traits; determining linkages between root traits and abiotic and biotic factors; and understanding ecosystem-level consequences of root traits. Focusing research efforts along these frontiers should help to fulfil the promise of trait-based ecology: enhanced predictive capacity across ecological scales.