Biochar, or pyrolyzed organic matter intended for use as a soil amendment, has a variety of properties of interest from the perspective of agroforestry, in particular its potential to increase soil C sequestration and enhance yields by increased retention of soil mineral nutrients. Nitrogen-fixing legumes commonly show large growth responses to biochar, and we hypothesized that combinations of legumes and non-legumes would show increased enhancement of yields in mixture due to enhanced facilitation related to increased nutrient retention. This hypothesis was tested in a glasshouse pot trial involving the leguminous tree leucaena (Leucaena leucocephala), and maize (Zea mays) grown for 120 days in mixture and monoculture in both an unamended soil and soil amended with a slow-pyrolysis coconut husk biochar at 10 t/ha. Responses were quantified in terms of biomass production and partitioning, leaf-level gas-exchange, leaf chlorophyll content, and (for leucaena) root nodule formation. Consistent with predictions, leucaena showed more pronounced growth and physiological responses to biochar than did maize. The system also showed increased total biomass yields relative to monocultures in the two-species mixtures with biochar (relative yield in mixture [RYM] = 1.69; 95% CI 1.41-1.81), but not in the control treatment (RYM = 1.25; 95% CI 0.93-1.76), and positive effects of biochar on yield were generally greater for both species when grown in mixture. Growth responses to biochar were most pronounced for stem and root biomass, and growth responses were consistent with changes in leaf-level photosynthesis and chlorophyll content. In addition, biochar had large effects on biomass partitioning, enhancing stem allocation in both species. Our results suggest that biochar additions may augment facilitative interactions in agroforestry and intercropped systems on acidic, nutrient-deficient soils by enhancing system nutrient retention.