Biochar is more and more widely recognized as a promising agricultural amendment improving yield and ecosystem services in a range of different contexts. However, underlying mechanisms contributing to biochars benefits, notably biochar–root interactions, and their mediation by biochar's diverse properties remain unclear and poorly quantified. This study aimed to examine and quantify the interactions between biochar properties and plant traits and their effect on plant performance. To gain a better understanding of biochar–plant interactions and their role in biochar overall effects, biochars with contrasted physical and chemical properties were applied to soils during a 3-month greenhouse experiment with barley (Hordeum vulgare L.). Barley biomass as well as several belowground morphological and physiological traits and aboveground traits related to nutrient acquisition were measured. A multivariate structural modeling approach was employed to quantify interactions between biochar properties and plant traits, and their feedback effect on plant biomass. Interactions between biochar chemical and physical properties and barley carboxylate release rate and their contribution to biochar effects were underlined. Among the plant traits examined the release of carboxylate appears as the best proxy to plant biomass following biochar addition, highlighting sparsely reported interactions between total carboxylate release rates and biochar ash content. Multivariate structural modeling offered elements of understanding for the complex interconnected mechanisms involved in biochar influence and their relative contribution. Adopting this approach across a wide range of species and contexts could contribute to ensure more reliable biochar benefits.