Increasing intra-specific diversity in agroecosystems is a promising practice, but belowground mechanisms shaping plant-plant interactions in mixtures remain unclear. We explored how belowground traits and associated microbial communities influence durum wheat performance under limited resources, testing whether traits in monogenotypic wheat stands could explain mixture performance. In a field experiment, 14 durum wheat genotypes were cultivated alone or in pairs under two stress levels (control versus combined watering and N-fertilizer reduction). We measured root morphological traits, mycorrhizal colonization rate and microbial communities (mycorrhizal fungi, fungi, and bacteria) via Illumina metabarcoding on wheat roots and rhizospheric soil. Wheat performances were assessed by shoot biomass, N and P uptake. Mixed models were used to assess which traits best explained wheat performances. In the stressed condition, mixtures exhibited 11% higher shoot biomass compared to monogenotypic stands. Also only under stress, wheat performances (shoot biomass and P uptake) were significantly correlated to root biomass and colonization by Mucoromycotina fungi, which are mycorrhizal fungi recently distinguished from the well-known Glomeromycota. However, belowground traits and microbiome composition of monogenotypic stands failed to explain mixture performances, pointing the need to explore different traits and integrate more diverse wheat genotypes as 11 were modern cultivars. Although a definitive framework is lacking, our study offers promising results, urging further research in designing mixtures meeting farmers' needs