Species choice and richness in intensively managed grassland mixtures regulate soil carbon (C) input via rhizodeposition, with potential consequences for long‐term soil organic carbon storage. Based on a field trial with different grass–legume–forb mixtures, we removed roots from the soil, which was then subjected to particle‐size fractionation to trace fresh organic carbon (net C rhizodeposition) into particulate organic matter (POM) and mineral‐associated organic matter (MAOM). We related these C input fractions to root traits. Using multiple‐pulse ¹³C–CO₂‐labeling, we captured the net formation of mineral‐associated organic carbon (MAOC) and particulate organic carbon (POC) at the end of the growing season. Pure stand perennial ryegrass (Lolium perenne) had higher quantities of rhizodeposited C allocated to MAOC and POC (0.21 and 0.13 g C kg⁻¹ dry soil, respectively) compared to grass–legume–forb mixtures (ranging from 0.10 to 0.12 for MAOC and 0.05 to 0.06 g C kg⁻¹ dry soil for POC). However, the proportion of MAOC (%MAOC of net C rhizodeposition) in relation to that of POC was higher in mixtures with legumes. Species richness did not affect the quantity of MAOC or POC, nor %MAOC. The quantities of MAOC and POC were positively associated with root length. In contrast, %MAOC was positively associated with root diameter and a lower root C:N ratio. Despite higher %MAOC in mixtures with legumes, the main driver of MAOC and POC quantities was the total amount of C rhizodeposition. These results highlight the importance of legumes in the formation of MAOC from rhizodeposition and of high root length for increasing both MAOC and POC quantities. Our study shows how plant community design can be used to increase MAOC and/or POC and facilitate soil C storage. By revealing the traits behind the relationships between plant communities and MAOC and POC formation, we provide a guide for species selection in intensively managed grasslands to mitigate climate change.