Summary The storage of carbon (C) and nitrogen (N) in soil is important ecosystem functions. Grassland biodiversity experiments have shown a positive effect of plant diversity on soil C and N storage. However, these experiments all included legumes, which constitute an important N input through N 2 ‐fixation. Indeed, the results of these experiments suggest that N 2 fixation by legumes is a major driver of soil C and N storage. We studied whether plant diversity affects soil C and N storage in the absence of legumes. In an 11‐year grassland biodiversity experiment without legumes, we measured soil C and N stocks. We further determined above‐ground biomass productivity, standing root biomass, soil organic matter decomposition and N mineralization rates to understand the mechanisms underlying the change in soil C and N stocks in relation to plant diversity and their feedbacks to plant productivity. We found that soil C and N stocks increased by 18% and 16% in eight‐species mixtures compared to the average of monocultures of the same species, respectively. Increased soil C and N stocks were mainly driven by increased C input and N retention, resulting from enhanced plant productivity, which surpassed enhanced C loss from decomposition. Importantly, higher soil C and N stocks were associated with enhanced soil N mineralization rates, which can explain the strengthening of the positive diversity–productivity relationship observed in the last years of the experiment. Synthesis . We demonstrated that also in the absence of legumes, plant species richness promotes soil carbon (C) and nitrogen (N) stocks via increased plant productivity. In turn, enhanced soil C and N stocks showed a positive feedback to plant productivity via enhanced N mineralization, which could further accelerate soil C and N storage in the long term.

Plant species richness promotes soil carbon and nitrogen stocks in grasslands without legumes