Legume-based cover crops (CC) provide a wide range of ecosystem services, including nitrate leaching reduction, nitrogen (N) fertilization and soil carbon (C) sequestration. However, the scarcity of information on belowground C and N increases the uncertainty on the magnitude of these effects. We quantified C and N input in soil (hereafter plant-deposition) from red clover and winter vetch CC in pure stands or in mixtures with non-legumes, under field conditions. To do so, we labeled CC plants in situ with ¹³C (CO₂) and ¹⁵N (leaf-labeling) throughout their growing period, with frequent multiple-pulses. After sampling, we incubated labeled CC roots and soil in the laboratory to investigate C and N mineralization. Cover crops produced 1.7–2.7 Mg ha⁻¹ aboveground biomass corresponding to 72–86 kg N ha⁻¹, and deposited to the soil 72–183 kg C ha⁻¹ and 29–113 kg N ha⁻¹. Cover crops allocated nearly equal proportions of C and N to above- and belowground fractions. Mineralization of root C was 11–18%, while N mineralization was 39–56% after four months of incubation at 10 °C. Mineral N derived from N plant-deposition to soil was on average 7 kg N ha⁻¹ after incubation, accounting for approximately 10% of mineral N release from red clover shoots and roots. Overall, C and N inputs from legume-based CC are substantially underestimated if plant-deposition is not taken into account. In addition, net mineral N derived from plant-deposition contributes significantly to the N fertilizer effect of legume-based CC.