| Summary: | Cover crops (CC), particularly legumes, are key to promote soil carbon (C) sequestration in no-tillage. Nevertheless, the mechanisms regulating this process need further elucidation within a broad comprehensive framework. Therefore, we investigated effects of CC quality: black oat (<i>Avena strigosa</i> Schreb) (oat), common vetch (<i>Vicia sativa</i> L.) (vetch), and oat + vetch on carbon dioxide-C (CO<sub>2</sub>-C) emission (124 days) under conventional- (CT), minimum- (MT) and no-tillage (NT) plots from a long-term experiment in Southern Brazil. Half-life time (t<sub>1/2</sub>) of CC residues and the apparent C balance (ACB) were obtained for CT and NT. We linked our data to long-term (22 years) soil C and nitrogen (N) stocks and crop yield data of our experimental field. Compared to CT, NT increased t<sub>1/2</sub> of oat, oat + vetch and vetch by 3.9-, 3.1- and 3-fold, respectively; reduced CO<sub>2</sub>-C emissions in oat, oat + vetch and vetch by 500, 600 and 642 kg ha<sup>−1</sup>, respectively; and increased the ACB (influx) in oat + vetch (195%) and vetch (207%). For vetch, CO<sub>2</sub>-C emission in MT was 77% greater than NT. Legume CC should be preferentially combined with NT to reduce CO<sub>2</sub>-C emissions and avoid a flush of N into the soil. The legume based-NT system showed the greatest soil C and N sequestration rates, which were significantly and positively related to soybean (<i>Glycine max</i> (L.) Merrill) and maize (<i>Zea mays</i> L.) yield. Soil C (0–90 cm depth) and N (0–100 cm depth) sequestration increments of 1 kg ha<sup>−1</sup> corresponded to soybean yield increments of 1.2 and 7.4 kg ha<sup>−1</sup>, respectively.
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