Inclusion of forbs into grassland mixtures is expected to have multiple benefits in animal nutrition, especially if those forbs are rich in secondary plant metabolites. Increased functional group diversity and the presence of deep-rooting forb species, in particular, may also increase soil carbon sequestration in temporary grasslands. In April 2020, we established four temporary grassland mixtures in Frick, Switzerland: grasses, grasses with legumes, grasses with plants rich in essential oils, and grasses with tannin-rich plants. Each mixture was replicated in four randomised plots that were grazed by dairy cows. In March 2020 and February 2024, soil cylinders were taken in the 0–10, 10–20 and 30–50 cm soil layers. Bulk density, soil organic carbon (SOC) and total soil nitrogen (STN) concentrations were measured for stock calculation. Across mixtures, SOC and STN stocks increased from 2020 to 2024 in the 0–10-cm layer as a consequence of increased soil density. Mixture affected neither SOC nor STN stocks or concentrations, but had a minor effect on bulk density in the 10–20-cm layer. The results show that an increase in functional group diversity may not always affect grassland carbon sequestration.