How does soil particulate organic carbon respond to grazing intensity in permanent grasslands?

International audience; Modification in grazing intensity causes functional changes in permanent grasslands, e.g. in carbon (C) cycling. However, we still know little about how the soil organic C of permanent grasslands responds to grazing intensity. In a grassland experiment with three levels of grazing intensity, we monitored root and rhizome C stocks, particulate organic C stocks, total soil C stocks, above-ground net primary production and plant species groups abundance over 7 years. A simple model was used to estimate the mortality of roots and rhizomes, decomposition rates of particulate organic C, and C fluxes under different grazing intensities. After 7 years, low grazing intensity and no grazing led to a modification in above-ground vegetation (production, plant species composition, nitrogen content) and a reduction in C transferred between roots and particulate organic matter fractions, while the C stocks of root and rhizomes, particulate organic matter and total soil were not significantly affected by grazing intensity. However, particulate organic C showed a strong interannual variability. Particulate organic C could have reacted more slowly than expected to changes in grazing intensity, or a marked interannual variability of particulate organic C stocks, through an increase in decomposition rates in all the grazing treatments, could have slowed down the accumulation of particulate organic C and masked the effect of the grazing intensity treatments.