The mitigation of climate change calls for a better understanding and management of the carbon sequestration potential of soils. This project will explore and unravel the yet unkown relationship between long-term P fertilization, resulting soil P-saturation and the content, nature and stability of soil organic carbon (SOC). We postulate that an excess available P in soil substantially lowers SOC stocks in agricultural soils. Higher P fertility in the topsoil lowers root development into the subsoil which, in turn, reduces the rhizodeposition of carbon into these deeper layers. In addition, there is an often-overlooked reduction of SOC protection in soil by competition with phosphate on SOC binding sites. First, we will establish the relationship between SOC stocks and available soil P by sampling 18 different long-term P trials in Europe and soils that are saturated with P. Second, a central experiment is a one year greenhouse trial with 13C-CO2 pulse labelled plants to identify P-status impacts on the belowground 13C inventory and the microbial community. Third, we will use state of the art physicochemical methods to understand P-SOC interactions. Finally, we will use and adapt a selected soil C model with the information from the experimental study to validate the long-term effect of P fertilisation on SOM sequestration. The applicants, including a German partner have a complementary expertise for speciation of P in soils and on organic matter quality and quantity in soils.