Efficient utilization of nutrients in agricultural systems is essential in order to meet a sustainable production of food. Grain and forage legumes add nitrogen (N) to the soil via biological N2-fixation which reduces the need for energy demanding mineral fertilizers. Management of the increased soil fertility found after legumes requires knowledge of the processes underlying soil fertility build-up. One of the most important processes in soil fertility build-up under legumes is the growth, death, and turnover of legume root systems, which is at present poorly understood. In particular there is a lack of knowledge of the longevity and turnover of legume roots in forage mixtures, due to methodological difficulties in investigating the roots of one specific specie in a species mixture. The aim of the proposed project is to develop and test a new approach to estimate single species root longevity in forage mixtures, and use this new approach to estimate the longevity of legume roots in forage mixtures as well as quantify the deposition of C and N from legume roots. The proposed project will add a new state-of-the-art approach to study root longevity in species mixtures, and furthermore increase our knowledge regarding the processes involved in soil fertility build-up. Thus, this project will aid a development towards greater self-supply in European agriculture by reducing the energy consumption needed in the agricultural production, which is important for solving the present climate change challenge.