Maintaining soil function and carbon storage represent key objectives in sustainable agriculture. Most C, N and P is returned to soil via exudation from live roots, and turnover of dead roots and their associated symbionts. Consequently, both the rate at which C fixed by plants is returned to the atmosphere as CO2 and the rate at which nutrients are recycled to plant-available forms are tightly coupled. However, quantitative separation of the three most important below-ground fluxes of C and nutrients (root exudation and root and mycorrhizal turnover) remains largely unachieved. This is due both to limited understanding of the fate of the range of C, N and P compounds and their polymers (e.g. protein, nucleic acids, cellulose, lignin and chitin) delivered to soil, and to a lack of suitable techniques to measure rates of delivery. Consequently, poor knowledge of these fluxes is a major impediment to understanding and modelling the storage of atmospheric C in soils and the factors controlling nutrient cycling. We propose to utilise recent technological advances to determine: (1) The relative quantities and types of C, N and P delivered to the soil in turnover of roots and mycorrhizas in temperate permanent grassland. (2) The rate and route of utilisation by soil microbes of the various forms of C delivered to the soil by these processes and how this controls the delivery of C back to the atmosphere as CO2. (3) How synchronous with mineralisation of plant C to CO2 is the return of N and P to plant-available forms. (4) How estimates of root and mycorrhizal turnover measured by state-of-the-art techniques compare with those from more traditional approaches.