Evaluation of the relative importance to carbon sequestration and nutrient cycling of root exudation, and turnover of roots and mycorrhizas

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.