The residence times of C, and the potential for C storage in some French cultivated soils.

In order to predict the potential of soils to store carbon in response to land use or climate changes, we measured the fluxes and distribution of residence times of C in French cultivated soils. We used the natural abundances in C-13 and C-14 to measure this distribution in long-term experiments of maize cultivation in France. 75% of the topsoil carbon had a mean residence time of 40 yr. Coarse particle-size fractions contained most of the younger carbon. A compartment of stable C was estimated using radiocarbon dating. Belowground plant material inputs stored as much as C as aboveground inputs. The effect of temperature on soil carbon mineralization affected only rate constants, with a Q(10) = 3.1 constant in the range 1-25 degrees C. The data were summerized in a simple simulation model, which predicted a nil or low effect of climatic change on soil carbon storage in the next 50 yr. In France, land use changes will have more influence than atmospheric changes on C storage.