Evidence from chronosequence studies for a low carbon-storage potential of soils

OVER most of the Earth's land surface, the amount of carbon stored in soil organic matter exceeds by a factor of two or three the amount stored in living vegetation. This pool of soil carbon is large (1.5 × 1018 g)1,2 and plays a dynamic part in the geochemical carbon cycle. Prentice and Fung3 have suggested that terrestrial vegetation and soils would act as a large sink for atmospheric carbon dioxide if its concentration were twice the present level. Here I use data from chronosequence studies to show that the production of refractory humus substances in soils sequesters only ∼0.4 × 1015 g C yr−1 from the atmosphere, accounting for just 0.7% of terrestrial net primary production. Moreover, agricultural practices tend, on balance, to cause a release of soil carbon to the atmosphere4,5. Thus if the terrestrial biosphere is indeed to act as a carbon sink under future elevated levels of carbon dioxide, this would be more likely to be the result of changes in the distribution and biomass of terrestrial vegetation than of changes in the accumulation of soil organic matter.