Soil warming and organic carbon content

Soils store two or three times more carbon than exists in the atmosphere as CO2, and it is thought that the temperature sensitivity of decomposing organic matter in soil partly determines how much carbon will be transferred to the atmosphere as a result of global warming1. Giardina and Ryan2 have questioned whether turnover times of soil carbon depend on temperature, however, on the basis of experiments involving isotope analysis and laboratory incubation of soils. We believe that their conclusions are undermined by methodological factors and also by their turnover times being estimated on the assumption that soil carbon exists as a single homogeneous pool, which can mask the dynamics of a smaller, temperature-dependent soil-carbon fraction. The real issue about release of carbon from soils to the atmosphere, however, is how temperature, soil water content and other factors interact to influence decomposition of soil organic matter. And, contrary to one interpretation3 of Giardina and Ryan's results, we believe that positive feedback to global warming is still a concern.