Soil carbon sequestration by Holocene fires inferred from soil charcoal inthe dry French Alps

The current global carbon budget has a missing sink, which is believed to b e in terrestrial ecosystems. At least one carbon sink, wood charcoal seques trated in soil, remains poorly detailed. We estimate the wood charcoal-carb on content in soils located in dry valleys within the French Alps. Soils we re sampled at five sites along altitudinal transects, from the conifer-domi nated subalpine forests to the alpine grasslands. The five sites were distr ibuted along a bioclimatic and biogeographic gradient from the southern Med iterranean to the northern continental Alps. The altitudinal distribution o f charcoal exhibits the same pattern in the five sites, despite stand fire history, and regional bioclimatic and biogeographic differences. Charcoal c oncentrations are low (0.01 to 10 g(char) m(-2)) in soils from the current treeless belt. while soils at lower elevation show high concentrations (10 to 2000 g(char) m(-2)). The results suggest that the landscape structure de termine the charcoal accumulation throughout variability of vegetation type and fire frequency. Charcoal concentrations recorded in the subalpine belt in the Alps are similar to those of Swedish boreal forests, but are 10 to 100 times lower than values from Mediterranean ecosystems. Dry to subhumid ecosystems contain subfossil carbon in the form of charcoal, which should b e explicitly taken into account in the global carbon budget.