Soil carbon stocks and their rates of accumulation and loss in a boreal forest landscape

Boreal forests and wetlands are thought to be significant carbon sinks, and they could become net C sources as the Earth warms. Most of the C of borea l forest ecosystems is stored in the moss layer and in the soil. The object ive of this study was to estimate soil C stocks (including moss layers) and rates of accumulation and loss for a 733 km(2) area of the BOReal Ecosyste m-Atmosphere Study site in northern Manitoba, using data from smaller-scale intensive field studies. A simple process-based model developed from measu rements of soil C inventories and radiocarbon was used to relate soil C sto rage and dynamics to soil drainage and forest stand age. Soil C stocks cova ry with soil drainage class, with the largest C stocks occurring in poorly drained sites. Estimated rates of soil C accumulation or loss are sensitive to the estimated decomposition constants for the large pool of deep soil C , and improved understanding of deep soil C decomposition is needed. While the upper moss layers regrow and accumulate C after fires, the deep C dynam ics vary across the landscape, from a small net sink to a significant sourc e. Estimated net soil C accumulation, averaged for the entire 733 km(2) are a, was 20 g C m(-2) yr(-1) (28 g C m(-2) yr(-1) accumulation in surface mos ses offset by 8 g C m(-2) yr(-1) lost from deep C pools) in a year with no fire. Most of the C accumulated in poorly and very poorly drained soils (pe atlands and wetlands). Burning of the moss layer in only 1% of uplands woul d offset the C stored in the remaining 99% of the area. Significant interan nual variability in C storage is expected because of the irregular occurren ce of fire in space and time. The effects of climate change and management on fire frequency and on decomposition of immense deep soil C stocks are ke y to understanding future C budgets in boreal forests.