Richard Birdsey & Linda S. Heath 
The Potential of U.S. Forest Soils to Sequester Carbon and Mitigate the Greenhouse Effect [PDF ebook] 

Much attention has been given to above ground biomass and its potential as a carbon sink, but in a mature forest ecosystem 40 to 60 percent of the stored carbon is below ground. As increasing numbers of forests are managed in a wide diversity of climates and soils, the importance of forest soils as a potential carbon sink grows.
The Potenti