How do you measure or estimate soil carbon? Here are three handbooks.

1. Pearson, Timothy, Sarah Walker, and Sandra Brown. 2006. Sourcebook for Land Use, Land-Use Change and Forestry Projects. Winrock International.
http://www.winrock.org/ecosystems/files/Winrock-BioCarbon_Fund_Sourceboo... (661 K pdf file; right click and "save link as" to download)

2. Stolbovoy, V., Montanarella, L., Filippi, N., Jones, A., Gallego, J., and Grassi, G. 2007. Soil sampling protocol to certify the changes of organic carbon stock in mineral soil of the European Union. Version 2. European Commission, Joint Research Centre. ISBN 978-92-79-05379-5
http://www.iiasa.ac.at/Research/FOR/INSEA/News/Stolbovoy/EUR21576.pdf

summary poster:
http://eusoils.jrc.it/ESDB_Archive/eusoils_docs/Poster/Soil_Sampling.pdf

3. McKenzie, N., Ryan, P., Fogarty, P., and Wood, J. 2000. Sampling, measurement, and analytical protocols for carbon estimation in soil, litter, and coarse woody debris. Australian Greenhouse Office, Technical Report 14.
http://www.greenhouse.gov.au/ncas/reports/tr14final.html

Winrock also has a sampling cost calculator available from
http://www.winrock.org/ecosystems/tools.asp