Here are some handbooks
1. Peter Donovan. Measuring soil carbon change: a flexible, practical, local method. 2010. A basic guide for do-it-yourselfers and the method for the Soil Carbon Challenge. Includes planning worksheet and plot data sheets.
2. 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_Sourcebook-compressed.pdf (661 K pdf file; right click and "save link as" to download)
Winrock also has a sampling cost calculator available from http://www.winrock.org/ecosystems/tools.asp
3. 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://eusoils.jrc.ec.europa.eu/esdb_archive/eusoils_docs/other/EUR21576_2.pdf
summary poster: http://eusoils.jrc.it/ESDB_Archive/eusoils_docs/Poster/Soil_Sampling.pdf
4. 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
5. Harnessing Farms and Forests in the Low Carbon Economy: How to create, measure, and verify greenhouse gas offsets edited by Zach Willey and Bill Chameides, Duke University Press, 2007.
6. Tugel, Arlene J., Skye A. Wills, and Jeffrey E. Herrick. 2008. Soil Change Guide: Procedures for Soil Survey and Resource Inventory, Version 1.1. USDA, Natural Resources Conservation Service, National Soil Survey Center, Lincoln, NE. Available from http://soils.usda.gov/technical/soil_change/index.html This page includes some sample size calculators in Excel, as well as planning aides and worksheets. This appears to be the most thorough of the lot.
Some relevant papers
Spatial variability of soil organic carbon in grasslands: implications for detecting change at different scales by Rich Conant and Keith Paustian, 2002. "Our results suggest that grassland soil C changes can be precisely quantified using current technology [soil sampling and dry combustion] at scales ranging from farms to the entire nation."
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