Another promising direct method is eddy covariance measurement of CO 2 fluxes. By correlating eddy size and CO 2 concentration for each upward and downward moving eddy, the net uptake or release of C by vegetation plus soil can be calculated. The accuracy and precision of this method is improving as more experience is gained. Direct methods are essential for determining changes in soil C content for different land management treatments at the plot scale. To determine soil C pools and rates of change for large areas, it is necessary to extrapolate the relationships developed at the plot and field scale.
Indirect methods are suitable for large areas.
Indirect methods include simple and stratified accounting where plot estimates are multiplied by all applicable areas as determined by soil survey, land cover, climate, and other spatial data. Environmental and topographic relationships with C storage and change and modeling approaches can also be applied when available. Remote sensing and other geographical information are valuable in appropriately representing areas in these spatially aggregated estimation methods. Accurate science-based methods are available for monitoring and verifying changes in soil C. Codifying a set of methods into protocols that can be used to account for C sequestration in public agreements or private trading contracts needs refinement and testing. The level of precision and effort required will vary with the purposes for which the measurements are applied.