In Australia there was a soil carbon research program and I would expect that there are associated sampling and calculation protocols. It is likely that there are sister programs in other countries and, e.g., in the EU.

ISRIC – World Soil Information https://www.isric.org/explore , and https://soilgrids.org/ - Standards: https://www.isric.org/about/standards

Main procedures and standards in use at ISRIC — World Soil Information cover the whole data life cycle from field sampling to serving quality-assessed soil data to the world community.