BACKGROUNDResearch on fractions of soil carbon can not only be helpful to realize the migration and transformation of carbon, but also provide the scientific basis for reducing global carbon emissions. There is lack of systematic research on organic carbon fractions, especially the chronic and inert fractions.

OBJECTIVESTo lay the foundation for future research through the summary of previous research on composition, distribution and roles of different carbon fractions.

METHODSIt proposes development directions for future works through a summary of previous research.

RESULTSThe main occurrence form of inorganic carbon was carbonate. The soil organic carbon pool covered the activated carbon pool (turnover time:0.1-4.5 year), chronic carbon pool (turnover time:5-50 year) and inert carbon pool (turnover time:50-3000 year). Previous studies showed that the activated carbon pool, which contained dissolved organic carbon, labile organic carbon and microbial biomass carbon, could reflect the small change of physical and chemical properties of soil sensitivity. Chronic carbon, as the important indicator of the conversion of organic matters in soil, contained light fraction organic carbon and particulate organic carbon. Heavy fraction organic carbon and mineral-associated organic carbon belonged to the inert carbon pool, which was one of the important mechanisms of organic carbon sequestration in soil. Carbonate in soil was always analyzed by gasometric method or titrimetry, while the organic carbon tended to be measured by the volumetric method, colorimetric method or gravimetric method.

CONCLUSIONSStudy on soil inorganic carbon and inert organic carbon should be strengthened. As far as the analysis methods, 'ramped combustion method' should be researched further. The in-situ measurement method of soil carbon (including organic carbon and inorganic carbon) also needs to be developed. Reference materials of soil carbon fractions with different soil type and different concentration are also necessary.