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Density-Based Soil Organic Carbon Fractionation: Experimental Method Comparison and Influential Factors


 Background: Because soil organic carbon (SOC) variation is a result of its physicochemical protection, fractionating SOC into different functional subpools according to its protection mechanism and studying the mechanism of different SOC fractions’ responses to environmental change will help guide the study of SOC dynamics. Therefore, we conducted an analysis of density-based SOC fractionation of 107 study sites from 35 literature sources to answer the following questions: (1) Will different fractionation methods yield different amounts in the three organic carbon pools: free organic carbon (FOC), occluded organic carbon (OOC) and mineral associated organic carbon (MOC)? (2) Does the distribution of these three SOC fractions differ with climate (mean annual temperature, MAT; mean annual precipitation, MAP), soil characteristics (e.g., soil layer, soil type, clay content) or vegetation type when controlling for any method differences?Results: Experimental method significantly affected OOC and MOC but not FOC results, and OOC separated by density and soil physical dispersion (density+disperse) was underestimated, thus a suitable SOC fractionation method should be carefully selected. SOC and MOC contents were negatively related to MAT; and highest SOC content appeared at moderate MAP, and when MAP increased or decreased, SOC decreased. SOC, FOC, and MOC were significantly affected by vegetation type; presumably due to anthropogenic disturbance or precipitation, plantations, grass and rainforest had the lower SOC contents and higher OOC and MOC percentages; and conifer, broadleaf, and mixed forests had similar FOC, OOC and MOC percentages, indicating less effect of tree species on SOC variation. The contents of both SOC and each fraction decreased in deeper sol layer; SOC, FOC and OOC contents were significantly affected by soil type; and SOC and MOC contents were negatively related to soil clay content, but the influences of soil characters on SOC and its fractions were less than experimental method and climate condition.Conclusion: Experimental methods for fractionation of SOC significantly affected fraction results. Climate, vegetation type and soil character also significantly influenced SOC and its factions, but the influences of soil characters on SOC and its fractions were not as strong as experimental method and climate condition.