Soil organic matter to soil organic carbon ratios of peatland soil substrates

• SOM/SOC ratios differ between peat types, peat conditions and soil horizons.
• The botanical origin of peat forming plants strongly influences SOM/SOC ratios.
• Peat type-related conversion factors are most appropriate for SOC assessments.
• The use of one single conversion factor causes considerable conversion errors.
• Limits for histic horizons after WRB need an adaptation considering SOM/SOC ratios.

Data of soil organic matter (SOM) content can be used for the assessment of stocks and changes of soil organic carbon (SOC) in peatland soils. Therefore, it is essentially necessary to convert SOM contents into SOC contents by SOM/SOC ratios (“conversion factors”). Various substrates of peatland soils in Northeastern Germany (n = 311) were analyzed in SOM/SOC ratios. Sphagnum peats showed significantly higher SOM/SOC ratios (2.05 ± 0.09) than peats of vascular plants (1.73 ± 0.09) and amorphous peats (1.93 ± 0.29). Amorphous peats and humic sands (2.41 ± 0.46) showed a high variability. The classification using WRB qualifiers featured significant differences (***P < 0.001) between humic, sapric, hemic and fibric substrates, except hemic and fibric peats of vascular plants. Moreover, impacts of drainage on pedogenesis could be proved in different SOM/SOC ratios of drained topsoils and water-saturated subsoils. Due to the high dependency of SOM/SOC ratios on the botanical origin, the implementation of peat type-related conversion factors is most suitable. In contrast, the application of one single conversion factor causes considerable conversion errors. As a consequence, many SOC assessment studies referring to peats should be reviewed.