Estimating organic carbon stocks of mineral soils in Denmark: Impact of bulk density and content of rock fragments

Management measures to reduce atmospheric carbon dioxide concentrations by increasing soil organic carbon (SOC) storage need verification, e.g., by periodic sampling of soils to estimate resulting changes in SOC stock. Estimates of SOC stocks are affected by content of rock fragments (systematic bias) and soil bulk density (random but significant effect), both of which may vary significantly between soils. We investigated the importance of using site-specific bulk density and correcting for rock fragment content on estimates of SOC stock in 0–50 cm depth of agricultural minerals soils, collected in 2019 in the Danish National Square Grid. We found that use of an average bulk density value for a given soil type category produced valid estimates of SOC stocks for regional/national inventories. However, large variations in bulk density were found within a given soil type category, which can result in over- or under-estimation at local sites. This calls for measurement of site-specific bulk density and rock fragment content to produce valid estimates of field-scale SOC stock, e.g., to be used in farm carbon credit schemes.