Abstract
Abstract. The global soil organic carbon (SOC) mass is relevant for the carbon cycle budget and thus atmospheric carbon concentrations. We review current estimates of SOC stocks and mass (stock × area) in wetlands, permafrost and tropical regions and the world in the upper 1 m of soil. The Harmonized World Soil Database (HWSD) v.1.2 provides one of the most recent and coherent global data sets of SOC, giving a total mass of 2476 Pg when using the original values for bulk density. Adjusting the HWSD's bulk density (BD) of soil high in organic carbon results in a mass of 1230 Pg, and additionally setting the BD of Histosols to 0.1 g cm−3 (typical of peat soils), results in a mass of 1062 Pg. The uncertainty in BD of Histosols alone introduces a range of −56 to +180 Pg C into the estimate of global SOC mass in the top 1 m, larger than estimates of global soil respiration. We report the spatial distribution of SOC stocks per 0.5 arcminutes; the areal masses of SOC; and the quantiles of SOC stocks by continents, wetland types, and permafrost types. Depending on the definition of "wetland", wetland soils contain between 82 and 158 Pg SOC. With more detailed estimates for permafrost from the Northern Circumpolar Soil Carbon Database (496 Pg SOC) and tropical peatland carbon incorporated, global soils contain 1325 Pg SOC in the upper 1 m, including 421 Pg in tropical soils, whereof 40 Pg occurs in tropical wetlands. Global SOC amounts to just under 3000 Pg when estimates for deeper soil layers are included. Variability in estimates is due to variation in definitions of soil units, differences in soil property databases, scarcity of information about soil carbon at depths > 1 m in peatlands, and variation in definitions of "peatland".
Highlights
The global mass of soil organic carbon (SOC; for a list of terms and acronyms see Table 1) is greater than the combined mass of carbon (C) contained in the atmosphere and in the living biomass (Ciais et al, 2013)
The Harmonized World Soil Database (HWSD) was the latest and most detailed inventory at the global scale when this study was begun and is still widely used as an international reference (e.g., Wieder et al, 2014; Yan et al, 2014) we describe the adjustments, especially those of bulk density (BD) of organic soils (Hiederer and Köchy, 2011), that are necessary for calculating the SOC stocks from the HWSD
We compare values of SOC masses derived from the adjusted HWSD to those given by other important sources for SOC-rich soils in the permafrost region and in peatlands
Summary
The global mass of soil organic carbon (SOC; for a list of terms and acronyms see Table 1) is greater than the combined mass of carbon (C) contained in the atmosphere and in the living biomass (Ciais et al, 2013). Temporal variation, and methodological differences in measuring any of the necessary soil characteristics (BD, Corg, volume of gravel and roots, forms of C, depth) contribute to the variability of SOC stock and mass estimates (Ellert et al, 2001). To calculate SOC stocks one needs Corg, BD, soil depth, and volumetric gravel fraction These are provided individually by 87, 32, 100, and 22 %, respectively, of the profiles (Batjes, 2009).
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