Mapping the distribution and magnitude of soil inorganic and organic carbon stocks across Australia

Abstract

Understanding the presence and dynamics of soil inorganic carbon (SIC) is essential, given its role as a significant sink for atmospheric carbon within the global carbon cycle. In arid and semi-arid regions such as Australia, soils may contain a higher proportion of SIC compared to soil organic carbon (SOC). However, the relative magnitudes of SIC and SOC in these areas remain unclear. This study resolves this uncertainty by estimating Australia’s total soil carbon stocks, with a particular focus on the inorganic carbon fraction. The SIC content was predicted using a two-step quantile regression forests mixture model of classification and regression for six depth intervals: 0–5 cm, 5–15 cm, 15–30 cm, 30–60 cm 60–100 cm, and 100–200 cm at 90 m × 90 m resolution. Equivalent SOC maps were derived from our previous study. The SIC models utilised a compilation of environmental covariates and inorganic carbon content related data from pH (n = 41,590), effervescence (n = 15,105) and calcium carbonate measurements (n = 5,776). Both the classification model (kappa = 0.533) and regression model (R² = 0.468) for SIC achieved fair accuracy. The elevated concentration of SIC is consistent with the distribution of calcareous soils, and mainly accumulates in the lower depth. Our estimates indicate that the total carbon stock in Australian soils (0–2 m) is 78.9 Pg, with 44 % comprised of SIC. In the upper 1 m depth, carbon stock from SIC is half that of SOC (17.57 Pg vs. 37.75 Pg); however, in the lower depth interval of 1–2 m, SIC is three times larger than SOC (17.48 Pg vs. 6.13 Pg). In the arid and semi-arid regions of Australia, the amount of SIC stock (34.1 Pg C) is slightly larger than that of SOC stock (29.82 Pg C). This study provides a baseline measure of soil as a carbon sink in the forms of organic carbon and inorganic carbon within Australia.