Australian Soil Carbon Sequestration Potential, 2023

Soil is a huge carbon (C) reservoir, but where and how much extra C can be stored is unknown. Here, using 5089 observations, we estimated that the uppermost 30 cm of Australian soil holds 13 Gt (10–18 Gt) of mineral-associated organic carbon (MAOC). Using a frontier line analyses, described in Viscarra Rossel et al. (2023), we estimated the maximum amounts of MAOC that Australian soils could store in their current environments, and calculated the MAOC deficit, or C sequestration potential. We propagated the uncertainties from the frontier fitting and mapped the estimates of these values over Australia using machine learning and kriging with external drift (KED). The maps show regions where the soil is more in MAOC deficit and has greater sequestration potential. The modelling shows that the variation over the whole continent is determined mainly by climate, linked to vegetation, and soil mineralogy. We find that the MAOC deficit in Australian soil is 40 Gt (25–60 Gt). The deficit in the vast rangelands is 20.84 Gt (13.97–29.70 Gt) and the deficit in cropping soil is 1.63 Gt (1.12–2.32 Gt). Our findings suggest that the C sequestration potential of Australian soil is limited by climate.