<p>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&nbsp;cm of Australian soil holds 13&nbsp;Gt (10–18&nbsp;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&nbsp;Gt (25–60&nbsp;Gt). The deficit in the vast rangelands is 20.84&nbsp;Gt (13.97–29.70&nbsp;Gt) and the deficit in cropping soil is 1.63&nbsp;Gt (1.12–2.32&nbsp;Gt). Our findings suggest that the C sequestration potential of Australian soil is limited by climate.

The soil in terrestrial and blue carbon ecosystems (BCE; mangroves, tidal marshes, seagrasses) is a significant carbon (C) sink. National assessments of C inventories are needed to protect them and aid nature-based strategies to sequester atmospheric carbon dioxide. We harmonised measurements from Australia's terrestrial and BCE and, using consistent multi-scale spatial machine learning, unravelled the drivers of soil organic carbon (SOC) variation and digitally mapped their stocks. The modelling shows that climate and vegetation are continentally the primary drivers of SOC variation. But the underlying regional drivers are ecosystem type, terrain, clay content, mineralogy, and nutrients. The digital soil maps indicate that in the 0-30&nbsp;cm soil layer, terrestrial ecosystems hold 27.6&nbsp;Gt (19.6-39.0&nbsp;Gt), and BCE 0.35&nbsp;Gt (0.20-0.62&nbsp;Gt). Tall open eucalypt and mangrove forests have the largest mean SOC per unit area. Eucalypt woodlands and hummock grassland, which occupy vast areas, store the largest total SOC stock. These ecosystems constitute important regions for conservation, emissions avoidance, and preservation because they also provide additional co-benefits.

Destructive sampling of 47 <em>Eucalyptus obliqua</em> trees was carried out in the Warra Tall Eucalypt site to determine a range of biomass measures that can be used to inform allometric equations.

This data contains diameter, volume and biomass measurements of all coarse woody debris pieces within the core 1 ha plot at the Tumbarumba Wet Eucalypt site in 2015.

This data contains diameter, volume and biomass measurements of all coarse woody debris pieces within the core 1 ha plot at the Wombat Stringybark Eucalypt site in 2015.

This data contains diameter, volume and biomass measurements of all coarse woody debris pieces and standing dead trees within the core 1 ha plot at the Cumberland Plain site in 2014 - 2015.

This data contains diameter, length volume, biomass and carbon measurements of all coarse woody debris pieces within the core 1 ha plot at the Warra Tall Eucalypt site in 2015.

This data contains diameter, volume and biomass measurements of all coarse woody debris pieces within the core 1 ha plot at the Samford Peri-Urban site in 2012 and 2014 - 2016.

This data contains diameter, volume and biomass measurements of all coarse woody debris pieces within the core 1 ha plot at the Robson Creek Rainforest site in 2012 - 2014 and 2017.

This data contains diameter, volume and biomass measurements of all coarse woody debris pieces within the core 1 ha plot at the Daintree, Cape Tribulation site in 2014 - 2016.