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 cm soil layer, terrestrial ecosystems hold 27.6 Gt (19.6-39.0 Gt), and BCE 0.35 Gt (0.20-0.62 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.

<p>The Biomass Plot Library is a collation of stem inventory data across federal, state and local government departments, universities, private companies and other agencies. It was motivated by the need for calibration/validation data to underpin national mapping of above-ground biomass from integration of Landsat time-series, ICESat/GLAS lidar, and ALOS PALSAR bacscatter data under the auspices of the JAXA Kyoto & Carbon (K&C) Initiative (Armston et al., 2016). At the time of Version 1.0 publication 1,073,837 hugs of 839,866 trees across 1,467 species had been collated. This has resulted from 16,391 visits to 12,663 sites across most of Australia's bioregions. Data provided for each project by the various source organisation were imported to a PostGIS database in their native form and then translated to a common set of tree, plot and site level observations with explicit plot footprints where available.</p> <p>Data can be downloaded from https://field.jrsrp.com/ by selecting the combinations Tree biomass and Site Level, Tree Biomass and Tree Level.</p>