Radioactive fallout legacy of nuclear tests in soils of the Southern Hemisphere

Fallout radionuclide (FRN) archived in soils are key tracers of soil redistribution, sediment dynamics, and associated carbon fluxes, yet their global application is limited by poorly constrained baselines in the Southern Hemisphere. Here, we present the first hemisphere-scale baseline and uncertainty maps of 137 Cs and 239+240 Pu soil inventories. Predictions exhibited lower spatial variability and relative uncertaintyfor 137 Cs across most landscapes, whereas 239+240 Pu showed greater spatial variability and larger relative uncertainty. The reconstructed inventories revealed strong latitudinal and topographic gradients, with the highest predicted fallout occurring in tropical and mountainous regions. We further show, under a standardized hypothetical 50% inventory-loss scenario, that uncertainty in low-inventory regions can propagate nonlinearly into soil erosion and carbon-budget estimates, producing carbon budget biases of up to 19.8 t C ha −1 . These findings provide a new benchmark for Southern Hemisphere fallout radionuclide applications and a scalable framework for integrating spatial uncertainty into sediment tracing, soil degradation assessment, and carbon cycle studies.