Radiocarbon Reveals Modern Carbon Exchange With Topsoil Inorganic Carbon in Drylands

Drylands store most of the global soil inorganic carbon (SIC), yet the extent to which this pool interacts with contemporary carbon (C) cycling remains poorly understood. To test whether SIC behaves primarily as an inert geological reservoir or instead bears a modern carbon imprint, we quantified SIC content and radiocarbon (∆¹⁴C) at 42 dryland sites spanning broad aridity gradients across Eurasia. We also evaluated the climatic, edaphic, and biotic factors associated with variation in ∆¹⁴C-SIC. Across all sites, topsoil SIC (~0-10 cm) was strongly depleted in ¹⁴C but consistently enriched relative to ¹⁴C-dead carbonates, indicating that it contains a measurable component derived from modern carbon inputs. In the Chinese drylands, ∆¹⁴C-SIC declined with increasing aridity, consistent with weaker modern carbon exchange under drier conditions and a greater contribution of inherited or ¹⁴C-depleted carbonate carbon. Soil pH and ∆¹⁴C of soil organic carbon were the strongest predictors of ∆¹⁴C-SIC, suggesting that carbonate dissolution-reprecipitation and the age of carbon entering soil CO₂ play key roles in determining SIC origins. At a subset of nine Chinese sites, ∆¹⁴C-SIC declined sharply with depth and approached ¹⁴C-dead values in subsoils, indicating little influence of modern carbon in deeper carbonate pools. The presence of mixed ¹⁴C-depleted and modern ¹⁴C signatures in SIC potentially complicates the use of SIC isotopic signatures as proxies of environmental conditions. Together, our results indicate that dryland topsoil SIC commonly carries a measurable modern carbon signature that is tightly linked to contemporary carbon cycling. This coupling weakens with increasing aridity and soil depth, suggesting that environmental change in drylands may reshape one of the planet's largest carbon pools not only through changes in SIC stocks, but also through shifts in carbonate radiocarbon signatures and sources.