Limited imprint of modern C sources on soil inorganic C

Drylands contain most of the global soil inorganic carbon (SIC), yet its sources and vulnerability are poorly constrained. We measured SIC content and its radiocarbon (14C) at 55 dryland sites across Asia and Europe to investigate the origins of SIC and its environmental factors. In the top 10 cm, SIC generally increased with aridity, but extremely dry sites with low soil organic carbon (SOC) often had low SIC, implying that low vegetation input limits both SOC accumulation and pedogenic carbonate formation. The Δ14C of topsoil SIC was positively related to the Δ14C of SOC and declined with aridity, consistent with reduced influence of modern C sources. SIC content was linked mainly to net primary productivity (NPP) and SOC, whereas Δ14C-SIC was controlled primarily by soil pH, which governs carbonate dissolution and precipitation reactions. Thus, a greater imprint of modern carbon is found in carbonates of less alkaline surface soils, either indicating a greater potential for forming new pedogenic carbonates or greater isotopic exchange. At a subset of sites on the Chinese Loess Plateau and the Inner Mongolia Plateau, soil samples were collected across multiple depth intervals. SIC contents remained relatively constant with depth, whereas Δ14C-SIC decreased, indicating reduced contributions of recent carbon to SIC. We estimate that carbonates reflecting the influence of modern C sources accounted for about 30% of topsoil SIC but only about 10% in subsoils. These results show that dryland subsoils retain older, more stable, and likely geogenic carbonates, whereas topsoils contain younger pedogenic carbonates that are more influenced by and potentially vulnerable to environmental change.