Unprotected carbon dominates decadal soil carbon increase.

Changes in the distribution of soil organic carbon (SOC) fractions - particulate organic carbon (POC; unprotected carbon) vs. mineral-associated organic carbon (MAOC; protected carbon) - affect SOC storage and stability. Here, we compile a SOC fraction dataset from 7219 soil samples across six continents. From 2000 to 2022, POC increases by 21.8% while MAOC decreases by 5.3%, leading to a net gain of 11.0% in SOC storage and a 29.1% increase in the POC/MAOC ratio. However, relative to undisturbed natural ecosystems, SOC declines in planted forests, grazed grasslands, and croplands with management practices such as heavy grazing and conventional tillage, primarily due to rapid declines in POC. Our results highlight that the global increase in SOC is mainly driven by POC, but warn of decreasing SOC stability associated with climate change and human activities. Thus, maintaining soil carbon sinks requires targeted strategies focusing on POC.