The particulate organic fraction dominates soil carbon in the mineral layer of cold ecosystems

Discriminating between soil particulate organic carbon (POC) and mineral-associated (MAOC) can constrain the uncertainty of global SOC estimates. Despite that a massive amount of the global SOC stock is stored in cold regions such as arctic, subarctic and alpine biomes, and that these regions are warming faster than the global average, how C is distributed among fractions is not considered when assessing the contribution of cold regions to global land C-climate feedbacks. Here we combined field observations of POC and MAOC fractions in the mineral layer with experimental warming, to investigate whether SOC in cold regions is predominantly stored in the POC fraction, and whether this relates to higher SOC losses with warming than in milder biomes. We show that SOC in surface mineral soils (0 to 30 cm) is largely dominated by the POC fraction in both permafrost and non-permafrost soils, and in arctic and alpine ecosystems but not in subarctic environments. The experimental findings indicate that SOC is more vulnerable to warming in cold ecosystems compared to milder biomes, as mediated by higher warming-induced POC losses, which agrees with the previous pattern of POC dominance. Our comprehensive analysis suggests that the massive SOC accumulation in cold ecosystems is not only warming faster that the global average, but predominantly stored in the POC fraction, the fraction most vulnerable to anthropogenic climate warming.