Climate-driven thresholds in reactive mineral retention of soil carbon at the global scale

Soil organic matter can release carbon dioxide to the atmosphere as the climate warms. Organic matter sorbed to reactive (iron- and aluminium-bearing) soil minerals is an important mechanism for long-term carbon storage. However, the global distribution of mineral-stored carbon across climate zones and consequently its overall contribution to the global soil carbon pool is poorly known. We measured carbon held by reactive minerals across a broad range of climates. Carbon retained by reactive minerals was found to contribute between 3 and 72% of organic carbon found in mineral soil, depending on mean annual precipitation and potential evapotranspiration. Globally, we estimate similar to 600 Gt of soil carbon is retained by reactive minerals, with most occurring in wet forested biomes. For many biomes, the fraction of organic carbon retained by reactive minerals is responsive to slight shifts in effective moisture, suggesting high sensitivity to future changes in climate.