Climate and mineral controls on global soil radiocarbon profiles

Radiocarbon measurements provide a powerful tool to assess the persistence of soil organic carbon (SOC). While soil depth is generally one of the most important predictors of soil radiocarbon age, it remains unclear whether this relationship is due to an overall decrease in C input with depth or to changing importance of climatic or mineralogical constraints on SOC decomposition. Due to this lack of mechanistic understanding, we argue that the relationship between soil radiocarbon age and SOC abundance may be a better proxy than depth to investigate SOC persistence. To test this hypothesis, we use globally collected soil radiocarbon data from the International Soil Radiocarbon Database (ISRaD) to examine the influence of climate and soil mineralogy on the relationship between SOC concentration and radiocarbon age at the profile level. Our analysis includes about 600 soil profiles covering all major climatic zones and soil types (except aridisols). We show that extreme climatic and mineralogical constraints can lead to a similar accumulation of old SOC throughout the soil profile but for very different reasons. Climatic extremes include soils from tundra/polar regions where C input and decomposition are constrained by low temperatures and water availability. Mineralogical extremes include volcanic soils (andisols) that are dominated by highly reactive amorphous minerals that limit SOC accessibility. Across all climate zones and for a given SOC concentration, arid soils tend to have younger radiocarbon ages compared to temperate soils. Tropical soils have the youngest SOC at the surface due to high C input and show a relatively uniform distribution of SOC and radiocarbon ages globally. In terms of mineralogy, soils dominated by low-activity clays (oxisols and ultisols) show younger radiocarbon ages than soils dominated by high-activity clays (all other soil types except andisols) for a given SOC concentration. These first results have far-reaching implications for better understanding SOC vulnerability and benchmarking soil carbon models for representing SOC turnover and persistence across climate zones and soil types.