Temperature effects on carbon storage are controlled by soil stabilisation capacities

Physical and chemical stabilisation mechanisms are now known to play critical roles in controlling carbon (C) storage in mineral soils. This has led to suggestions that climate warming-induced C losses may be lower than previously predicted. However, evidence has also been produced that the decomposition of older, and more protected soil organic matter (SOM) is more sensitive to temperature than unprotected and more rapidly decomposing SOM pools. Thus, the extent to which temperature controls C storage in mineral soils remains controversial, and it is not known whether the C stores in soils with large capacities for stabilising C are more, or less, vulnerable to climate warming than the C stored in soils with more limited stabilisation capacities.

By analysing data on >9,000 soil profiles from the World Soil Information Database, we found that, overall, C storage declines strongly with mean annual temperature. However, we observed very large differences in the effect of temperature on C storage in soils with different capacities for stabilising SOM, as indicated by their textural properties. In coarse-textured soils (clay contents less than 20%) with more limited stabilisation capacities, C storage declined strongly with temperature, decreasing by a factor of 1.6 to 2.0 for every 10 ^oC increase in temperature. However, in fine-textured soils (clay contents greater than 35%) with greater stabilisation capacities, the effect of temperature on C storage was more than three times smaller. This pattern was observed independently in cool and warm regions, and after accounting for potentially confounding factors including plant productivity, precipitation, aridity, cation exchange capacity and pH. The difference in the effects of temperature on C storage in soils with contrasting stabilisation capacities could not, however, be represented by an established Earth system model (ESM). To reduce uncertainties in projections of the effect of climate change on soil C losses, we suggest that ESMs could be evaluated against their ability to simulate the differences in the effects of temperature on C storage in soils with contrasting textural properties.

In conclusion, our results suggest that there are stabilised pools of SOM in fine-textured soils that may be relatively insensitive to the impacts of climate change, but that less protected pools in coarser-textured soils may be substantially more vulnerable to global warming than currently predicted. Finally, given the mismatches between data and model outputs, ESMs may not be predicting accurately the potential magnitude of soil C losses in responses to climate warming or which stocks are most vulnerable.