Radiocarbon constraints imply reduced carbon uptake by soils during the 21st century
Soil is the largest terrestrial carbon reservoir and may influence the sign and magnitude of carbon cycle feedbacks with climate change. Earth system Models (ESMs) generally estimate a significant soil carbon sink under 21st century climate change scenarios, primarily as a consequence of soil responses to increases in net primary production. However, turnover times of soil organic matter dynamics have rarely been evaluated against observations, thus raising questions about the accuracy of future projections. We optimized 2-box (fast and slow pool) soil decomposition models to 5 ESMs that contributed simulations to CMIP5. In all cases, the reduced-complexity model was able to capture the starting pool size and transient behavior of soil carbon in the ESM forced with increasing levels of atmospheric CO2. We then used the reduced-complexity model to simulate D14C at 48 sites across the globe.
The D14C of the reduced-complexity models was considerably higher than the observations when integrated to a depth of 1m. To match the observed D14C, the models required longer turnover times of the slow pool (by a factor of 13±3) and much lower carbon flow rates into this pool (about a 90% reduction). These adjustments yielded a 56±25% decrease in the soil carbon sink under a quadrupling of CO2, and indicated that current ESMs are overestimating the potential of soils to accumulate carbon under climate change. The reduction in sink capacity identified here is independent from other recently identified mechanisms (e.g. nutrient limitation), and suggests the need for better representation of mineral stabilization of slow soil organic matter.