Evaluating the simulated mean soil carbon transit times by Earth system models using observations

One known bias in current Earth system models (ESMs) is the underestimation of global mean soil carbon (C) transit time (τ_soil), which quantifies the age of the C atoms at the time they leave the soil. However, it remains unclear where such underestimations are located globally. Here, we constructed a global database of measured τ_soil across 187 sites to evaluate results from 12 ESMs. The observations showed that the estimated τ_soil was dramatically shorter from the soil incubation studies in the laboratory environment (median = 4 years; interquartile range = 1 to 25 years) than that derived from field in situ measurements (31; 5 to 84 years) with shifts in stable isotopic C (¹³C) or the stock-over-flux approach. In comparison with the field observations, the multi-model ensemble simulated a shorter median (19 years) and a smaller spatial variation (6 to 29 years) of τ_soil across the same site locations. We then found a significant and negative linear correlation between the in situ measured τ_soil and mean annual air temperature. The underestimations of modeled τ_soil are mainly located in cold and dry biomes, especially tundra and desert. Furthermore, we showed that one ESM (i.e., CESM) has improved its τ_soil estimate by incorporation of the soil vertical profile. These findings indicate that the spatial variation of τ_soil is a useful benchmark for ESMs, and we recommend more observations and modeling efforts on soil C dynamics in regions limited by temperature and moisture.