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

• Published in: Biogeosciences
• Main Authors: Wang, Jing, Xia, Jianyang, Zhou, Xuhui, Huang, Kun, Zhou, Jian, Huang, Yuanyuan, Jiang, Lifen, Xu, Xia, Liang, Junyi, Wang, Ying-Ping, Cheng, Xiaoli, Luo, Yiqi
• Format: Text
• Language: English
• Published: 2019
• Subjects: Tundra
• Online Access: https://doi.org/10.5194/bg-16-917-2019; https://www.biogeosciences.net/16/917/2019/

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 ( 13 C ) 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.