How has soil carbon stock changed over recent decades?

Soil is the largest stock of carbon (C) in the terrestrial biosphere, so even slight changes in soil C stock may induce significant fluctuations in the atmospheric C dioxide (CO2) concentration. Early coupled C-climate models predicted that positive C-climate feedback would be triggered due to the acceleration of C release to the atmosphere under future climate warming (Cox et al., 2000). However, due to the omission of key microbial components and biogeochemical mechanisms in these models (Wieder et al., 2013), these predictions remain controversial, because soil C dynamics is still highly uncertain among results simulated by 11 Earth system models (ESMs) involved in CMIP5 (Ciais et al., 2013). Likewise, experimental evidence is also contradictory, revealing increasing, decreasing, or nonsignificant changes among individual experiments (Lu et al., 2013). Given the very mixed results from both modelling and experimental studies, we present a global synthesis of soil C changes to evaluate a central tendency.