On the formation of high-latitude soil carbon stocks: Effects of cryoturbation and insulation by organic matter in a land surface model

We modify the soil component of the ORCHIDEE terrestrial carbon cycle model to include vertically-discretized soil carbon. With this model, we investigate the feedback of considering thermal insulation by soil carbon, which modifies the soil thermal regime by lowering the thermal conductivity and increasing the heat capacity of a carbon-rich soil, on the total carbon stocks the model builds up. In addition, we demonstrate the effects of diffusive vertical mixing of soil organic matter by cryoturbation on the total carbon stocks that the model builds up in mineral soils in equilibrium with a steady climate. We show that including these two effects together leads to up to 30% higher soil carbon stocks in the top meter of permafrost soils, as well as large stocks of carbon below 1m in the upper permafrost soil layers. The vertical profile of partitioning of carbon between different lability pools is also affected, as the slower pools are more deeply mixed; also the time to reach equilibrium lengthens considerably. These effects are largest in the coldest regions such as Eastern Siberia. The inclusion of cryoturbative mixing and insulation by soil carbon leads to better agreement with estimates of high-latitude soil carbon stocks, where substantial amounts of carbon are found in permafrost regions, to depths of three meters; however we do not include peat, Yedoma, or alluvial deposition processes here, so the total carbon stocks are still lower than observed.