Impact of Soil Moisture on Carbon Cycle Changes in Permafrost Regions in CESM2

Permafrost regions store vast amounts of soil carbon, and warming-driven thaw can enhance microbial decomposition and carbon dioxide release, strengthening the permafrost carbon feedback. Although this feedback has been widely studied under continued warming, the processes governing its magnitude and persistence remain highly uncertain. Here we use the Community Earth System Model version 2 (CESM2) to examine permafrost carbon-cycle dynamics under both warming and mitigation, and apply multiple regression to quantify the controls on net biome production. We identify soil moisture as a key regulator of carbon cycle dynamics. Thaw-driven increases in liquid soil water enhance plant photosynthesis, but more strongly accelerate microbial decomposition, shifting ecosystems toward net carbon release and reducing net biome production. Sensitivity to soil moisture is strongly heterogeneous, with the largest response in central Siberia where high litter carbon coincides with relatively low climatological soil moisture, whereas North America shows weaker sensitivity under a wetter background state. Soil moisture also delays recovery after mitigation by slowing permafrost refreezing and sustaining anomalously wet soils during the net-zero period. This maintains elevated heterotrophic respiration and prolongs negative net biome production even after mitigation. In conclusion, our results show that soil moisture can amplify and sustain permafrost carbon losses along mitigation pathways, highlighting improved representation of coupled soil hydrology and permafrost processes as a priority for reducing uncertainty in future carbon budget assessments.