Climate sensitivities of carbon turnover times in soil and vegetation: understanding their effects on forest carbon sequestration

The high uncertainty associated with the response of terrestrial carbon (C) cycle to climate is dominated by ecosystem C turnover time (τeco). Although the relationship between τeco and climate has been extensively studied, significant knowledge gaps remain regarding the differential climate sensitivities of turnover time in major biomass (τveg) and soil (τsoil) pools, and their effects on vegetation and soil C sequestration under climate change are poorly understood. Here, we collected multiple time-series observations on soil and vegetation C from permanent plots in ten Chinese forests and used model-data fusion to retrieve key C cycle process parameters that regulate τsoil and τveg. Our analysis showed that 𝜏veg and 𝜏soil both decreased with increasing temperature and precipitation, and τsoil was more than twice as sensitive (1.27 yr/°C, 1.70 yr/100 mm) than τveg (0.53 yr/°C, 0.40 yr/100 mm). The higher climate sensitivity of τsoil caused a more rapid decrease in τsoil than in τveg with increasing temperature and precipitation, thereby significantly reducing the difference between τsoil and τveg (τdiff) under warm and humid conditions. τdiff, an indicator of the balance between the soil C input and exit rate, was strongly responsible for the variation (more than 50%) in soil C sequestration. Therefore, a smaller τdiff under warm and humid conditions suggests a relatively lower contribution from soil C sequestration. This information has strong implications for understanding forest C-climate feedback, predicting forest C sink distributions in soil and vegetation under climate change, and implementing C mitigation policies in forest plantations or soil conservation.