Shifts in Microbial Thermal Traits Mitigate Heat-Induced Carbon Losses in Soils.

Global warming is expected to transfer carbon from soil organic matter to atmospheric CO2, with microbial communities playing a crucial role in regulating this exchange. While the immediate impact of temperature on microbial functions is well understood and causes soil carbon losses, the long-term response remains unclear, with losses stabilising over time, reducing the overall effect of chronic warming on soil organic carbon (SOC) stocks. Here, we examined the temperature dependence of microbial respiration and growth after 9 years of +5°C warming in a temperate forest. Using these temperature dependences and field temperature data, we modelled in situ carbon fluxes and changes in SOC stocks. Results showed that the direct effect of temperature initially increased respiration and growth, projecting a potential 31% SOC stock loss if the trend had persisted. However, the gradual optimisation of microbial traits to warming balanced the direct temperature effects, enhanced carbon use efficiency and offset CO2 emissions. Together, these microbial trait shifts limited the heat-induced SOC loss to 15%, closely aligning with empirical observations. These findings suggest that microbial trait optimisation can moderate carbon emissions, providing a parsimonious mechanistic explanation for observations worldwide and underscoring the need to integrate microbial dynamics into models.