Intensified warming effects on soil respiration upon thermokarst formation

As global temperatures continue to rise, a key uncertainty of terrestrial carbon (C) climate feedback is the rates of C loss upon abrupt permafrost thaw. This type of thawing - termed thermokarst - may in turn accelerate or dampen the response of microbial degradation of soil organic matter and carbon dioxide (CO2) release to climate warming. However, such impacts have not yet been explored in experimental studies. Here, by experimentally warming three thermo-erosion gullies in an upland thermokarst site combined with incubating soils from another five thermokarst-impacted sites on the Tibetan Plateau, we investigate whether and how abrupt permafrost thaw would influence the responses of soil CO2 release to climate warming. Our results show that warming-induced increase in soil CO2 release is higher in thermokarst features than the adjacent non-thermokarst landforms. This larger warming response is mainly attributed to the lower substrate quality and higher abundance of microbial functional genes for recalcitrant C degradation in thermokarst-affected soils. Taken together, our study provides experimental evidence that abrupt permafrost thaw aggravates the warming-associated soil CO2 loss, which will exacerbate the positive soil C-climate feedback in permafrost-affected regions under future warming scenarios.