Simulating Arctic Greening: Microbial Responses in Thawing Yedoma Permafrost

Arctic permafrost soils and sediments store vast amounts of organic carbon. With global warming intensifying, these soils are increasingly exposed to microbial decomposition. Further, Arctic warming and longer growing seasons drive regional greening, potentially accelerating the degradation of the previously frozen organic matter through rhizosphere priming of the soil microbial community. However, little is known about how fresh plant‐derived inputs shape microbial functioning in thawing and eroding Yedoma deposits. We simulated vegetation inputs to sediments from a retrogressive thaw slump in northeast Siberia in two thaw mounds (Pleistocene Yedoma) and one slump‐floor site (mixed Yedoma and Holocene material). The microbial response to two substrate levels (1% and 4% of total organic carbon) was evaluated in a 7‐week aerobic incubation by measuring heterotrophic respiration, bacterial and fungal growth rates, extracellular enzyme activities, and carbon use efficiency (CUE). Overall, substrate additions stimulated respiration, bacterial growth, and enzyme production, consistent with microbial activation, while microbial CUE remained unaffected. Growth responses shifted in favor of bacteria despite the high C:N ratio and mostly polymeric nature of the substrate mixture. Throughout the incubation, however, fungal growth increased as labile substrates were likely depleted. In addition, sustained increases in nutrient‐acquiring enzymes suggest that nutrient‐mining‐induced priming of native soil organic matter may have occurred. Both thaw mounds showed varying microbial activity, likely reflecting differences in organic matter quality, whereas the slump floor was characterized by a stronger emphasis on resource acquisition than growth, likely linked to more acidic conditions. Together, these results support microbial activation and nutrient mining theories and suggest that plant‐derived inputs may enhance microbial processing of thawed Yedoma carbon, while site‐specific conditions shape distinct microbial activities.