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Megan L. Purchase, Richard P. Phillips, Jonathan D. Raff, Amy I. Phelps, Elizabeth Huenupi, Ryan M. Mushinski

The emergence of periodical cicadas from soil every 13 or 17 years is a unique ecological phenomenon with the potential to affect soil biogeochemistry in forests, with increased emissions of climate-relevant gases as a consequence. While it's well-known that cicada carcasses create resource pulses of carbon and nitrogen (N) in soil when they die in mass, the processes underlying these effects, as well as the consequences of these effects for N losses, are poorly known. We investigated how the emergence of Brood X cicadas (Magicicada spp.) in 2021 affected soil microbial communities – particularly N cycling taxa - in forests of the United States. We found that decaying carcasses led to emissions of nitrous oxide (N₂O) and ammonia (NH₃) gas at around 0.53 mg-N m^−2 h^−1, estimated to be a ∼ 35-fold increase over ∼21 days from the annual average emissions from US forest soils (0.015 mg-N m^−2 h^−1), with the greatest effects occurring at the interface between carcasses and soil surface. Using amplicon sequencing and qPCR, we determined the potential microbial mechanisms behind N₂O and NH₃ production, including correlations between taxa capable of carrying out less well studied processes DNRA and nitrifier denitrification, and increased emissions of N₂O and NH₃. Although distinguishing the relative contributions of DNRA, denitrification, and nitrifier denitrification requires direct rate measurements, our results suggest these processes working together contribute to previously unrecognised greenhouse gas emissions following insect emergence events. Collectively, our results indicate that cicadas significantly affect nutrient cycling in forests with the potential to alter soil microbial communities in ways that may enhance ecosystem N emissions.