Abstract

Nitrous oxide (N2O) is a potent greenhouse gas with more warming potential than carbon dioxide (CO2) and soils are known to be the dominant source of atmospheric N2O. N2O is produced and emitted during microbial transformations of nitrogen (N) species that make up the N cycle. The synchronous emergence of periodical cicadas represents one of the largest insect emergence events. The year 2021 saw the emergence of the 17-year “Brood X” cicadas (Magicicada septendecim, M. cassinii, and M.Septendecula). The exoskeletons of cicadas are primarily made up of chitin, a polymer of N-acetylglucosamine (GlcNac). The aboveground lifespan of cicadas is just ~6 weeks, after which the large deposition of low C:N content cicada carcasses is a significant N-rich input into the environment, particularly to forest soils. 

Decomposition of insect carcasses occurs more readily than decomposition of other organic material, making insects an important and currently understudied input of N to soils that can contribute to emissions of N species from soils.It has been found that emissions of N2O are enhanced when cicada carcasses are added to soil chambers. We have examined the soil microbiome, and the cicada necrobiome using quantitative PCR (qPCR) and 16S rRNA amplicon sequencing to elucidate the potential microbial source of these enhanced emissions. We suggest a currently unknown microbial pathway for the decomposition of cicada chitin and the resulting enhanced N cycle processes that lead to “hotspots” of N2O emissions from forest soils following cicada emergence. Future work will involve time-series experiments to ascertain the links between decomposition state, microbial community of the soil and carcasses, and N2O emissions.

Abstract

Volatile reactive nitrogen oxides (NOy) are significant atmospheric pollutants, including NO + NO2 (NOx) and HONO + HNO3 + NO3(NOz). Biogenic sources, including soil, account for over 50% of natural NOy emissions to the atmosphere. Despite their importance, NOy emissions from soils are generally not included in atmospheric models due to a lack of mechanistic data.

Spatial heterogeneity amongst landscapes and across population gradients likely influences NOy fluxes due to differences in atmospheric deposition rates and anthropogenic soil modifications – likely altering microbial and abiotic cycling of NOy; however, this has not been explored in mechanistic detail. Here, we link nitrifying and denitrifying soil microbes, across a gradient of urbanisation and land-use, to NOy gas fluxes from soil. To resolve temporal discrepancies, soil sampling (0-10 cm) occurred seasonally.

Results show significant changes in relative abundances of microbial orders associated with nitrification, denitrification and nitrogen fixation across a human population gradient, and between land-use types, suggesting anthropogenic impact on soil microbiology. From structural equation modelling (SEM), we see NOy and NO fluxes significantly affected by land-use type. Soil physicochemistry is a major influence on other measured variables and is therefore likely to be an important predictor of soil NOy fluxes. 

Keywords: Soil, Biogeochemical cycling, Anthropogenic impacts

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