N2O production is influenced by the abundance of nitrite-reducers and N2O-reducers in casts produced by a large variety of tropical earthworm species

Abstract

We investigated the potential of earthworm casts to emit N₂O, hypothesizing that emission levels are influenced by the species of earthworm and their ecological category. This study examined casts a broad taxonomic and ecological coverage of tropical earthworms, i.e., 16 different species across four ecological categories. We quantified the potential nitrification, N₂O production and consumption as well as the abundance of N-related microbial functional groups, including ammonia-oxidizers, nitrite-reducers, and distinct clades of N₂O-reducers, along with casts chemical properties to determine cast organic matter quality and substrate availability. Earthworm casts exhibited significantly higher concentrations of carbon, nitrogen, and nitrate compared to control soil, while humification index were lower. A negative correlation between humification index and potential N₂O production suggests that more labile substrates in the casts promote higher N₂O flux. Net potential N₂O emissions were higher in the casts of 7 out of 16 species compared to control soil, and all species’ casts showed higher gross potential N₂O production, with substantial interspecific variability. The abundance of nitrite and N₂O reducers was significantly higher in the casts and positively correlated with potential N₂O emissions. Casts from epigeic and mixed categories displayed higher carbon and nitrogen content, abundance of nitrite and N₂O reducers, ammonia-oxidizing bacteria, and potential N₂O production compared to anecic and endogeic categories, which exhibited higher values of humification index. Structural equation modeling indicated that gross potential N₂O production was primarily explained by the abundance of nitrite reducers and substrate availability indicators such as humification index and nitrate concentration. Our study demonstrates significant interspecific variability in N₂O potential emissions from a broad range of tropical earthworm casts, influenced by species feeding behavior, microbial communities, and substrate availability.