Aggravation of nitrous oxide emissions driven by burrowing crab activities in intertidal marsh soils: Mechanisms and environmental implications

Coastal wetlands are hotspots for nitrogen (N) cycling and a significant natural source of the potent greenhouse gas nitrous oxide (N2O). Burrowing benthos are known to transform N in intertidal marsh soils, but their contribution to N2O emissions and the underlying molecular mechanisms remain unclear. Here, the effects of crab bioturbation on N2O emissions in coastal marshes, where Chiromantes dehaani and Helice tridens tientsinensis were the dominant crab species, were investigated in field and indoor experiments. The N2O emissions increased significantly in the presence of crabs, being positively correlated with the intensity of crab bioturbation but differing among crab species. Natural-abundance isotope analyses indicated that crab bioturbation greatly promoted the contribution of hydroxylamine oxidation to N2O production; nevertheless, bacterial denitrification (including heterotrophic denitrification and nitrifier denitrification) remained the dominant pathway. Molecular analyses suggested that the increased N2O emissions in crab-disturbed habitats might be driven by underlying changes to microbial communities, especially by the disproportionate stimulation of nitric oxide reductase-carrying microbes over nitrous oxide reductase-carrying microbes. This study highlights the importance of considering the burrowing activity, density, and species of benthos when evaluating the N cycle in coastal wetlands.