Crab bioturbation alters nitrogen cycling and promotes nitrous oxide emission in intertidal wetlands: Influence and microbial mechanism.

Intertidal wetlands provide important ecosystem functions by acting as nitrogen (N) cycling hotspots, which can reduce anthropogenic N loading from land to coastal waters. Benthic bioturbations are thought to play an important role in mediating N cycling in intertidal marshes. However, how the burrowing activity of benthos and their microbial symbionts affect N transformation and greenhouse gas nitrous oxide (NO) emission remains unclear in these environments. Here, we show that bioturbation of crabs reshaped the structure of intertidal microbial communities and their N cycling function. Molecular analyses suggested that the microbially-driven N cycling might be accelerated by crab bioturbation, as the abundances of most of the N related functional genes were higher on the burrow wall than those in the surrounding bulk sediments, except for genes involved in N fixation, dissimilatory nitrate reduction to ammonium (DNRA), and NO reduction, which were further confirmed by isotope-tracing experiments. Especially, the potential rates of the main NO production pathways, nitrification and denitrification, were 2-3 times higher in the burrow wall sediments. However, even higher NO emission rates (approximately 6 times higher) were observed in this unique microhabitat, which was due to a disproportionate increase in NO production over NO consumption driven by burrowing activity. In addition, the sources of NO were also significantly affected by crab bioturbation, which increased the contribution of hydroxylamine oxidation pathway. This study reveals the mechanism through which benthic bioturbations mediate N cycling and highlights the importance of considering burrowing activity when evaluating the ecological function of intertidal wetlands.