Nitrogen loss from anaerobic ammonium oxidation coupled to Iron(III) reduction activity across estuarine and coastal wetlands of China: Spatial variations, controlling factors, and environmental implications

Anaerobic ammonium oxidation coupled with ferric iron reduction (Feammox) is a newly identified microbial nitrogen (N) removal process in natural ecosystems. However, little is known about the distributions of Feammox activity and their environmental drivers across estuarine and coastal wetlands at a continental scale. In this study, we determined the potential Feammox rates and related microbial diversities and abundances along the estuarine and coastal wetlands of China encompassing subtropical to temperate climate zones. Measured potential Feammox activity ranged from 0.05 to 0.23 mg N kg⁻¹ d^-1, and the process rates at the subtropical sites were generally higher than at the temperate sites. Partial least squares path model (PLS-PM) showed that the abundance of Fe reducing bacteria significantly and directly affected the potential Feammox rates, while sediment characteristics (mainly including sediment water content, Fe(III), TOC and NO₃^–) and geographical factors (longitude and latitude) had an indirect effect on the process rates. The N loss via the Feammox was approximately 1.72–7.89 kg N ha⁻¹ year⁻¹, accounting for 0.41–1.91 % of multi-annual average land-based inorganic N transported into the estuarine and coastal wetlands of China. Overall, this study reveals the distribution patterns and controlling factors of the Feammox process in estuarine and coastal wetlands of China, and provides a valuable database for N management in estuarine and coastal ecosystems.