Effects of reclamation on the distribution and diversity of comammox along the coastal wetlands of China

Reclamation of estuarine wetlands for paddy fields (PF) and aquaculture ponds (AP) used to be a common practice in China, which has changed land types and significantly affected microbial nitrogen transformations. However, its impacts on nitrification process especially complete ammonia oxidizers (comammox) remain poorly understood. Our study investigated the distribution and diversity of comammox across the major estuarine wetland systems along the coast of China by comparing different land types, including paddy fields (PF), aquaculture ponds (AP), and wetlands. The results showed that reclamation for PF and AP (1.94 × 10⁶ copies g⁻¹ soil) significantly reduced the gene abundance of comammox compared with wetlands (3.19 × 10⁶ copies g⁻¹ soil), with higher ammonia-oxidizing archaea (AOA) and bacteria (AOB) abundances and the rates of nitrification in PF than in AP. Phylogenetic analysis revealed different community structures in the two reclamation types. The distribution pattern of comammox genes in PF was similar to that in estuarine wetlands, with clade A1 and clade A2.1 being the dominant branches. Water content, iron, sulfide, and salinity were identified as the key factors affecting the distribution of comammox. This study highlights the impact of reclamation on comammox in estuarine wetlands and expands our understanding of nitrogen cycling processes in estuarine systems.