Abstract: Land reclamation for aquaculture, a major driver of coastal wetland degradation and loss, may influence nitrogen biogeochemical cycling at the region scale. Using acetylene inhibition method, we measured soil denitrification potential (SDP) of a Cyperus malaccensis marsh and the converted earthen aquaculture pond in the Shanyutan wetland of the Miniiang River Estuary, southeastern China. Results showed that, compared with the C. malaccensis marsh, the conversion of coastal wetlands into aquaculture ponds significantly reduced SDP by 61.2% to 71.7%. Total soil carbon, total nitrogen, NO₃^--N, and NH₄⁺-N in pore water significantly decreased by 5.1%, 62.3%, 64.8%, and 53.6%, respectively. The abundances of four denitrification functional genes (nirK, nirS, nosZⅠ, and nosZⅡ) significantly decreased by 85.7%, 92.6%, 91.6%, and 94.1%, respectively, while soil carbon-to-nitrogen ratio significantly increased by 170.4%. Random forest analysis identified soil total nitrogen, porewater NO₃^--N and NH₄⁺-N, and the abundances of nirK and nirS genes as the main factors driving the variation in SDP following the wetland conversion. Overall, reclamation for aquaculture markedly suppressed soil denitrification potential, which in turn diminished the coastal wetland’s natural nitrogen removal and its buffering capacity against exogenous nitrogen loading, consequently raising the risks of nitrogen retention and eutrophication under equivalent nitrogen inputs.

Key words: denitrification potential, denitrifier, carbon-nitrogen substrate, aquaculture reclamation, estuarine wetland