Conversion of coastal wetland to aquaculture ponds decreased NO emission: Evidence from a multi-year field study.

Land reclamation is a major threat to the world's coastal wetlands, and it may influence the biogeochemical cycling of nitrogen in coastal regions. Conversion of coastal marshes into aquaculture ponds is common in the Asian Pacific region, but its impacts on the production and emission of nitrogen greenhouse gases remain poorly understood. In this study, we compared NO emission from a brackish marsh and converted shrimp aquaculture ponds in the Shanyutan wetland, the Min River Estuary in Southeast China over a three-year period. We also measured sediment and porewater properties, relevant functional gene abundance, sediment NO production potential and denitrification potential in the two habitats. Results indicated that the pond sediment had lower N-substrate availability, lower ammonia oxidation (AOA and comammox Nitrospira amoA), nitrite reduction (nirK and nirS) and nitrous oxide reduction (nosZ Ⅰ and nosZ Ⅱ) gene abundance and lower NO production and denitrification potentials than in marsh sediments. Consequently, NO emission fluxes from the aquaculture ponds (range 5.4-251.8 μg m h) were significantly lower than those from the marsh (12.6-570.7 μg m h). Overall, our results show that conversion from marsh to shrimp aquaculture ponds in the Shanyutan wetland may have diminished nutrient input from the catchment, impacted the N-cycling microbial community and lowered NO production capacity of the sediment, leading to lower NO emissions. Better post-harvesting management of pond water and sediment may further mitigate NO emissions caused by the aquaculture operation.