Conversion of coastal wetland to aquaculture ponds decreased N2O 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 N2O 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 N2O production potential and denitrification potential in the two habitats. Results indicated that the pond sediment had lower Nsubstrate 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 N2O production and denitrification potentials than in marsh sediments. Consequently, N2O emission fluxes from the aquaculture ponds (range 5.4-251.8 mu g m-2 h-1) were significantly lower than those from the marsh (12.6-570.7 mu g m-2 h-1). 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 N2O production capacity of the sediment, leading to lower N2O emissions. Better post-harvesting management of pond water and sediment may further mitigate N2O emissions caused by the aquaculture operation.

Automated summary

Land reclamation poses a major threat to coastal wetlands worldwide and its effects on the nitrogen biogeochemical cycling in coastal regions are still poorly understood. In this study, the N2O emissions from a brackish marsh and converted shrimp aquaculture ponds in Southeast China were compared over a three-year period. The results showed that the aquaculture ponds had significantly lower N2O emissions compared to the marsh, which could be attributed to diminished nutrient input, changes in microbial community, and lower N2O production capacity of the sediment. Better management of pond water and sediment could further mitigate N2O emissions from aquaculture operations.