Conversion of natural coastal wetlands to mariculture ponds dramatically decreased methane production by reducing substrate availability

Coastal wetlands are increasingly being converted into aquaculture ponds to meet growing global demand for fish protein. Coastal wetlands conversion has been predicted to result in significant carbon (C) emissions; however, a mechanistic understanding of the effects of coastal wetland conversion on soil’s capacity to produce greenhouse gases remains lacking. Here, integrated biogeochemical investigations on methanogenic substrates, community structures, and activities were conducted in a coastal Spartina alterniflora saltmarsh and three saltmarsh-converted mariculture ponds aged 6, 13, and 20 years. Saltmarsh conversion into mariculture ponds decreased CH4 production potential from 353 to 16.3–78.3 µg CH4 kg−1 d−1. The concentrations of dissolved organic C and non-competitive methanogenic substrate, trimethylamine, were reduced by 84.8% and 79.7%, respectively, whereas methanogens abundance decreased by 43.7%, probably due to the decreased C input following conversion. Among the ponds, CH4 production potential, methanogenic substrates (acetate and trimethylamine), and methanogen abundance decreased with conversion chronosequence. However, the dominant methanogen group did not shift after land conversion, and the potential methylamine-utilizing Methanosarcinaceae accounted for 86.5% and 85.3–89.7% of the total populations in the S. alterniflora saltmarsh and mariculture ponds, respectively, implying that methylotrophic methanogenesis was the predominant pathway of CH4 production in both ecosystems. The results indicated that the conversion of natural coastal saltmarsh to mariculture ponds decreased CH4 production by reducing substrate availability and methanogen abundance, and CH4 production potential in mariculture ponds was more efficiently suppressed with conversion chronosequence.