Abstract: Sulfur cycle is an important component of fundamental biogeochemical cycles, which deeply affects the cycles of carbon, iron, phosphorus and other elements as well as the ecological function of coastal wetlands. In this study, we measured the content of acid volatile sulfide (AVS), elemental sulfur (ES), pyrite sulfur (FeS₂), organic carbon, and active iron in the sediment cores collected from the Beidagang wetland (in Tianjin) with different salinity, to better understand the migration and transformation processes of sulfur in coastal wetland sediments. The vertical distribution characteristics of sulfur species, active iron, and organic carbon in the wetlands with different salinity were clarified. The influencing factors of sulfur migration and transformation in different forms were examined. The results showed that AVS was the main reduced inorganic sulfur in the sediments of mesohaline and polyhaline wetlands, followed by FeS₂ and ES. The relative contributions of AVS to reduced inorganic sulfur (RIS) were 71.45% ± 18.97% and 58.96% ± 28.88% for the mesohaline and polyhaline wetland sediments, respectively. However, the content of FeS₂ was the highest inorganic sulfur in the sediments of oligohaline wetland, with an average content of 83.41 ± 33.03 μmol·g^-1, which accounted for 71.98% ± 13.01% of RIS in the sediments. These results indicated that the spatial distribution of reduced inorganic sulfur species in wetland sediments highly depended on salinity. Furthermore, the high sulfate concentration in the water of mesohaline and polyhaline wetlands led to an enhanced sulfate reduction driven by methane and organic matter in the sediments, producing more AVS. The formation of pyrite in the mesohaline and polyhaline wetlands was mainly limited by the content of active iron and ES, while the accumulation of FeS₂ in the sediments of oligohaline wetland may be mainly affected by the reservoir retention effect, particle input, and high sulfur content. Our results contribute to the restoration and management of ecological environment in wetlands.

Key words: coastal wetland, sediment, reduced inorganic sulfur, acid volatile sulfide, elemental sulfur, pyrite, active iron