Organic carbon and reduced inorganic sulfur accumulation in subtropical saltmarsh sediments along a dynamic coast, Yancheng, China

The carbon and sulfur cycles in coastal wetland sediments are intricately linked. We measured total organic carbon (TOC) and total reduced inorganic sulfur (TRIS) in sulfate-rich wetland sediments at middle and low elevation saltmarshs along the Yancheng coast, China. Results show that the average TOC concentrations (5.01 ± 2.31 kg C/m³) associated with Spartina sediments in mid elevation sites is much higher than that seen in low elevation mudflats (0.65 ± 0.25 kg C/m³). TRIS concentrations in Spartina sediments ranged from 21.7 ± 11.2 μmol·cm⁻³ to 3.0 ± 2.0 μmol·cm⁻³. Within the TRIS pool, elemental sulfur (ES) constituted the lowest fraction (3.6% to 31.4%) and chromium reduced sulfur (CRS) and acid volatile sulfur (AVS) fractions were higher particularly at coasts characterized by siltation, ranging from 29.9% to 62.2% and from 28% to 59%, respectively. AVS or CRS contents correlated positively with dissolved iron/SO₄²⁻ and pH, while no clear relationship was present between the TRIS fractions and the abundance of sulfate reduction genes or between the TRIS fractions and TOC. Massive pyrite was the dominant authigenic pyrite present at all sites. Our results suggest that with sea level rise, prolonged inundation and rapid siltation would increase TRIS (especially AVS) and TOC accumulation concurrently while coastal erosion and strong waves would reduce both TOC and TRIS accumulation but increase the ES proportion due to exposure to aerobic conditions.