Patterns of sulfate reduction and the sulfur cycle in a South Carolina salt marsh1,2

Rates of sulfate reduction and the magnitude of various sediment parameters related to the sulfur cycle were measured at about quarterly intervals for a 3‐yr period at sites in a South Carolina salt marsh containing tall‐ or short‐form Spartina alterniflora. Sulfate reduction rates were greater at all times in the short‐form sites and decreased markedly with depth. Seasonal changes were highly correlated with ambient temperature except in May and August when interactions with plants may have affected rates directly. In the short‐form site, the annual integrated sulfate reduction rate accounted for about 47.5–67.0% of the estimated belowground plant production. Dissolved sulfides and pH varied seasonally in the short‐ but not tall‐form sites; seasonal minima in the short‐form site occurred during May and were probably due to oxidation of the sediment with the onset of plant production. Mechanisms resulting in oxidation were not evident however. Chromium‐reducible sulfur, which is primarily pyrite, was substantially greater in the tall‐ than short‐form site but apparently stable temporally at both sites. The differences in pool sizes of pyrite were probably due to differences in iron dynamics and the extent of sulfide oxidation. Microprofiles of dissolved oxygen were consistent with greater oxidation at the short‐form site since the penetration of oxygen was deeper. Differences in sediment texture and tidal inundation may have contributed to greater oxygen inputs at the short‐form site.