Sulfate reduction in a New England salt marsh1

Abstract

Sulfate reduction rates were measured for 2 years in the peat of a salt marsh by a radiotracer technique. Rates are high throughout the peat, from the surface to more than 20 cm deep. The integrated annual rate is about 75 mol SO₄²⁻·m⁻²·yr⁻¹, the highest yet reported for any natural ecosystem. Sulfate reduction accounts for the consumption of 1,800 g org‐C·m⁻²·yr⁻¹, about equal to net primary production in the marsh. Respiration using other electron acceptors (such as oxygen or nitrate) is much less important. Sulfate reduction rates in the peat of the salt marsh are probably high for at least three reasons: the belowground production of Spartina alterniflora provides a large, annual input of organic substrates over a depth of some 20 cm; sulfate is rapidly resupplied to the peat in infiltrating tidal waters, so that sulfate depletion never limits the rate of reduction; and sulfide concentrations remain below toxic levels. The stable pyrite (FeS₂) is a major end product of sulfate reduction in the marsh peat while iron monosulfide (FeS) is not. If the incorporation of ³⁵S into pyrite were not measured, the [³⁵S]SO₄²⁻ reduction measurements would greatly underestimate the true rate of sulfate reduction. Pyrite acts largely as a temporary store of reduced sulfur, with seasonal changes in its concentration.