Carbon flow through oxygen and sulfate reduction pathways in salt marsh sediments1

Abstract

We measured surface O 2 uptake, 35 SO 4 2− reduction, and total sediment metabolism (CO 2 production) in sediments (0–30 cm) supporting stands of short Spartina alterniflora in a New England salt marsh. Sediment CO 2 production was highest at the surface where O 2 was present and declined rapidly with depth. In deeper sediments (below 5 cm) CO 2 production was equal to 35 SO 4 2− reduction as determined by the Cr 2+ reduction technique. Time‐course experiments using CO 2 production and 35 SO 4 2− reduction (by Cr 2+ reduction and aqua regia digest) indicate that the aqua regia technique is not reliable for measuring SO 4 2− reduction and that the rate of SO 4 2− reduction is much less than previously reported for this marsh. Our estimates of annual O 2 uptake conform with those for CO 2 production and belowground production. We estimate carbon mineralization to average about 180 mmol C·m −2 ·d 1 , among the highest measured for marine sediments. Our simultaneous measurements of O 2 uptake, CO 2 production, and SO 4 2− reduction suggest that at least half of this decomposition occurs via SO4 2− reduction. We conclude that the mechanisms underlying carbon and sulfur cycling in salt marsh sediments are much more similar to subtidal sediments than previously recognized.