Effect of plants on sulphur geochemistry in the Tagus salt-marshes sediments

Depth distributions of salinity, pH, Eh, sulphate (SO42−), thiosulfate (S2O32−), sulphite (SO32−), hydrogen sulphide (H2S) and acid volatile sulphides (AVS) were measured seasonally in pore waters of vegetated and non-vegetated sediments of a Tagus salt-marsh colonized by Spartina maritima. Near-bottom water was also analyzed. The vertical distributions obtained in both sites were compared. The vegetated site was more acid (6.0–7.0) and oxidative (reaching + 360 mV vs. H), and presented higher SO42− concentrations (10–31 mM). At depth of higher root density, AVS and H2S levels were low (0.1–53 μmol g−1 and < 0.1–0.5 μM, respectively) in comparison to vegetated sediments (30–225 μmol g−1 and 0.5–17 μM). S2O32− was only detected in anoxic non-vegetated sediments (11–54 μM) and concentrations increased with depth. Differences of sulphur speciation between the two sites were recorded during all seasons as result of Spartina activity almost the entire year.