Variation with depth and season in metal sulfides in salt marsh soils

Abstract

Seasonal monitoring of metal sulfides was carried out in four soils of the Ria de Ortigueira salt marshes. Soils from the high salt marsh (with suboxic redox conditions at the surface), had low concentrations of iron sulfides (AVS and pyrite fraction) and thus a low degree of trace metal pyritiza- tion (DTMP) in surface layers (0-10 cm), but concentrations of metals associated with the pyrite frac- tion increased considerably at depth (27.5 cm). In the low salt marsh soils (with anoxic conditions at the surface) maximum concentrations of metal sulfides were found in the surface layers of soils colonized by Spartina maritima. These results are explained by the double effect exerted by roots in strongly re- duced soils. On the one hand, they stimulate the activity of sulfate-reducing bacteria and on the other, they favour the partial oxidation of the soil, thus generating polysulfides with which Fe 2+ immediately precipitates as pyrite, whereas in the deepest, permanently anoxic layers, pyrite must be formed in a reaction in which FeS is an intermediate, as follows: FeS + H2S → FeS2 +H 2. Concentrations of metal sulfides also varied greatly with the season, with two patterns being distinguished. In soils colonized by S. maritima in both high and low salt marshes, the lowest concentrations were found in summer. At this time of the year there is a net loss of metal sulfides throughout the profile, presumably due to physi- ological activity of plants (evapotranspiration and release of oxygen from roots). In contrast, maximum concentrations of AVS and pyritic metals were found in the summer in the low salt marsh soils not colonized by vascular plants (creek bottom). In this case, the higher temperatures increased the activity of sulfur-reducing bacteria leading to synthesis and accumulation of metal sulfides in the soil.