Are hypersaline tidal flat soils potential silicon sinks in coastal wetlands?

Hypersaline tidal flats (HTF) are transitional ecosystems which play an important role over Si biogeochemistry in wetlands. Sources and sinks of Si in these ecosystems are directly dependent on pedogenesis and are strongly influenced by the readily soluble Si components, such as dissolved Si, adsorbed Si, and amorphous silicates. The aim of this study was to identify and assess the distribution of readily soluble Si pool in two HTFs on the Brazilian coast. We based our studies on sequential extractions of iron and silicon, modeling of the soil aqueous phase, and X-ray diffraction of the fine clay fraction. Our results show that mobile Si and adsorbed Si are present at very low levels in HTF soils, comprising <3% of the readily soluble Si pool. Silicon associated with Fe oxyhydroxides is the second largest component of the readily soluble Si pool, making up 8.7-40.8% of the Si extractions. The correlation between the ferrihydrite fraction and Si associated with Fe oxyhydroxides indicates a large control of Si by ferrihydrite. Amorphous Si makes up the dominant fraction, comprising 59-90% of the Si extractions. SEM images show the presence of diatoms, sponges, and phytoliths in the soils. Precipitation of pedogenic opal seems to occur in the soils due to a dry season at both sites. The clay assemblages are composed of the mixed-layer kaolinite-smectite (K-S), kaolinite-illite (K-I), and illite-smectite (I-S), and lesser amounts of kaolinite (K) and smectite (S) end-members. The presence of these clays indicates that HTF soils are conducive to the formation of mixed-layer minerals, which partially control Si dynamics. Our data indicate that HTF soils tend to accumulate Si by adsorption, (co)precipitation, and reactions involving clay minerals.