Effects of bioturbation by root and crab activity on iron and sulfur biogeochemistry in mangrove substrate

Solid-phase analysis was undertaken to obtain information on the effects of bioturbation (activity of plants and macrofauna) on the distribution of different forms of iron and sulfur in a mangrove ecosystem in Brazil. A seasonal study involving four compartments from a mangrove forest (UN: an unvegetated mudflat; SP: a low intertidal zone colonized by Spartina alterniflora; RH: monospecific forest of Rhizophora mangle and; AV: monospecific forest of Avicennia schauerianna) was carried out. Pyrite-Fe, oxyhydroxides-Fe, acid volatile sulfide (AVS), degree of pyritization (DOP), pH and Eh were determined. The results show an intense effect of mangrove vegetation on soil redox processes. Higher live root densities (56–132 g m− 2) in surface layers (0–10 cm) from vegetated sites led to suboxic/oxic conditions (129–374 mV), pyrite oxidation, and acidic (3.8–5.5) conditions, showing the role of plant activity on iron sulfide biogeochemistry. The combination of more oxidizing conditions and pyrite oxidation process also favored oxyhydroxide precipitation, as indicated by higher values of oxyhydroxides-Fe found in superficial layers. In contrast, vegetated zones presented higher values of pyrite-Fe than UN, indicating that root exudades may stimulate the activity of sulfate-reducing bacteria. The results show a clear seasonal variation in AVS and pyritic fractions. In summer, the elevated temperatures cause higher evapotranspiration rates and plant activity which intensified the oxidation of iron sulfides. Thus, at all sites the AVS and pyrite-Fe values were substantially lower during summer (especially in superficial layers). A clear seasonal pattern was also observed in the unvegetated mudflat compartment. The lowest pyrite-Fe, DOP and AVS values, observed in summer, appear to be related to increased crab activity (Uca spp), probably because of more intense foraging and reproductive activity.