Masked diversity and contrasting soil processes in tropical seagrass meadows: the control of environmental settings

Abstract. Seagrass meadows are among the most valuable ecosystems on Earth. However, in tropical countries, there is a substantial knowledge gap on “seagrass science”. To address this gap, seagrass soils from three Brazilian coastal regions were investigated (the NE, SE, and S coasts). Soil profiles from different geological and bioclimatic settings were sampled, described, and analyzed. Detailed macromorphological descriptions, soil classification, physicochemical analysis (soil particle size, soil pH, pH_oxidation, Eh, total organic carbon; TOC), Fe partitioning, and X-ray diffractometry were performed. Water samples were analyzed for pH, salinity, and ion concentrations. Different environmental settings in the coastal compartments produced contrasting geochemical conditions, which caused different intensities of pedogenetic processes. On the NE coast, the denser plant coverage favored higher TOC contents (2.5±0.1 %) and an anaerobic environment (Eh = +134 ± 142 mV), prone to an intense sulfidization (i.e., pyritic Fe formation; Py-Fe). Py-Fe contents in NE soils were 6-fold and 2-fold higher than those in the SE and S coastal soils, respectively. Conversely, lower TOC contents (0.35 ± 0.15 %) and a suboxic environment (Eh +203 ± 55 mV) in the SE soils, along with the Fe-rich geological surroundings, decreased the intensity of gleization. The contrasting intensities in the soil processes, related to the (seemingly subtle) differences in the geochemistry of each environment, ultimately caused relevant pedodiversity among the studied sites. Our findings contribute to a better understanding of the general functioning of tropical seagrass meadows but also have important environmental implications for studies focused on carbon sequestration in these ecosystems.