Nutrients and trace elements of semi-arid dwarf and fully developed mangrove soils, northwestern Venezuela

Mangrove forests are characterized by a high carbon storage capacity. Anthropogenic activities and the climate change are threatening the stability of these ecosystems, specially those that are surviving on extreme environmental conditions, representing the most susceptible groups. This study aims to understand the geochemical differences of soils of mangrove forest with diverse degree of development of Rhizophora mangle and Avicennia germinans, located in two semi-arid regions from the Falcon state, Venezuela. The soil samples were collected in Punta Caiman region (dwarf mangroves of less than 2 meters height) and Boca de cano lagoon (fully developed mangroves of up to 8 meters height). The trace elements and macronutrients were determined in two sequential extraction, first with 1M HCl followed by an extraction with ultrapure grade HNO3. Results showed that, in general, dwarf mangroves have limited capability to accumulate organic matter in the soil (average TOC of 0.6 +/- 0.2% ) and that the sampled surface soils from the dwarf mangrove region had no differences with deeper material. On the other hand, the TOC of soils dominated by developed mangroves is 6 +/- 2% , representative of semiarid conditions. Moreover, the soils dominated by dwarf mangroves are characterized high concentrations of trace elements related to the chalcophile group. The difference in soil chemistry between Rhizophora mangle and Avicennia germinans becomes significant when mangroves are fully developed. Finally, soils of dwarf mangrove forests presented a greater concentration of trace elements than soils of fully developed mangroves, which might be due to a different sediment source, to an anthropogenic influence or to the difference in organic matter.