Soil organic carbon of mangrove forests (Rhizophora and Avicennia) of the Venezuelan Caribbean coast

Abstract

In order to understand the processes that control organic matter preservation in tropical wetlands, we have evaluated the mineralogy, total organic carbon (TOC wt%), soluble organic matter (SOM wt%), bulk density (g/cm3), and carbon stock (Mg/ha), and used FTIR to identify functional groups in SOM of soil in mangrove forest dominated by Rhizophora or Avicennia with different conditions (live, deteriorated and dead). Six locations along the Cuare Inlet and Morrocoy National Park were studied. Mineralogical analysis showed the presence of minerals, such as pyrite and rhodochrosite, from anoxic environments. Rhizophora mangrove soils have higher TOC compared with Avicennia, but we did not find significant differences in SOM. TOC/SOM ratios were lower for Avicennia soils. The carbon content ranged from 11.30 to 59.84 Mg/ha for the first 10 cm of soil. Regardless of stand condition, the TOC/SOM ratio was lower at a depth of 20–40 cm. The results of the TOC/SOM ratio are attributable to: (a) the association of TOC with clays and non crystalline minerals; (b) the leaching processes of soluble compounds of the OM; (c) a higher proportion of recalcitrant compounds; (d) a lower decomposition rate for recalcitrant or non-recalcitrant compounds; and (e) physicochemical conditions that limit biological activity, such as high salinity, soil anoxia and hypoxia. The soils can be divided into three groups according to the presence and intensity of functional groups detected by FTIR. The functional groups identified could not be related to the sampling sites, to species composition or conditions. These differences may be due to other sources of organic matter, as well as to degree of preservation. Information of soil organic matter properties and their relationship with mangrove composition and conditions is important to understand carbon sequestration and storage potential in Venezuelan Caribbean mangrove systems.