4.5 Other

4.5.1 Ferreira et al, 2007

“Redox processes in mangrove soils under Rhizophora mangle in relation to different environmental conditions” (Ferreira et al. 2007)

Key contribution: Upon recognizing that redox conditions may be highly variable over space and across different environmental settings, this study seeks to examine redox patterns in pure Rhizophora stands across three different environmental settings. They find that different redox patterns occur, and that particular characteristics that may be associated with particular settings (e.g., degree of periodic oxidation or soil texture) may greatly influence redox potentials.

Key notes:

Bacterial reduction of sulfate is commonly considered the main form of respiration in tidal environments, but recent work has shown that the reduction of Fe may also be very important in mineralization of organic matter.

Study design

Pure Rhizophora forests in three different environmental settings (fringe, riverine and basin) were selected for examining soil conditions and redox conditions.

Half-meter cores were taken with 6 replicates, and pH and Eh were measured in the field.

Cores were preserved and one was cut into fine-scaled sections of variable depth, whereas the other five were cut into two depth sections (0-10 & 10-30 cm).

Pore water analysis was conducted after sampling and from the middle of each soil sample. HS- and Fe2+ concentrations were determined.

Soild-phase Fe was partitioned to determine the contributions of seven operationally derived Fe pools:

  1. Exchangeable Fe
  2. Carbonate-associated Fe
  3. Ferrihydrite Fe
  4. Lepidocrocite Fe
  5. Crstalline oxides
  6. Fe oxyhydroxides (goethite, hematite)
  7. Pyrite Fe

Total organic carbon, total S, total Fe, and percentage sand were determined for each soil sample.

Results

The different environmental settings produced differences in soil composition and physicochemical conditions.

Redox potential values decreased significantly with depth in all studied profiles.

A highly significant correlation between pH and Eh existed, with lowest pH values corresponding to the highest redox potentials.

For Fe2+ and HS- concentrations, high variation over both space as well as with depth were observed. Concentrations of HS- increased significantly with depth, whereas highest values of Fe2+ were found in surficial layers.

Iron partitioning was found to be variable between the fringe site versus the riverine and basin sites:

  • Fringe site - Within the fringe site, Fe oxyhydroxides predominated, especially in surficial layers. Pyrite at this site increased notably with depth.
  • Basin and riverine sites - Relatively lower amounts of Fe oxyhydroxides existed. Pyrite was much higher in the riverine site both at surface and at depth, whereas pyrite approximated Fe oxyhydroxides at surface but pyrite predominated at depth

Total exchangeable Fe was small across all sites and not considered in the study.

Discussion

The author attribute the spatial variation in geochemical conditions to different periods of inundation in which soils are exposed, response to positions within the mangrove forest, and the composition (texturally and chemically) of the substrate.

  • Fringe forest - convex profile and high density of roots induces rapid drainage, aeration and oxidation in low tide; extremely acidic conditions in surface soils coincide with tidal cycle. High tide induces an increase in pH due to diluting and buffering effect of sea water, which favors precipitation of dissolved Fe as Fe oxyhydroxides.
  • Basin forest - strongly reduced conditions due to long periods of inundation from concave profile; anoxic conditions within the profile favor synthesis of pyrite rather than Fe2+ and HS- in pore water. Predominance of pyrite synthesis is due to high availability of OM, which maintains activity of sulfate-reducing bacteria.
    • High rates of sulfate reduction may occur with presence of reactive Fe, as different species of Fe oxyhydroxides have different surface areas (goethite and hematite are reduced), that may limit actions of microorganisms.
  • Riverine forest - highly reduced despite position along river. Total Fe and free Fe was very low relative to the other sites, and was consistent with sandy, quartz-based texture. Thus, sulfate reduction was primary reduction pathway of OM.

Conclusions

The dominant redox pathways are highly variable throughout space and depth. Occurrence or dominance or a particular process over others may significantly affect chemical composition of pore water and solid phases.

References

Ferreira, TO, XL Otero, P Vidal-Torrado, and F Macías. 2007. “Redox Processes in Mangrove Soils Under in Relation to Different Environmental Conditions.” Soil Science Society of America Journal 71 (2): 484–91. doi:10.2136/sssaj2006.0078.