Geochemical alterations in contaminated mangrove soils and effects on the bioavailability of metals and microbial community

Abstract

Mangrove soils in their natural conditions are known as a sink of contamination for retaining heavy metals in solid and organic particles in non-available forms. In the northern littoral of Pernambuco, the Botafogo River estuary has a history of contamination by mercury (Hg), chrome (Cr) and zinc (Zn) but it is needed to evaluate how changes on the soil pH, redox potential, and iron (Fe) contents of these soils can affect the availability of metals and the soil microbiota. The objectives of this study were to determine Hg, Cr, Zn and Fe contents in mangrove soils and analyze changes in compartmentalization of these metals under 10 times of incubation exposing the soils to oxic conditions, as well as the interferences of geochemical alterations on microbial respiration and soil carbon biomass. Soils from three areas (A1, A2 and A3) of the Botafogo River were collected and selected for ten times of incubation (0, 5, 15, 30, 45, 60, 75, 90, 105 and 120 days). After incubation times the samples were divided in two depths (0-5 cm and 5-10 cm). Potential redox (Eh) and pH were determined on site and after every collection from incubation times. Soil particle size distribution was performed for each area. Environmental available contents of Hg, Cr, Zn and Fe were determined by the 3051A method and sequential extraction of these metals was performed. Soil microbial biomass, soil respiration and microbial metabolic quotient were also determined. These soils are classified as clay and silty clay. After 120 days of decreased humidity, all areas showed acidification and an increase in Eh from anoxic/suboxic to oxic conditions. Alterations of soil pH and redox potential did not cause significant redistribution of Hg, Cr and Fe in A1, A2 and A3. There was a decrease of Zn associated with carbonate fractions and increased association with residual fractions under incubation times for all three areas. Iron was mostly associated with the pyrite fraction and had significant contribution to holding metals in recalcitrant fractions of soils. Acidification and changes from anoxic to oxic conditions had the main effect on microbial biomass increase and decrease on soil respiration in the most contaminated area, possibly because these new conditions favored fungi communities over bacteria. Besides the changes in soil pH and Eh, there was no correlation between heavy metals and microorganismal attributes analyzed in this study. The microbial metabolic quotient remained low under different areas and times of incubation, showing little impact from the geochemical alterations on microorganisms from the Botafogo River soils.