Organism-induced accumulation of iron, zinc and arsenic in wetland soils.

The aim of this study was to gain a better understanding of the impact of rhizosphere/burrow oxidation by wetland plants and burrowing invertebrates on the biogeochemistry of metals and metalloids in salt marsh ecosystems. It was hypothesised that salt marsh plants and burrowing invertebrates could considerably affect the retention capacity of wetlands for metals through oxidation of the rhizosphere/burrow wall. Various soil, plant and porewater samples were collected from areas dominated by the plant species Spartina townsendii and Atriplex portulacoides and by the lugworm Arenicola marina, and from corresponding nearby unvegetated/uninhabited sites at North Bull Island salt marsh, Dublin Bay, Ireland. Samples were analysed for total Fe, Zn and As. The organic matter content (LOI), bulk density, water content and dry/fresh weight ratio of rhizosphere, burrow wall and bulk soil was measured for each species. DCB-extractable Fe, Zn and As, associated with the iron plaque on the roots of the two plant species were also determined. The presence of vegetation and, to a lesser extent, burrowing organisms were shown to have a significant effect on the concentration and accumulation of heavy metals in salt marsh soils. Iron and arsenic concentrations were significantly higher in vegetated/inhabited soils compared to nearby unvegetated/uninhabited areas. Zinc showed the same trend but the difference was not statistically significant. The concentrations of Fe and As were also significantly higher in the rhizosphere soil around the plant roots and in the burrow walls of Arenicola compared to the bulk soil. For zinc, the same pattern was significant only for S. townsendii-dominated soils. Atriplex stands appeared to have the greatest potential for heavy metal accumulation with concentrations reaching 1238 micromol Fe g(-1), 4.9 micromol Zn g(-1) and 512 nmol As g(-1) in the rhizosphere. The Zn/Fe ratio for S. townsendii and the As/Fe ratios for both plant species also increased from the bulk soil towards the roots. Concentrations of Zn and As appeared to correlate with both Fe concentrations and LOI values. However, covariation was significant only with Fe, indicating that it is the oxidation of Fe, rather than the binding to organic matter, that drives the accumulation of Zn and As. The amount of each element present in the various compartments associated with the plants (the sum of the element concentrations in the rhizosphere, ironplaque and roots) in 1 litre of the top 20 cm of soil, amounted to 0.84 % for Fe, 3.6% for Zn and 2.8% for As for S. townsendii, and 12.5% for Fe, 19% for Zn and 18.3% for As for A. portulacoides. Densities of A. marina were never higher than 1 per litre of top soil so the small volume of burrow wall soil would therefore render that pool of negligible size compared to the rhizospheres of plants. It is likely that lugworms affect the movement of metals more importantly through bioturbation.