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 inver tebrates 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 wetl ands for metals through oxidation of the rhizosphere/burrow wall. Vari ous soil, plant and porewater samples were collected from areas domina ted by the plant species Spartina townsendii and Atriplex portulacoide s and by the lugworm Arenicola marina, and from corresponding nearby u nvegetated/uninhabited sites at North Bull Island salt marsh, Dublin B ay, Ireland. Samples were analysed for total Fe, Zn and As. The organi c matter content (LOT), bulk density, water content and dry/fresh weig ht ratio of rhizosphere, burrow wall and bulk soil was measured for ea ch species. DCB-extractable Fe, Zn and As, associated with the iron pl aque on the roots of the two plant species were also determined. The p resence of vegetation and, to a lesser extent, burrowing organisms wer e shown to have a significant effect on the concentration and accumula tion of heavy metals in salt marsh soils. Iron and arsenic concentrati ons were significantly higher in vegetated/inhabited sails compared to nearby unvegetated/uninhabited areas. Zinc showed the same trend but the difference was not statistically significant. The concentrations o f Fe and As were also significantly higher in the rhizosphere soil aro und the plant roots and in the burrow walls of Arenicola compared to t he bulk soil. For zinc, the same pattern was significant only for S. t ownsendii-dominated soils. Atriplex stands appeared to have the greate st potential for heavy metal accumulation with concentrations reaching 1238 mu mol Fe g(-1), 4.9 mu mol Zn g(-1) and 512 nmol As g(-1) in th e 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 r oots. Concentrations of Zn and As appeared to correlate with both Fe c oncentrations and LOI values. However, covariation was significant onl y 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 associa ted with the plants (the sum of the element concentrations in the rhiz osphere, ironplaque and roots) in I litre of the top 20 cm of soil, am ounted 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. D ensities of A. marina were never higher than 1 per litre of top sail s o the small volume of burrow wall soil would therefore render that poo l of negligible size compared to the rhizospheres of plants. It is lik ely that lugworms affect the movement of metals more importantly throu gh bioturbation. (C) 1997 Elsevier Science Ltd.