Iron oxides control sorption and mobilisation of iodine in a tropical rainforest catchment

Abstract. Iodine is an essential trace element for all mammals and its bioavailability in terrestrial systems depends on its accumulation in soils but also on its release into the aquatic system. In tropical systems retention and mobilization of iodine in soils and related concentrations in streamflow are poorly understood. We, therefore, investigated the relationship between solid phase iodine binding on hillslope soils and the iodine and dissolved organic carbon (DOC) mobilisation to a connected stream. Our study was conducted in a pristine pre-montane rainforest in Costa Rica with old (up to nine My) and highly weathered volcanic soils. A total of nine soil profiles from two tributary sub-catchments to the main stream were sampled. Solid phase sequential extraction was used to identify iodine binding forms in soils. The water leachable iodine fraction was additionally assessed by batch soil leaching experiments. Stream water was sampled randomly over a period of five weeks. Results showed extremely high iodine concentrations in soils, ranging from 53–130 mg kg⁻¹ (median: 69 mg kg⁻¹), which is 13-fold higher than in temperate soils. In contrast, median water-soluble iodine was only 0.01 % (range: 0–0.4 %) of total soil iodine. Solid phase sequential extractions revealed iodine sorption to iron oxides (median: 79 % of total iodine) as the main retention factor. High enrichment and low mobilisation of iodine in soils caused relatively low iodine concentrations (0.77–1.26 μg L⁻¹) in stream waters during base and even moderate high flow conditions. The significant correlation of iodine and DOC in soil leachates suggested transport of organically bound iodine from upper- to subsoil horizons and strong sorption of DOC-iodine complexes to iron oxides. Our results showed that the old and highly weathered tropical soils in the study area were highly enriched in iodine caused by strong retention of DOC bound iodine to iron oxides. As a result, iodine release from soils was low which resulted in low stream water iodine concentrations and subsequently in a low bioavailability.