Chloride and Organic Chlorine in Forest Soils: Storage, Residence Times, And Influence of Ecological Conditions

摘要

Recent studies have shown that extensive chlorination of natural organic matter significantly affects chlorine (Cl) residence time in soils. This natural biogeochemical process must be considered when developing the conceptual models used as the basis for safety assessments regarding the potential health impacts of 36-chlorine released from present and planned radioactive waste disposal facilities. In this study, we surveyed 51 French forested areas to determine the variability in chlorine speciation and storage in soils. Concentrations of total chlorine (Cl(tot)) and organic chlorine (Cl(org)) were determined in litterfall, forest floor and mineral soil samples. Cl_(org) constituted 11-100% of Cl_(tot) with the highest concentrations being found in the humus layer (34-689 mg Cl_(org) kg~(-1)). In terms of areal storage (53-400 kg Cl_(org) ha~(-1)) the mineral soil dominated due to its greater thickness (40 cm). Cl_(org) concentrations and estimated retention of organochlorine in the humus layer were correlated with Cl input, total Cl concentration, organic carbon content, soil pH and the dominant tree species. Cl_(org) concentration in mineral soil was not significantly influenced by the studied environmental factors, however increasing C1:C ratios with depth could indicate selective preservation of chlorinated organic molecules. Litterfall contributions of Cl were significant but generally minor compared to other fluxes and stocks. Assuming steady-state conditions, known annual wet deposition and measured inventories in soil, the theoretical average residence time calculated for total chlorine (inorganic (Cl_(in)) and organic) was 5-fold higher than that estimated for Cl_(in) alone. Consideration of the Cl_(org) pool is therefore dearly important in studies of overall Cl cycling in terrestrial ecosystems.