Recycling and Persistence of Iodine 127 and 129 in Forested Environments : A Modelling Approach

Differences in the source and behaviour of 129 I versus 127 I isotopes have been described for a variety of surface environments but little is known about their cycling rates in terrestrial ecosystems. We developed a compartment model of iodine cycle in forest ecosystem, with a labile and non-labile pool to simplify the complex fate of iodine in forest floor and soil. Simulations were performed using different atmospheric 127 I and 129 I inputs for diverse environmental conditions. In general, consideration of dry deposits in addition to wet deposits improved model simulations. Model results support the view that soil is the sink for atmospheric iodine deposits in forest ecosystems while tree vegetation has little influence on long-term iodine budgets. Modelling also showed that iodine cycling reaches equilibrium after a period of about 5000 years, mainly due to a gradual iodine incorporation into the bulk stabilized soil organic matter. At steady-state, this pool of non-labile iodine in soil can retain about 20% of total deposition with a mean residence time of 900 years, while the labile iodine pool is recycled with a mean residence time of 90 years. The proportions of modern anthropogenic 129 I in each pool reflect those of stable 127 I after at least several decades, explaining why isotopic disequilibrium is common in field data analysis. Volatilization plays a central role in regulating iodine storage in soil and thus its residence time, while drainage is a minor export pathway, except at some calcareous sites. Dynamic modelling has been particularly helpful to apprehend the long-term response of iodine partitioning to continuous, single or even varying deposition. At specific-sites, meaningful simulations of iodine cycling may require better estimates of dry atmospheric deposits, parent rock weathering and volatilization from soil and vegetation, as these processes remain insufficiently documented in general