Iodine distribution and cycling in a beech (Fagus sylvatica) temperate forest

Highlights: • Beech forest monitoring realised to describe the biogeochemical cycling of iodine. • Soil is the main I reservoir of studied forest stand. • Iodine is poorly stored in beech trees. • Litterfall is a major pathway in the I biological cycling. Radioiodine is of health concerns in case of nuclear events. Possible pathways and rates of flow are essential information for risk assessment. Forest ecosystems could influence the global cycle of long-lived radioiodine isotope (¹²⁹I) with transfer processes similar to stable isotope (¹²⁷I). Understanding iodine cycling in forest involves study of the ecosystem as a whole. In this context, we determined the ¹²⁷I contents and distribution in soil, tree compartments and atmospheric inputs during a three years in situ monitoring of a temperate beech forest stand. The iodine cycle was first characterized in terms of stocks by measuring its concentrations in: tree, litterfall, humus, soil, rainfall, throughfall, stemflow and soil solutions. Main annual fluxes (requirement, uptake and internal transfers) and forest input-output budget were also estimated using conceptual model calculations. Our findings show that: (i) soil is the main I reservoir accounting for about 99.9% of ecosystem total stock; (ii) iodine uptake by tree represents a minor fraction of the available pool in soil (<0.2%); (iii) iodine allocation between tree compartments involves low immobilization in wood and restricted location in the roots; (iv) translocation of excess iodine towards senescing foliage appears as an elimination process for trees, and (v) litterfall is a major pathway in the I biological cycling. In our soil conditions, the input – output budget shows that the ecosystem behaves as a potential source of I for groundwater.