The accumulation of 137Cs in the biological compartment of forest soils

Soil samples were collected in various forest stands, located about 40 km north-west from Uppsala. The various stands were: (1) Clear cut area made in 1987, (2) Normal forest with 50–100 old Norway spruce and Scots pine and with a thick humic layer of about 10 cm; (3) Raised bog with 50-year-old Scots pine and Sphagnum moss layer over peat soil. (4) Rocky area with old Scots pine, growing on a shallow soil, mainly of organic origin. (5) Normal forest with nearly 100-year-old spruce and pine, growing a shallow humic layer over sandy soil. Soil blocks of about 20×20 cm and down to a depth of 10–15 cm were collected on each site. The soil samples were mechanically separated into various fractions: bulk, rhizosphere and soil–root interface. The results showed that 137Cs was unevenly distributed between the three soil fractions. The highest activity concentrations — 3–4 times higher than in the other two fractions — as well as the highest organic content — usually more than 95% — were found in the soil–root interface fraction. Of the total 137Cs activity in the soil, 18% as a mean value was found in the soil–root interface fraction. The results thus show that a substantial fraction of the 137Cs in the soils in some way associated with the biological part of the soil, probably with the fungal component.