Radioprotection - Collogues, volume 37, CI (2002) C1-569 Effect of fertilisation and drainage intensity on the foliar radiocaesium and potassium concentrations of Scots pine on a drained peatland S. Kaunisto, L. Aro and A. Rantavaara 1 Finnish Forest Research Institute, METLA, Pari<ano Research Station, 39700 Pankano, Finland 1 Radiation and Nuclear Safety Authority, STUK, P.O. Box 14, 00881 Helsinki, Finland Abstract. This study aims at finding out the effects of fertilisation and drainage intensity on the needle IM Cs and IJ7 Cs activity in Pinus sylvestris on a pine mire in Finland. The fertilisation treatments were as follows: a control, fertilised once in 1965 or fertilised twice in 1965 and 1977, with N, P and K. The potassium concentrations of the current year needles in 1990 showed severe potassium deficiency on the controls (K 3.6 mg g" 1 ) and slight deficiency on the plots fertilised only once (K 4.2 mg g ' ) but no deficiency on the plots fertilised twice (K 4.8 mg g" 1 ). Fertilisation decreased the radiocaesium concentrations and amounts in the needles. The concentrations of 1M Cs were 821 in the control, 442 in the single fertilisation and 485 Bq kg"' in the repeated fertilisation treatment, and the l57 Cs concentrations 5378, 2734 and 2104 Bq kg" 1 , respectively. Drainage intensity (10-100 m) had no significant effect on the needle K concentration or radiocaesium activity. 1. INTRODUCTION Peatlands suitable for growing forests have adequate nitrogen status but often only limited amounts of mineral nutrients in Finnish conditions. Potassium is often a growth-limiting nutrient especially on originally wet and sparsely stocked peatlands [1]. Tree roots penetrate seldom deeper than 20 cm even into drained peat soils [2]. This, 0-20 cm, peat layer may contain as much as [3,4] but often less potassium than a tree stand of 150 m 3 [5]. Some investigations have shown that potassium deficiency enhances 137 Cs uptake by trees and that potassium fertilisation decreases it in adequate drainage conditions used also in forestry practice [6,7]. It is not known if this is true also in more intensive or less intensive drainage conditions. This study aims at finding out the effect of potassium fertilisation on the needle l37 Cs and potassium concentrations of Scots pine on a pine mire in a wider range of drainage conditions than used in forestry practice. 2. MATERIAL AND METHODS 2.1 Site and treatments The material was collected from a ditch spacing and fertilisation experiment in western Finland (61°59'N, 23°15'E; [8,9]). The experiment was established in the middle of the 1950's. The peatland site type was originally a low/tall sedge pine mire, presently stocked by uneven-aged, 50-80-year-old Scots pines and downy birches. Scots pine was the dominating tree species. Nitrogen concentrations in peat varied between 1.7 and 2.3%, phosphorus concentrations between 863 and 1281 mg kg" 1 , and potassium concentrations between 144 and 451 mg kg" 1 in the 0 andlO cm surface peat layer. The lowest potassium concentrations were in the unfertilised treatments. The peat layer was more than one meter thick in the whole area. Partly the same fertilisation treatments as in the study by Kaunisto et al. [7] but instead of three six ditch spacing treatments were selected for this study. The fertilisation treatments were as follows: a control, single fertilisation with PK or NPK in 1965, and fertilisation with PK or NPK in 1965 and with NPK in 1977. In 1965 36 kg of P and 69 kg of K or 50 kg of N, 26 kg of P and 25 kg of K ha" 1 were given as broadcast fertilisation. In 1977 the amounts of elements were 110 kg of N, 42 kg of P and 79 kg ha" 1 of K. The total phosphorus and potassium application rates were low in 1965 compared with the present practice (about 45 kg ha 1 of P and 80 kg ha" 1 of K). Potassium was given as KG. The ditch spacing treatments were 10, 20, 40, 60, 80 and 100 m.