Plant andSoil 170: 315-322, 1995. (g) 1995 Kluwer Academic Publishers. Printed in the Netherlands. Radiocaesium in an organic soil and the effect of treatment with the fungicide 'Captan' C. A. Shand 1, M. V. Cheshire I , S. Smith I , C. D. Campbell 1, P. Anderson 2, C. M. Davidson 2, D. Littlejohn 2 and N. Jamieson 3'4 lMacaulay Land Use Research Institute, Craigiebuckler, Aberdeen, AB9 2QJ, 2Department of Pure and Applied Chemistry, University of Strathclyde, Glasgow, G1 1XL and 3Department of Plant and Soil Science, University of Aberdeen, AB9 2UE, UK. 4Present address: Department of Soil Science, University of Reading, RGI 5AQ, UK Received 27 May 1994; Accepted in revised form 9 September 1994 Key words: 'Captan', ergosterol, fungi, plant availability, organic soil, radiocaesium Abstract Soil fungi accumulate radiocaesium from contaminated soil and it has been hypothesised that this may alter the plant availability and movement of the radionuclide in soil. The effect of twice-monthly addition of an aqueous suspension of the fungicide 'Captan' on the changes in a peaty podzol soil at 2 sites, contaminated 2 or 3 years earlier by the injection of 134Cs, has been quantified. The sites had different soil acidity and vegetation cover. The less acid soil (pHwater 5.0) had been improved by the addition of lime and fertilizer and was reseeded with grass and clover. The more acid soil (pHwate~ 3.8) was under hill grasses, herbs and heather. On both sites the addition of fungicide did not alter the amount or concentration of radiocaesium in plant material sampled monthly or the depth distribution of radiocaesium in the soil profile. The concentration of the fungal constituent, ergosterol, in the soil, measured monthly, was unaffected by the fungicide treatment but evidence was obtained from a pot experiment to show that ergosterol decomposes slowly in cold, wet soils. On the more acid soil, two weeks after the last application of fungicide, there was a decline in active fungi as measured by fluorescein diacetate staining. Chloroform fumigation of the more acid soil resulted in a small increase in the amount of 134Cs exchangeable with 1 M ammonium acetate. Radiocaesium in seven different fungi grown in pure culture was found to be almost entirely extractable (> 95%) with 1 M ammonium acetate. Another, Amanita rubescens, showed some retention and 88% was extractable. These findings do not preclude the fungal biomass as an important soil component controlling plant availability of radiocaesium from acid, organic soils by maintaining radiocaesium in a biological cycle, but make it unlikely that any fixation by fungi in a chemical sense is involved. Introduction Soil fungi accumulate caesium and may be responsible for the retention and altered plant availability of radio- caesium in acid organic soils. It has been estimated that fungi in upland grassland soil may act as a sink for all (Dighton et al., 1991) or part (Bakken and Olsen, 1990) of the radiocaesium deposited from the Cher- nobyl accident. Direct determination of the association of radionuclides with different soil phases is general- ly not possible but can be assessed using chemical extractions. Using this approach, Shand et al. (1994) have studied the distribution of 134Cs in contaminated organic soils but the behaviour of radiocaesium bound to soil fungi in such schemes has not been quantified. In soil, fungi largely exist as microscopic hyphae inti- mately associated with other soil components. Several methods, reviewed by Dighton and Kooistra (1993) and Newell (1992), can be used to estimate their mass and activity. Living fungi contain ergosterol and the amount in soil can give a measure of fungal activi- ty. The relative numbers of active and inactive fungal hyphae can also be measured by staining with fluores- cein diacetate (FDA). To determine the influence of soil fungi on the movement and plant uptake of radiocaesium in an