The effect of climate change on biologically mediated soil processes is likely to be complex and difficult to predict. The direct effect of changes in soil moisture and temperature may be obscured or even reversed by changes in the return of organic substrates to the soil. This paper is the first in a series reporting the results from a medium‐term investigation into the effect of simulated climate change on soil N mineralization in a seminatural calcareous grassland in southern England. Gross rates of N mineralization were determined by isotope dilution in plots subjected to winter warming (3 °C above ambient), enhanced or decreased summer rainfall and combinations of these treatments. The results from the control treatment, reported here, show a strong seasonality of gross N mineralization with rates highest in the Spring and Autumn and lowest in Summer. They indicate that water availability is the main restraint on microbial processes and plant growth and that, in the short term, enhanced summer rainfall is likely to be the main factor influencing nutrient turnover. Further papers will report the results from the imposed treatments.

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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 Cs-134, has been quantified. The sites had different soil acidity and vegetation cover. The less acid soil (pH(water) 5.0) had been improved by the addition of lime and fertilizer and was reseeded with grass and clover. The more acid soil (pH(water) 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 Cs-134 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