Forest soils show a great degree of temporal and spatial variation of nitrogen mineralization. The aim of the present study was to explain temporal variation in nitrate leaching from a nitrogen-saturated coniferous forest soil by potential nitrification, mineralization rates and nitrate uptake by roots. Variation in nitrate production in time and space, between the different organic horizons, has been related to temperature, moisture content, substrate availability and pH. Temporal variation in concentrations of nitrate and ammonium in the forest floor was significant during a one-year cycle, when randomly taken samples were pooled. Nitrogen concentrations differed between the different organic horizons with highest concentrations found in the litter layer, decreasing with increasing depth. Ammonium concentrations always exceeded nitrate concentrations by a factor ten, indicating that ammonium was not limiting nitrification. Nitrification potential, the nitrate production at field moisture at 25°C, was highest in the litter layer, lower in the fragmentation layer and hardly measurable in the mineral soil. Uptake of nitrate by roots and changes in mineralization rates turned out to be unimportant to explain variation in time, as seasonal fluctuations seem to be less important than spatial variation. We found that horizontal spatial variation in potential nitrate production, leaching of nitrate and nitrogen concentrations from non-pooled field samples was higher than variation in time. All this reflects the actual spatial variation in the field, which is not explained by differences in moisture content or temperature. Overall neither pH nor substrate availability could explain this observed variation, however, local variation in microsites may be responsible for small-scale spatial variation. Allelopathic compounds and/or the composition of the microbial community are suggested as factors possibly affecting nitrate production. (C) 2000 Elsevier Science Ltd.