European gorse (Ulex europaeus L.) N-2 fixation rate (% Ndfa) was studied in a maritime pine (Pinus pinaster Ait.) oligotrophic forest. Fertilization field trials were carried out on 5 sites with various inputs of phosphorus (0-240 kg P2O5. ha(-1)). Seven to ten years after pine planting, gorse were sampled to evaluate the effect of P fertilization on gorse % Ndfa, determined using the N-15 natural abundance method. One of the prerequisites of this method is the existence of a significant difference between the N-15/N-14 ratios in the atmospheric N reference and in the stand soil N references. This prerequisite was satisfied for 80 of 120 cases. The average % Ndfa was high (70 +/- 3%) but with high local variability. No significant difference in % Ndfa was detected among P treatments. Nitrogen concentration of gorse was significantly higher in the highest dose treatments compared to the control.