In March 1986, four 2x2 m treeless plots were trenched all around in an evergreen oak stand in the Montseny mountains (NE Spain). Responses to perturbation were monitorised through: 1) net mineralisation and nitrification rates measured monthly in situ incubation in polyethylene bags (020 cm); 2) N03- concentration in soil solution. In control plots, 80 kg N/ha/year were mineralised and 9 per cent of them were nitrificated. In trenched plots 325 kg N/ha/year were mineralised and 73 per cent of them were nitrificated. The results obtained in control and trenched plots reveal the potential for very high nitrate losses following disturbance.