Increased nitrogen (N) deposition rates were simulated for 20 years by sprinkling rain water enriched with NH 4 NO 3 (?22 kg ha -1 year -1 N) in a small headwater catchment within a spruce (Picea abies) forest at Alptal (central Switzerland). The added N was labelled with 15 NH 4 15 NO 3 during the first year. A control catchment was labelled in years 6 and 8. NO 3 leaching doubled in the N-addition catchment during the first year, resulting almost entirely from loss of 15 Nlabelled NO 3 -. The proportion of added N leached as NO 3 --N reached 1/3 after 5 years, then remained stable for the next 10 years, at 6.6-10 kg ha -1 year -1 compared with 1.4-2.9 kg ha -1 year -1 in the control catchment. In each catchment, half of the large trees were girdled during the 15th year and felled 1 year later to assess their role in soil N retention. Girdling and felling resulted in a strong increase in NO 3 --N leaching from the N-addition catchment (up to 19 kg ha -1 year -1 ). The increase was significantly smaller in the control (up to 4 kg ha -1 year -1 ). NO 3 leached after tree girdling contained only 0.18% of the originally applied tracer in the control and none detectable in the N-addition catchment. The increased leaching was thus mainly from new N and due to reduced N immobilisation by trees rather than to increased N mineralisation and nitrification in the soil. Dissolved organic N leaching (average 6.1 kg ha -1 year -1 ), in contrast, was unaffected by the treatment.

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