Nitrate leaching from a sub-alpine coniferous forest subjected to experimentally increased N deposition for 20 years, and effects of tree girdling and felling

Increased nitrogen (N) deposition rates were simulated for 20 years (1995-2015) by sprinkling rain water enriched with NH4NO3 (+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 (NH415)-N-15 NO3 during the first year. A control catchment was labelled in years 6 and 8. NO3- leaching doubled in the N-addition catchment during the first year, resulting almost entirely from loss of N-15-labelled NO3 (-). The proportion of added N leached as NO3--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 NO3--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)). NO3- 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.