1 Swiss Fed. Inst. for Forest, Snow and Landscape Res. (WSL), CH-8903 Birmensdorf
2 DISAFA, NatRisk-LNSA, Univerity of Turin, IT-10095 Grugliasco, Torino
3 Swiss Fed. Inst. for Forest, Snow and Landscape Res. (WSL), CH-8903 Birmensdorf (present address: Kaden & Partner AG, CH-8500 Frauenfeld)
Increased nitrogen (N) deposition rates were simulated for 20 years (1995-2015) by sprinkling rain water enriched with NH4NO3 (+22 kg ha-1 y-1 N) in a small headwater catchment within a spruce (Picea abies) forest at Alptal (central Switzerland). The added N was labelled with 15NH415NO3 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 15N-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 to 10 kg ha-1 y-1 compared with 1.4 to 2.9 kg ha-1 y-1 in the control catchment. In each catchment, half of the large trees were girdled during the 15th year and felled one 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 y-1). The increase was significantly smaller in the control (up to 4 kg ha-1 y-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 12.3 kg ha-1 y-1), in contrast, was unaffected by the treatment.
Keywords: forest ecosystem, nitrate leaching, nitrogen deposition, paired-catchment experiment, tree girdling, 15N isotope