『Abstract
　Information about the impact of nitrogen (N) deposition on the fate of deposited N in peatland ecosystems is lacking. Thus we investigated the fate of experimentally added ¹⁵N in long-term N-fertilized treatments in a Sphagnum-dominated ombrotrophic bog. Fertilization significantly stimulated vascular plant and suppressed Sphagnum and Polytrichum moss growth. N content in peat, mosses, and vascular plants was raised by the fertilizer addition and reached a maximum at 3.2 g m^-2 N input level with phosphorus (P) and potassium (K) addition. Most of N was retained in the vegetation and upper 10 cm of the peat. When N deposition equalled 1.6 g m^-2 and less, or 3.2 g m^-2 N with P and K addition, no inorganic N leaching was observed on the plots. This result indicates that co-fertilization with P and K raised the N retention capacity and that critical N loads with respect to N saturation depend on P and K availability. Most of the deposited ¹⁵N was recovered in the bulk peat, which may be related to a rapid immobilization of inorganic N by microorganisms and mycorrhizal assimilation. Increase of N, P, and K fertilization increased the contribution of vascular plants to N retention significantly and reduced those of mosses. The increase was mainly related to enhanced productivity, vascular biomass and N content in tissues; the reduced retention by mosses resulted from both reduced moss biomass and assimilation. The study shows that the N filter function of ombrotrophic bogs will be influenced by interactions with other nutrients and shifts in plant community structure.

Keywords: Peatland; Bog; Nitrogen deposition; Nitrogen saturation; Nitrogen retention』

Introduction
Materials and methods
　Study sites
　Experimental design and field sampling
　Chemical analyses
　Calculations and statistics
Results
　Effects of fertilization on ecosystem structure and biomass
　Nitrogen contents and concentrations
　Retention of ¹⁵N
　¹⁵N balance and ¹⁵N retention efficiency
Discussion
　N retention by mosses
　N retention in peat
　N retention in vascular plants
　N saturation and importance of P and K
Conclusions
Acknowledgements
References