『Abstract
　In the last decades, in particular forest ecosystems became increasingly N saturated due to elevated atmospheric N deposition, resulting from anthropogenic N emission. This led to serious consequences for the environment such as N leaching to the groundwater. Recent efforts to reduce N emissions raise the question if, and over what timescale, ecosystems recover to previous conditions. In order to study the effects on N distribution and N transformation processes under the lowered N deposition treatment, we investigated the fate of deposited NH4⁺-¹⁵N in soil of a N-saturated Norway spruce forest (current N deposition: 34 kg ha^-1 year^-1; critical n load: 14 kg ha^-1 year^-1), where N deposition has been reduced to 11.5 kg ha^-1 year^-1 since 14.5 years. We traced the deposited ¹⁵N in needle litter, bulk soil, and amino acids, microbial biomass and inorganic N in soil. Under reduced N deposition, 123±23% of the deposited N was retained in bulk soil, while this was only 72±15% under ambient deposition. We presume that with reduced deposition the amount of deposited N was small enough to become completely immobilized in plant and soil and no leaching losses occurred. Trees receiving reduced N deposition showed a decline in N content as well as in ¹⁵N incorporation into needle litter, indicating reduced N plant uptake. In contrast, the distribution of ¹⁵N within the soil over active microbial biomass, microbial residues and inorganic N was not affected by the reduced N deposition. We conclude that the reduction in N deposition impacted only plant uptake and drainage losses, while microbial N transformation processes were not influenced. We assume changes in the biological n turnover to start with the onset of the decomposition of the new, N-depleted litter.

Keywords: Amino acids; microbial biomass; N deposition; ¹⁵N tracer; Solling roof project』

Abbreviations
Introduction
Materials and methods
　Site characterization
　Experimental design
　¹⁵N abundance and N content in different soil constituents
　　Ammonium and nitrate
　　Microbial biomass
　　Amino acids
　　Bulk soil and needle litter
　　¹⁵N content and recovery in soil constituents
　Statistics
Results
　Basic soil properties and total, inorganic and organic N
　Nitrogen in microbial biomass
　Nitrogen in amino acids
　¹⁵N recoveries of the added tracer throughout soil profiles under reduced and ambient deposition
　¹⁵N distribution under reduced and ambient N deposition
Discussion
　¹⁵N recovery and retention in the ecosystem
　¹⁵N distribution in relation to N deposition
　Nitrogen transformation processes
Conclusion
Acknowledgments
References