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TECHNICAL REPORTS

Dynamics of Atmospheric Nitrogen Deposition in a Temperate Calcareous Forest Soil

^a Swiss Federal Inst. of Technology (EPFL), Lab. of Ecological Systems (ECOS), CH-1015 Lausanne, Switzerland
^b Paul Scherrer Inst. (PSI), Lab. of Atmospheric Chemistry, CH-5232 Villigen-PSI, Switzerland
^c Swiss Federal Inst. for Forest, Snow and Landscape Research (WSL), CH-8903 Birmensdorf, Switzerland

^* Corresponding author (patrick.schleppi{at}wsl.ch).

Received for publication October 25, 2007. In temperate forest ecosystems, soil acts as a major sink for atmospheric N deposition. A ¹⁵N labeling experiment in a hardwood forest on calcareous fluvisol was performed to study the processes involved. Low amounts of ammonium (¹⁵NH₄⁺) or nitrate (¹⁵NO₃^–) were added to small plots. Soil samples were taken after periods ranging from 1 h to 1 yr. After 1 d, the litter layer retained approximately 28% of the ¹⁵NH₄⁺ tracer and 19% of ¹⁵NO₃^–. The major fraction of deposited N went through the litter layer to reach the soil within the first hours following the tracer application. During the first day, a decrease in extractable ¹⁵N in the soil was observed (¹⁵NH₄⁺: 50 to 5%; ¹⁵NO₃^–: 60 to 12%). During the same time, the amount of microbial ¹⁵N remained almost constant and the ¹⁵N immobilized in the soil (i.e., total ¹⁵N recovered in the bulk soil minus extractable ¹⁵N minus microbial ¹⁵N) also decreased. Such results can therefore be understood as a net loss of ¹⁵N from the soil. Such N loss is probably explained by NO₃^– leaching, which is enhanced by the well-developed soil structure. We presume that the N immobilization mainly occurs as an incorporation of deposited N into the soil organic matter. One year after the ¹⁵N addition, recovery rates were similar and approximately three-quarters of the deposited N was recovered in the soil. We conclude that the processes relevant for the fate of atmospherically deposited N take place rapidly and that N recycling within the microbes–plants–soil organic matter (SOM) system prevents further losses in the long term.

Abbreviations: DON, dissolved organic nitrogen • ISN, immobilized soil nitrogen • SOM, soil organic matter