『Abstract
　Despite long-term enhanced nitrogen (N) inputs, forests can retain considerable amounts of n. While rates of N input via throughfall and N leaching are increased in coniferous stands relative to deciduous stands at comparable sites, N leaching below coniferous stands is disproportionally enhanced relative to the N input. A better understanding of factors affecting N retention is needed to assess the impact of changing N deposition on N cycling and N loss of forests. Therefore, gross N transformation pathways were quantified in undisturbed well-drained sandy soils of adjacent equal-aged deciduous (pedunculate oak (Quercus robur L.)) and coniferous (Scots pine (Pinus sylvestris L.)) planted forest stands located in a region with high N deposition (north Belgium). In situ inorganic ¹⁵N labelling of the mineral topsoil (0-10 cm) combined with numerical data analysis demonstrated that (i) all gross N transformations differed significantly (p＜0.05) between the two forest soils, (ii) gross N mineralization in the pine soil was less than half the rate in the oak soil, (iii) meaningful N immobilization was only observed for ammonium, (iv) nitrate production via oxidation of organic N occurred three times faster in the pine soil while ammonium oxidation was similar in both soils, and (v) dissimilatory nitrate reduction to ammonium was detected in both soils but was higher in the oak soil. We conclude that the higher gross nitrification (including oxidation of organic N) in the pine soil compared to the oak soil, combined with negligible nitrate immobilization, is in line with the observed higher nitrate leaching under the pine forest.

Keywords: Forest type; Mineralization; Nitrification; ¹⁵N; Nutrient cycling; Tracing model』

Introduction
Materials and methods
　Study site
　Plot installation and N addition
　Soil sampling and analysis
　Calculations, data analysis, and ¹⁵N tracing model
Results
　Nitrogen pool sizes and ¹⁵N enrichments
　Soil and root ¹⁵N recovery and model fit
　Gross nitrogen transformation rates
Discussion
　Production and consumption of ammonium
　Production and consumption of nitrate
　Forest type as driver of N dynamics
Acknowledgments
References