『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