Data from: Forest nitrogen dynamics in response to increasing nitrogen deposition: comparing above-canopy and soil fertilizations in a mature beech forest

Nitrogen (N) fertilization experiments provide critical insights into how
increasing N deposition alters the balance between N retention and
saturation in forest ecosystems. However, most studies have considered
soil N applications, overlooking tree canopy-atmosphere interactions,
leading to an incomplete understanding of the fate of N in forests. We
investigated ecosystem N dynamics four years after the establishment of a
nitrogen manipulation experiment in a beech forest, involving Control
(N0), 30 kg N ha^-1^ y^-1^, distributed to soil (N30) and above tree
canopies (N30A), and 60 kg N ha^-1^ y^-1^ applied to the soil (N60). We
assessed N concentration and δ15N across forest compartments (leaves, fine
roots, ectomycorrhizal root tips, soil, and litter) and quantified
microbial functional genes related to soil N processes. N concentrations
were minimally affected by treatments, whereas δ15N increased along
compartments, particularly in the N60, indicating enhanced soil N losses.
Both N30 and N60 increased N concentrations and d15N values in
ectomycorrhizal root tips and soil, suggesting enhanced fungal N
immobilization but limited transfer to the host plants. In contrast, N30A
led to 15N depletion in fine roots and litter, reflecting stronger plant
reliance on ectomycorrhizal activity and potential alterations in litter
quality, which may inhibit decomposition. Soil nitrifiers and denitrifiers
were abundant, regardless of the treatments. Our findings highlight the
need for future experiments to simulate realistic N deposition scenarios,
including canopy interactions, to better understand ecosystem N dynamics
and forest responses under global change.