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 4 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 δ¹⁵N values in ectomycorrhizal root tips (ERT) 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.