Canopy 15N fertilization increases short-term plant N retention compared to ground fertilization in an oak forest

The effects of atmospheric nitrogen (N) deposition on forests ecosystems depend on the fate of N in the ecosystem, and the role of tree canopy in intercepting, transforming, and assimilating atmospheric N inputs. In this study, increased in N deposition was simulated with the application of N fertilizer with two contrasting strategies: above and below the canopy, in a sessile oak (Quercus petraea (Matt.) Liebl.) stand. Three weeks after the labeled fertilization, more than twice as much fertilizer was recovered in plants in the above-canopy application (12 ± 5 %), than in the below-canopy application (5 ± 2 %). By contrast, the litter layer on the forest floor retained more fertilizer in the below-canopy (37 ± 8%) than in the above-canopy (7 ± 5 %) application. Seven months later, more fertilizer was recovered in wood and fine roots in below-canopy (8 ± 1 %) than in above-canopy (4 ± 1 %) fertilization. Wood recovery was higher in the below-canopy (6.7 ± 2.7 % vs 3.6 ± 0.6 % in the above-canopy). This experiment shows that fertilization approach alters the short-term fate of the added N in a broadleaved forest, highlighting the importance of the forest canopy in intercepting atmospheric N deposition also in these widely represented forests.