Abstract
Human voracity for energy and global food production are the two main processes capable of breaking down the triple-bond of diatomic nitrogen (N2), generating reactive forms of this element (Nr). The accumulation of anthropogenically enhanced Nr input to forest ecosystems could relief their N limitation, thus boosting net primary productivity and carbon (C) capture. Yet, in recent decades a growing body of research shed a light onto the possible detrimental effect of forest N saturation, when growing Nr quantities satisfy both biotic and abiotic sinks and additional Nr molecules trigger a cascade of ecological consequences. Several manipulative experiments showed that, when artificially high doses of N fertilizer are supplied directly to the forest floor, soil pH decreases, toxic Al3+ can solubilize in soil solution, nutrient imbalances occur, nitrate leaches to both surface and groundwater enhancing eutrophication, greenhouse gas emissions augments, and an overall forest decline can eventually occur. This highly nonlinear dose-response process is strongly dependent on the fate of atmospheric deposited N in the ecosystem, which must pass through the forest canopy layer before reaching the ground. Past tracer studies show that litter and soils dominate the short-term fate of added 15N when the fertilizer is applied directly to the forest floor, yet few have examined the role of the canopy in intercepting, transforming, and assimilating atmospheric N deposition before they reach these sinks. This thesis aims at comparing the fate of N between a classic to-the-ground fertilization approach and the innovative canopy-added N, in two representative forest types of Italian Alps’ Mountain environment: an almost pure European beech forest and a Sessile Oak stand. The goal of the study is to highlight how the fate of N can be modify by the canopy layer, thus investigating if the canopy-fertilization approach better simulates future scenarios of increased N deposition. This is a policy-relevant matter when it comes to evaluate the forest ecosystem sequestration of pollution-derived N, calculate critical loads, and put in place management strategy. The thesis first offers a state-of-the-art overview on the canopy role in the forest N cycling, acknowledging how interference of the canopy may be significant; however, the experimental studies including the canopy are scarce. The thesis later explores a pulse labeling performed in the Sessile oak forest and the fate of N at different timing, highlighting a canopy interception of 12 % of total delivered N. However, long-living tissues recovery in the NAB after 7 months is not higher than in the NBL, thus the canopy added N seems not to affect storing capacity. A third section investigating the same hypothesis in a European beech forest, shows that N retention in canopy could be up to a third of total simulated wet N deposition, however this study is still a short-term experiment, and storage in woody tissues doesn’t seems to be affected. We conclude that canopy interception of atmospheric N deposition doesn’t affect the plant recovery, however a follow-up is needed to trace remobilization of stocks in organs that typically reacts on longer timespans (eg. years to decades).