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Atmospheric nitrogen (N) depositions have increased sharply since the industrial revolution. Numerous authors forecasted severe impacts on forest ecosystem services and functioning when new N input exceeds both biotic and abiotic holding capacity, in what is known as the N saturation process. However, most of the studies performed so far to quantify these impacts have largely neglected the potential role of the canopy and associated biota in biogeochemical processes. The present review collects and summarizes the existing literature about all the interactions of atmospheric N deposition and the canopy, evidencing different processes that may interfere with them. By summarizing the research of the last three decades, we found that a share of both wet and dry N deposition can be retained within the canopy, partially absorbed by the leaves or other aerial parts and utilized for plant metabolism, substantially changing the fate of N in the ecosystem. Focusing on canopy-associated biota, we analyzed how this compartment can represent a highly reactive layer and have an active role in different processes involved in the N cycle such as nitrification, fixation and volatilization. It emerges clearly that the canopy hosts a complex network of processes; however, despite this, most of the manipulative experiments failed to include the canopy into the studies. The documented manipulative experiments which included the canopy are mainly short-term and represent very few forest types. Moreover, we highlighted a lack of research concerning the interaction between the complex network of canopy processes, atmospheric N deposition and other global change factors, such as increasing atmospheric CO2 concentration, increasing temperature and drought events.

Abstract. Forests ability to store carbon is strongly connected with the amount of nitrogen (N) that forest ecosystems can retain; N is indeed considered the most limiting nutrient for terrestrial ecosystem's net primary productivity. Since the industrial revolution, human activities have more than doubled the rate of N input into the nitrogen cycle and this could alleviate N limitation thus stimulating plant growth. However, it has been suggested that when N availability exceeds biotic demand and abiotic sinks, additional N can trigger a negative cascade effect: nutrient imbalance, reduced productivity, increased losses of N, eutrophication and acidification of soil and water, leading toward forest decline and net greenhouse gases emissions. The consequences of increased N deposition on forest depend in large share on the fate of N in the ecosystem, which can be simulated and quantified by a fertilization at a known isotopic signature. Nevertheless, most of the tracer experiments performed so far added the fertilizer directly to the forest floor, neglecting the potential role of N uptake by the forest canopy. In the Italian Alps, we are conducting an experiment where both types of N additions (above and below the canopy layer) are performed in two different forest stands, to understand if canopy fertilization better simulates ecological consequences of increased atmospheric N deposition. These field-scale manipulation experiments are willing to test two different hypotheses: i) the N uptake by trees in the above-canopy N addition experimental sites is higher than under-canopy N addition ii) forest growth rate varies with the type of treatment. To describe the fate of the applied N, stable isotope techniques have been adopted: the forest sites, fertilized with NH4NO3 at a known isotopic signature, are sampled for all the ecosystem components (plant, soil and water) periodically to determine the total N content and its isotopic signature. The δ15N values permit to calculate the recovery of N-fertilizer in tree tissues, soil and leaching-water, allowing us to understand how N allocation varies under these two fertilization strategies and how this affects C sequestration potential. Results regarding the short-term effects over the first 6 years of data collection will be presented.

International audience; The mineral reserves of mountain environments have been exploited since the beginning of metallurgy and legacy contamination from activities such as mining persist to this day. This is particularly the case in the soils of the European mountains where potential harmful trace elements (such as Pb, Sb, As, and Hg) of anthropogenic origin have accumulated since Antiquity. The French Pyrenees are no exception to this, as many mine sites in the region date back to the Bronze Age, resulting in landscape alternations and anthropogenic environmental impacts on a millennial scale. The mountain critical zone is sensitive both to human‐induced environmental changes (e.g., agriculture, mining, clear‐cutting) as well as to climate‐induced rapid environmental fluctuations. The legacy of trace metal contamination in other environments has been documented at individual sites in Europe and around the world, however, the fate of such legacy metals over time, in particular within mountainous regions, is poorly understood. This is despite the fact that a large proportion of metals was deposited and stored before 1800 CE in these areas. Using a case study from the Central French Pyrenees as a specific example, we here show that legacy metal (e.g., Pb) contamination in mountain environments is still persistent and a potential threat to mountain ecosystem health. We emphasize methods that aim to understand, in an interdisciplinary and coordinated way, the fate of legacy metals in the Central Pyrenees and beyond. We highlight the importance of research in the mountain critical zone for the whole of Europe, as mountains are the source of water and provide regional economic and socio‐ecological resources. The goal of this chapter is, therefore, to draw attention to and provide fellow researchers with, the background information and methodologies needed to address the problem of legacy metal accumulation, transport, storage, remobilization, and redeposition in mountain watersheds, as well as potential subsequent environmental impacts downstream.