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 abovecanopy
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 Nfertilizer
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.