Guerrieri R, Mencuccini M, Sheppard LJ, Saurer M, Perks MP, Levy P, Sutton MA, Borghetti M, Grace J
Contatto: Rossella Guerrieri (email@example.com)
Abstract: Forests store about 25% of the CO2 emitted by land use change and fossil-fuels combustion, thus contributing to the mitigation of climate change. However, the forest C-sink may be altered by the increase in nitrogen deposition (Ndep), which represents the return of the huge amount of reactive N emitted by industrial and agricultural activities from the atmosphere to the biosphere. A beneficial role of Ndep on the C uptake by N-limited forests in temperate and boreal regions has been hypothesized. How much of the forest C-sink is explained by Ndep has been hotly debated. One of the main unresolved issues is whether Ndep is transformed and absorbed by tree canopies, a process excluded in previous experiments. Our study aimed to evaluate the effects of long-term, high frequency, aerial N and sulphur (S) misting over tree canopies of a Sitka spruce plantation in Scotland (UK), where ambient Ndep was relatively low (8-10 kg N ha-1 yr-1). The effect of different treatments on the ratio between A and gs, i.e., the intrinsic water-use efficiency (WUEi), was assessed through d13C in tree rings. Further, combining d13C and d18O enabled us to suggest which physiological traits, i.e., A and/or gs contributed most to the variations in WUEi. In addition, d15N provided indications of changes in N cycling in response to the different treatments. Finally we compared our results with those reported in previous studies, where no N was added to the soil as either ammonium nitrate or urea. In our experiment, the addition of N, independent of the presence of S, increased the WUEi, by enhancing CO2 assimilation, thus supporting the fertiliser effect of N. Based on our WUEi values, trees largely returned to pre-treatment values within 6 years after the cessation of N treatment. However, this was not the case for 2NS treatment. In fact, S excess had a longer lasting detrimental effect on WUEi, by reducing A, even six years after the spray treatment stopped. This suggests that the beneficial role of Ndep could be down regulated by the negative effect of S, which may still be operating in forests that previously received high loads of acid rain. Spraying N over tree canopies led to a significant reduction of Ã¯ÂÂ¤15N in tree rings compared to the NS treatments and the wet control, which is in contrast to observations from soil fertilization experiments. In fact, soil applications contributed to increasing the opening of N cycles, i.e., losses of 15N-depleted compounds, through denitrification, NO3- leaching or NH3 volatilization, the prevailing effect of which was to enrich Ã¯ÂÂ¤15N signal of tree pools compared to the control. In our experiment, the variations of Ã¯ÂÂ¤15N in tree rings carried the effect of different canopy N dynamics, rather than changes in the soil processes. Indeed, the substantial level of canopy N retention (up to 60% in the high-N treatment) would have restricted the amount of N washed through to the soil, limiting any soil N accumulation or loss. Hence investigating canopy N processes and the way they link C and N cycles in forests, under different level of Ndep and climate conditions, is the crucial step forward towards a more comprehensive understanding of the indirect human-induced C-sinks.
Parole chiave: Annual Tree Rings, Atmospheric Deposition, Stable Isotopes, Water Use Efficiency
Citazione: Guerrieri R, Mencuccini M, Sheppard LJ, Saurer M, Perks MP, Levy P, Sutton MA, Borghetti M, Grace J (2011). Impacts of nitrogen deposition on C and N cycling in forests: the case study of manipulation of canopy deposition at Deepsyke, Scotland . 8° Congresso Nazionale SISEF, Rende (CS), 04 – 07 Ott 2011, Contributo no. #c8.11.1