Leaf microbes and nitrification in the canopies of European forests: evidence from stable isotopes, meta-barcoding and qPCR

Forest canopies influence our climate through carbon, water and energy exchanges with the atmosphere. However, less investigated is whether and how tree canopies change the chemical composition of precipitation, with important implications on forest nutrient cycling. Recently, we provided for the first time isotopic evidence that biological nitrification in tree canopies was responsible for significant changes in the amount of NO3 from rainfall to throughfall across two UK forests at high nitrogen (N) deposition. This finding strongly suggested that bacteria and/or Archaea species of the phyllosphere are responsible for transforming atmospheric N before it reaches the soil. Our study aims to 1) characterize microbial communities harboured in tree canopies for two of the most dominant species in Europe (Fagus sylvatica L. and Pinus sylvestris L.) using meta-barcoding techniques, 2) quantify the functional genes related to nitrification, and 3) estimate the relative contribution of NO3 derived from biological canopy nitrification vs. atmospheric deposition by using ?18O and ?17O of NO3 in forest water. We considered twelve sites included in the European ICP forests monitoring network, chosen along a climate and N deposition gradients, spanning from Fennoscandia to the Mediterranean. Preliminary results indicate differences in bacterial structure and composition across phyllosphere (and between the two species considered), water samples and soil. Moreover, presence of nitrifying bacteria and Archea (as assessed through qPCR) and their activity (as derived from ?18O and ?17O) were detected across all the samples, regardless the level of N deposition.