Isotopic evidence for the occurrence of biological nitrification and nitrogen deposition processing in forest canopies

Rossella Guerrieri, Elena I. Vanguelova, Greg Michalski, Timothy H. E. Heaton, Maurizio Mencuccini

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This study examines the role of tree canopies in processing atmospheric nitrogen (N-dep) for four forests in the United Kingdom subjected to different N-dep: Scots pine and beech stands under high N-dep (HN, 13-19kgNha(-1)yr(-1)), compared to Scots pine and beech stands under low N-dep (LN, 9kgNha(-1)yr(-1)). Changes of NO3-N and NH4-N concentrations in rainfall (RF) and throughfall (TF) together with a quadruple isotope approach, which combines delta O-18, Delta O-17 and delta N-15 in NO3- and delta N-15 in NH4+, were used to assess N transformations by the canopies. Generally, HN sites showed higher NH4-N and NO3-N concentrations in RF compared to the LN sites. Similar values of delta N-15-NO3- and delta O-18 in RF suggested similar source of atmospheric NO3- (i.e. local traffic), while more positive values for delta N-15-NH4+ at HN compared to LN likely reflected the contribution of dry NHx deposition from intensive local farming. The isotopic signatures of the N-forms changed after interacting with tree canopies. Indeed, delta N-15-enriched NH4+ in TF compared to RF at all sites suggested that canopies played an important role in buffering dry N-dep also at the low N-dep site. Using two independent methods, based on delta O-18 and Delta O-17, we quantified for the first time the proportion of NO3- in TF, which derived from nitrification occurring in tree canopies at the HN site. Specifically, for Scots pine, all the considered isotope approaches detected biological nitrification. By contrast for the beech, only using the mixing model with Delta O-17, we were able to depict the occurrence of nitrification within canopies. Our study suggests that tree canopies play an active role in the N cycling within forest ecosystems. Processing of N-dep within canopies should not be neglected and needs further exploration, with the combination of multiple isotope tracers, with particular reference to Delta O-17.

Original languageEnglish
Pages (from-to)4613-4626
Number of pages14
JournalGlobal Change Biology
Issue number12
Early online date3 Jul 2015
Publication statusPublished - Dec 2015


  • delta N-15
  • delta O-18
  • Delta O-17
  • Beech
  • canopy nitrification
  • dissolved organic nitrogen
  • forest canopy interception
  • NH4-N
  • nitrogen deposition
  • NO3-N
  • Scots pine
  • N-15/N-14 RATIOS

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