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Isotopic evidence for the occurrence of biological nitrification and nitrogen deposition processing in forest canopies
Author(s) -
Guerrieri Rossella,
Vanguelova Elena I.,
Michalski Greg,
Heaton Timothy H. E.,
Mencuccini Maurizio
Publication year - 2015
Publication title -
global change biology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 4.146
H-Index - 255
eISSN - 1365-2486
pISSN - 1354-1013
DOI - 10.1111/gcb.13018
Subject(s) - beech , throughfall , scots pine , nitrification , nitrogen , deposition (geology) , environmental chemistry , stable isotope ratio , canopy , chemistry , environmental science , botany , biology , pinus <genus> , physics , paleontology , organic chemistry , sediment , quantum mechanics
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, 13–19 kg N ha −1 yr −1 ), compared to Scots pine and beech stands under low N dep (LN, 9 kg N ha −1 yr −1 ). Changes of NO 3 ‐N and NH 4 ‐N concentrations in rainfall ( RF ) and throughfall ( TF ) together with a quadruple isotope approach, which combines δ 18 O, Δ 17 O and δ 15 N in NO 3 − and δ 15 N in NH 4 + , were used to assess N transformations by the canopies. Generally, HN sites showed higher NH 4 ‐N and NO 3 ‐N concentrations in RF compared to the LN sites. Similar values of δ 15 N‐NO 3 − and δ 18 O in RF suggested similar source of atmospheric NO 3 − (i.e. local traffic), while more positive values for δ 15 N‐ NH 4 + at HN compared to LN likely reflected the contribution of dry NH x deposition from intensive local farming. The isotopic signatures of the N‐forms changed after interacting with tree canopies. Indeed, 15 N‐enriched NH 4 + 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 δ 18 O and Δ 17 O, we quantified for the first time the proportion of NO 3 − 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 Δ 17 O, 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 Δ 17 O.