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

This study examines the role of tree canopies in processing atmospheric nitrogen (Ndₑₚ) for four forests in the United Kingdom subjected to different Ndₑₚ: Scots pine and beech stands under high Ndₑₚ (HN, 13–19 kg N ha⁻¹ yr⁻¹), compared to Scots pine and beech stands under low Ndₑₚ (LN, 9 kg N ha⁻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 δ¹⁸O, Δ¹⁷O and δ¹⁵N in NO₃ ⁻ and δ¹⁵N 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 δ¹⁵N‐NO₃ ⁻ and δ¹⁸O in RF suggested similar source of atmospheric NO₃ ⁻ (i.e. local traffic), while more positive values for δ¹⁵N‐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, ¹⁵N‐enriched NH₄ ⁺ in TF compared to RF at all sites suggested that canopies played an important role in buffering dry Ndₑₚ also at the low Ndₑₚ site. Using two independent methods, based on δ¹⁸O and Δ¹⁷O, 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 Δ¹⁷O, 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 Ndₑₚ within canopies should not be neglected and needs further exploration, with the combination of multiple isotope tracers, with particular reference to Δ¹⁷O.