17 research outputs found
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, 13â19 kg N haâ1 yrâ1), compared to Scots pine and beech stands under low Ndep (LN, 9 kg N haâ1 yrâ1). Changes of NO3-N and NH4-N concentrations in rainfall (RF) and throughfall (TF) together with a quadruple isotope approach, which combines ÎŽ18O, Î17O and ÎŽ15N in NO3â and ÎŽ15N 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 ÎŽ15N-NO3â and ÎŽ18O in RF suggested similar source of atmospheric NO3â (i.e. local traffic), while more positive values for ÎŽ15N-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 ÎŽ18O and Î17O, we quantified for the first time the proportion of NO3â 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 Î17O, 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 Î17O
InterCarb: A Community Effort to Improve Interlaboratory Standardization of the Carbonate Clumped Isotope Thermometer Using Carbonate Standards
International audienceIncreased use and improved methodology of carbonate clumped isotope thermometry has greatly enhanced our ability to interrogate a suite of Earth-system processes. However, interlaboratory discrepancies in quantifying carbonate clumped isotope (Î47) measurements persist, and their specific sources remain unclear. To address interlaboratory differences, we first provide consensus values from the clumped isotope community for four carbonate standards relative to heated and equilibrated gases with 1,819 individual analyses from 10 laboratories. Then we analyzed the four carbonate standards along with three additional standards, spanning a broad range of ÎŽ47 and Î47 values, for a total of 5,329 analyses on 25 individual mass spectrometers from 22 different laboratories. Treating three of the materials as known standards and the other four as unknowns, we find that the use of carbonate reference materials is a robust method for standardization that yields interlaboratory discrepancies entirely consistent with intralaboratory analytical uncertainties. Carbonate reference materials, along with measurement and data processing practices described herein, provide the carbonate clumped isotope community with a robust approach to achieve interlaboratory agreement as we continue to use and improve this powerful geochemical tool. We propose that carbonate clumped isotope data normalized to the carbonate reference materials described in this publication should be reported as Î47 (I-CDES) values for Intercarb-Carbon Dioxide Equilibrium Scale