Sirvi Autor "Sepp, Holar" järgi

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15N tracers and microbial analyses reveal in situ N2O sources in contrasting water regimes of a drained peatland forest
(2024) Masta, Mohit; Espenberg, Mikk; Kuusemets, Laura; Pärn, Jaan; Thayamkottu, Sandeep; Sepp, Holar; Kirsimäe, Kalle; Sgouridis, Fotis; Kasak, Kuno; Soosaar, Kaido; Mander, Ülo
Managed peatlands are a significant source of nitrous oxide (N2O), a powerful greenhouse gas and stratospheric ozone depleter. Due to the complexity and diversity of microbial N2O processes, different methods such as tracer, isotopomer, and microbiological technologies are required to understand these processes. The combined application of different methods helps to precisely estimate these processes, which is crucial for the future management of drained peatlands, and to mitigate soil degradation and negative atmospheric impact. In this study, we investigated N2O sources by combining tracer, isotopomer, and microbial analysis in a drained peatland forest under flooded and drained treatments. On average, the nitrification genes showed higher abundances in the drained treatment, and the denitrification genes showed higher abundances in the flooded treatment. This is consistent with the underlying chemistry, as nitrification requires oxygen while denitrification is anaerobic. We observed significant differences in labelled N2O fluxes between the drained and flooded treatments. The emissions of N2O from the flooded treatment were nearly negligible, whereas the N2O evolved from the nitrogen-15 (15N)-labelled ammonium (15NH4+) in the drained treatment peaked at 147 μg 15N m-2 h-1. This initially suggested nitrification as the driving mechanism behind N2O fluxes in drained peatlands, but based on the genetic data, isotopic analysis, and N2O mass enrichment, we conclude that hybrid N2O formation involving ammonia oxidation was the main source of N2O emissions in the drained treatment. Based on the 15N-labelled nitrate (15NO3-) tracer addition and gene copy numbers, the low N2O emissions in the flooded treatment came possibly from complete denitrification producing inert dinitrogen. At atomic level, we observed selective enrichment of mass 45 of N2O molecule under 15NH4+ amendment in the drained treatment and enrichment of both masses 45 and 46 under 15NO3- amendment in the flooded treatment. The selective enrichment of mass 45 in the drained treatment indicated the presence of hybrid N2O formation, which was also supported by the high abundances of archaeal genes.
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Holotseeni paleokeskkonna muutused Loode-Eestis järvesetete stabiilsete isotoopide ja jälgelementide põhjal Turvaste Valgejärve läbilõikest
(Tartu Ülikool, 2013) Sepp, Holar; Sohar, Kadri, juhendaja; Laumets, Liina, juhendaja; Tartu Ülikool. Loodus- ja tehnoloogiateaduskond; Tartu Ülikool. Geoloogia osakond
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Isotopic stability of mass reference materials and possible use as isotopic reference materials
(Tartu Ülikool, 2020) Gonzales Ferraz, Margarita Esmeralda; Kirsimäe, Kalle; Sepp, Holar; Tartu Ülikool. Loodus- ja täppisteaduste valdkond
Natural isotope variation or fractionation depends on equilibrium and kinetic processes affecting the individual isotope. The purpose of the of this work is to determine the stability and reproducibility of historical data of δ(15N/14N) abbreviated as δ15N, and the δ(13C/12C), abbreviated as δ13C, of total N and C in three solid samples (Aspartic acid, Nicotinamide and Acetanilide). These three samples contain N and C. The primary reference material for relative N isotope-ratio measurements (δ15N) used were atmospheric nitrogen gas (N2), which is widespread and homogeneous and by convention, has a δ15N consensus value of 0‰. The primary reference material for relative C isotope-ratio measurements (δ13C) is the L-SVEC lithium carbonate which have consensus value of -46.6±0‰ on the Vienna Peedee Belemnite (VPDB scale. The secondary references used were IAEAN1 (+0.4±0.2‰), IAEAN2 (+20.3±0.2‰) for δ15N and IAEACH3 (-24.724±0.041‰), IAEACH6 (-10.449±0.033‰) for δ13C. The δ15N and δ13C data were provided for the Laboratory of Isotope Ration Mass Spectrometry (IRMS) of the Department of Geology University of Tartu. The δ15N and δ13C measurements were made with a Delta V Plus CF-IRMS, which alternately measures the isotope-amount ratios of the sample N2 and CO2 gases and one or more injections of the working reference N2 and CO2 gases. From the data and results obtained for uncertainties for the data provided for the 2014-2020, we obtained the following results: for Aspartic acid δ15N (-7.0±1.6 ‰), δ13C (2.0±28.6‰), for Nicotinamide δ15N (-2.0±1.0 ‰), δ13C (-34.5±0.7‰) and for Acetanilide δ15N (1.1±1.1 ‰), δ13C ( -27.0±0.9‰). These uncertainties probably could be due to fractionation process happening during sample preparation and transformation to gas before entering the IRMS. From the analysis of the stability we can conclude that the Aspartic acid (+35.1774 to -31.2543), Nicotinamide (-34.1254 to-34.7113) and Acetanilide (-26.5022 to -27.2740) are more stable for δ13C, but unstable for δ15N during, during 2014-2020. Lastly there is a good long-term reproducibility (SRW) only for Acetanilide and Nicotinamide with % RSD pooled of 1% for δ13C.