Jinuntuya, M. 2007. Isotopologue fractionation during microbial reduction of N2O in soil. MS Thesis, Michigan State University, East Lansing, Michigan, USA.
Reduction of N2O is a challenge to studies using isotope values to resolve global budgets and microbial sources of this critical greenhouse gas. Prior research has demonstrated that the difference in δ 15N between the central (α) and outer (β) N atoms in the N2O can be used to distinguish N2O derived from nitrification and denitrification (Sutka et al., 2003; 2006; Toyoda et al., 2005). If intramolecular distribution of 15N, however, is altered during reduction, apportionment of N2O to nitrification and denitrification will be inaccurate. Isotopologue analyses of N2O within soil mesocosm experiments were used to investigate fractionation during N2O reduction at four levels of water filled pore space (WFPS) 60, 80, 100% (saturation) and 10% in excess. Soils were obtained from the Kellogg Biological Station Long Term Ecological Research Site (Michigan). Isotopic enrichment factors (ϵ) for δ15N, δ18O, δ 15Nα and δ15Nβ ranged from -4.2 to -9.0, -12.5 to -23.6, -6.4 to -10.0 and -2.0 to -7.9, respectively. With the exception of site preference (SP), lower fractionation factors were observed at higher WFPS demonstrating the importance of diffusion in limiting the expression of enzymatic fractionation. Isotopic discrimination in SP was small and the ϵ values varied between -4.5 and 0‰. Strong correlations were evident between δ18O and δ 15N and δ18O and δ15N α, with slopes of 2.7 and 2.0, respectively. These relationships (1) provide a definitive means for establishing that isotope effects during reduction are present and (2) may provide a means to determine the source signatures even when reduction occurs.
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