Hall Steven J, Weintraub Samantha R, Bowling David R
Global Change and Sustainability Center, University of Utah, Salt Lake City, UT, USA.
Department of Ecology, Evolution, and Organismal Biology, Iowa State University, 251 Bessey Hall, Ames, IA, 50011, USA.
Oecologia. 2016 Aug;181(4):1221-31. doi: 10.1007/s00442-016-3626-1. Epub 2016 Apr 21.
Natural abundance nitrate (NO3 (-)) isotopes represent a powerful tool for assessing denitrification, yet the scale and context dependence of relationships between isotopes and denitrification have received little attention, especially in surface soils. We measured the NO3 (-) isotope compositions in soil extractions and lysimeter water from a semi-arid meadow and lawn during snowmelt, along with the denitrification potential, bulk O2, and a proxy for anaerobic microsites. Denitrification potential varied by three orders of magnitude and the slope of δ(18)O/δ(15)N in soil-extracted NO3 (-) from all samples measured 1.04 ± 0.12 (R (2) = 0.64, p < 0.0001), consistent with fractionation from denitrification. However, δ(15)N of extracted NO3 (-) was often lower than bulk soil δ(15)N (by up to 24 ‰), indicative of fractionation during nitrification that was partially overprinted by denitrification. Mean NO3 (-) isotopes in lysimeter water differed from soil extractions by up to 19 ‰ in δ(18)O and 12 ‰ in δ(15)N, indicating distinct biogeochemical processing in relatively mobile water versus soil microsites. This implies that NO3 (-) isotopes in streams, which are predominantly fed by mobile water, do not fully reflect terrestrial soil N cycling. Relationships between potential denitrification and δ(15)N of extracted NO3 (-) showed a strong threshold effect culminating in a null relationship at high denitrification rates. Our observations of (1) competing fractionation from nitrification and denitrification in redox-heterogeneous surface soils, (2) large NO3 (-) isotopic differences between relatively immobile and mobile water pools, (3) and the spatial dependence of δ(18)O/δ(15)N relationships suggest caution in using NO3 (-) isotopes to infer site or watershed-scale patterns in denitrification.
天然丰度的硝酸盐（NO₃⁻）同位素是评估反硝化作用的有力工具，但同位素与反硝化作用之间关系的尺度和背景依赖性很少受到关注，尤其是在表层土壤中。我们测量了半干旱草甸和草坪在融雪期间土壤提取物和蒸渗仪水中的NO₃⁻同位素组成，以及反硝化潜力、大气O₂和厌氧微位点的替代指标。反硝化潜力变化了三个数量级，所有测量样品中土壤提取的NO₃⁻中δ¹⁸O/δ¹⁵N的斜率为1.04±0.12（R² = 0.64，p < 0.0001），与反硝化作用的分馏一致。然而，提取的NO₃⁻的δ¹⁵N通常低于土壤总δ¹⁵N（相差高达24‰），表明硝化过程中的分馏被反硝化作用部分覆盖。蒸渗仪水中的平均NO₃⁻同位素与土壤提取物相比，δ¹⁸O相差高达19‰，δ¹⁵N相差12‰，表明相对流动的水与土壤微位点中的生物地球化学过程不同。这意味着主要由流动水补给的溪流中的NO₃⁻同位素不能完全反映陆地土壤氮循环。潜在反硝化作用与提取的NO₃⁻的δ¹⁵N之间的关系显示出强烈的阈值效应，在高反硝化速率下最终呈现零相关关系。我们的观察结果包括：（1）氧化还原异质表层土壤中硝化作用和反硝化作用的竞争分馏；（2）相对不流动和流动的水池之间NO₃⁻同位素的巨大差异；（3）δ¹⁸O/δ¹⁵N关系对空间的依赖性，这表明在使用NO₃⁻同位素推断反硝化作用的场地或流域尺度模式时应谨慎。
