Scale-dependent linkages between nitrate isotopes and denitrification in surface soils: implications for isotope measurements and models
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- 作者：Steven J. Hall ; Samantha R. Weintraub ; David R. Bowling
- 刊名：Oecologia
- 出版年：2016
- 出版时间：August 2016
- 年：2016
- 卷：181
- 期：4
- 页码：1221-1231
- 全文大小：534 KB
- 刊物类别：Biomedical and Life Sciences
- 刊物主题：Life Sciences
Ecology
Plant Sciences - 出版者：Springer Berlin / Heidelberg
- ISSN：1432-1939
- 卷排序：181
文摘
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 δ18O/δ15N in soil-extracted NO3− from all samples measured 1.04 ± 0.12 (R2 = 0.64, p < 0.0001), consistent with fractionation from denitrification. However, δ15N of extracted NO3− was often lower than bulk soil δ15N (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 δ18O and 12 ‰ in δ15N, 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 δ15N 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 δ18O/δ15N relationships suggest caution in using NO3− isotopes to infer site or watershed-scale patterns in denitrification.KeywordsIsotope mass balance modelMobile waterNitrificationRedoxSnowmelt