Probing soil nitrification and nitrate consumption using Δ17O of soil nitrate

Zhongjie Yu, Emily M. Elliott

Research output: Contribution to journalArticlepeer-review

Abstract

Recent analytical and conceptual advances related to the nitrate (NO3 ) 17O anomaly (Δ17O) have opened the door to a new method that probes soil nitrification and NO3 consumption using Δ17O of soil NO3 . Because biological NO3 production and consumption processes in soil obey the mass-dependent fractionation law, Δ17O of soil NO3 , an index of excess 17O over that expected from 18O, can be used to trace gross nitrification and NO3 consumption in a way analogous to the 15NO3 tracer typically employed in studies of soil NO3 cycling. Moreover, coupling Δ17O with the dual NO3 isotopes (δ15N and δ18O) at natural abundances offers additional valuable insights into mechanisms that underlie soil NO3 dynamics. In this study, we conducted both laboratory and field experiments to assess the use of Δ17O-NO3 - for tracing soil nitrification and NO3 consumption. Soil samples spanning a wide range of physical and chemical properties were sampled from four sites for batch incubations and amendments with a Δ17O-enriched NO3 fertilizer. After amendments, the triple isotopes (δ15N, δ18O, and Δ17O) of soil NO3 were measured periodically and used in a developed Δ17O-based numerical model to simultaneously derive gross rates and isotope effects of soil nitrification and NO3 consumption. The measured Δ17O-NO3 - was also used in the classical isotope dilution model to estimate gross NO3 turnover rates. In situ field soil sampling was conducted in a temperate upland meadow following snowmelt input of Δ17O-enriched atmospheric NO3 to assess the robustness of Δ17O-NO3 - as a natural tracer. The results show that the temporal dynamics of Δ17O-NO3 - can provide quantitative information on soil nitrification and NO3 consumption. In the laboratory incubations, a wide range of gross nitrification and NO3 consumption rates were estimated for the four soils using the Δ17O-based models. The estimated rates are well within the range reported in previous 15N tracer-based studies and not sensitive to oxygen isotopic fractionations during nitrification and NO3 consumption. Coupling Δ17O-NO3 - with the dual NO3 isotopes using the numerical model placed strong constraints on the δ15N and δ18O endmembers of nitrification-produced NO3 and revealed soil-specific N isotope effects for nitrification and NO3 consumption, consistent with the inferred differences in soil microbial community structure among these soils. Non-zero Δ17O-NO3 - values, up to 4.7‰ were measured in the meadow soil following the snowmelt event. Although soil heterogeneity in the field prevents quantitative rate estimation using Δ17O-NO3 -, active NO3 cycling via co-occurring nitrification and denitrification was revealed by the covariations in the triple NO3 isotopes. Integrating the field observations with the incubation results uncovered isotopic overprinting of nitrification on denitrification in the surface soil following the snowmelt, which has important implications for explaining the discrepancies between field- and laboratory-derived isotope systematics of denitrification. We conclude that Δ17O-NO3 - is a conservative and powerful tracer of soil nitrification and NO3 consumption and future applications are expected to help disentangle soil NO3 cycling complexity at various scales.

Original languageEnglish (US)
Pages (from-to)187-199
Number of pages13
JournalSoil Biology and Biochemistry
Volume127
DOIs
StatePublished - Dec 2018
Externally publishedYes

Keywords

  • Denitrification
  • Dual nitrate isotopes
  • Nitrate consumption
  • Nitrification
  • Snowmelt
  • ΔO

ASJC Scopus subject areas

  • Microbiology
  • Soil Science

Fingerprint Dive into the research topics of 'Probing soil nitrification and nitrate consumption using Δ<sup>17</sup>O of soil nitrate'. Together they form a unique fingerprint.

Cite this