Probing soil nitrification and nitrate consumption using Δ17O of soil nitrate

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.