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.