Identifying the pathways of N₂O produced through nitrification and denitrification in soil is crucial for effective mitigation. Thus, we monitored a new natural isotopic signature (Δ¹⁷O) of N₂O in forested soil, which is almost stable during various biogeochemical processes, to identify the pathways. Our results suggest Δ¹⁷O is a promising signature for identifying pathways of N₂O production, revealing that increased denitrification drives the high emission of N₂O in soil on rainy days.

By monitoring the concentration and Δ¹⁷O of stream nitrate in three forested streams, the new nitrogen saturation index of forested catchments (M_atm/D_atm ratio) was estimated. We found that (1) the unprocessed atmospheric nitrate in our studied forested stream (FK1 catchment) was the highest ever reported in forested streams; (2) the M_atm/D_atm ratio can be used as a robust index for evaluating nitrogen saturation in forested catchments as the M_atm/D_atm ratio is independent of the precipitation.

Past studies used the Δ¹⁷O of stream nitrate to estimate GNR in each forested catchment. However, the GNR estimated from the Δ¹⁷O of stream nitrate using the equations was more than six times the actual GNR in our simulated calculation for a forested catchment. As a result, it must be essential to clarify/verify the distribution of the Δ¹⁷O values of NO₃⁻ in forested soils before applying the Δ¹⁷O values of stream NO₃⁻ to estimate GNR.

Excessive leaching of nitrate from forested catchments during storm events degrades water quality and causes eutrophication in downstream areas. Thus, tracing the source of nitrate increase during storm events in forested streams is important for sustainable forest management. Based on the isotopic compositions of stream nitrate, including Δ¹⁷O, this study clarifies that the source of stream nitrate increase during storm events was soil nitrate in the riparian zone.

Using triple oxygen isotopic composition (Δ¹⁷O) of ozone as a new tracer, we estimated the absolute concentrations of stratospheric ozone supplied through stratosphere-troposphere transport in the troposphere. We observed the diurnal variations in the Δ¹⁷O of ozone, which could have affected studies (field measurements, atmospheric modeling) using Δ¹⁷O to constrain atmospheric chemical paths. Our study provides an important basis for a better understanding of ozone behavior in the troposphere.