Nitrous oxide (N₂O) reduction is a key process controlling and mitigating the highly heterogeneous N₂O emission from soils. We here developed a new method (¹⁵N₂O reduction tracing) to quantify potential gross reduction rates of N₂O by incubating only a few grams of soil samples, spiking single-labeled ¹⁵N₂O tracer as the direct substrate for N₂O reduction, and analyzing its direct product (the ¹⁵N/¹⁴N ratio of N₂). First, the mass balance between ¹⁵N₂O consumption and ¹⁵N₂ production was confirmed (recovery rate: 107% ± 10%) using pure cultures of complete denitrifying bacteria. Second, we tested our method’s applicability to soil profiles at a secondary forest (O, 0–5 cm A1, 5–20 cm A2 horizons), no-tillage agricultural plot (O, A1, A2), and conventional tillage plot (0–20 cm Ap horizon). Their N₂O reduction potentials under a controlled soil water potential (−1 kPa) and 0.1% ¹⁵N₂O air varied across orders of magnitude: higher in the shallower, carbon-rich horizons (O–A1). Our method allowed the direct comparison between the N₂O reductions and copy numbers of nosZ (the functional gene responsible for N₂O reduction), which revealed no clear relationship across the studied samples. Instead, the variation in N₂O reduction potential co-varied with the soil total carbon (C) content, C/N ratio, and 16S rRNA gene copy number, suggesting C substrate control on the N₂O reduction. By further reducing the required soil mass, the current method may help to identify N₂O reduction hotspots at smaller scales and clarify mechanisms behind the heterogenous N₂O dynamics in soils.

The XRF core scanner, which facilitates non-destructive, high-speed, and high spatial resolution measurements, has recently been extensively used to measure element variability in sediments. Although identifying the precision of the measurement and concentration of the elements in the sample is essential to elucidating the variations recorded in the sediments, the element peak area count ratios are most commonly used to report the results of XRF core scanners in many previous studies, and element concentration is not discussed. This is mainly because quantitative calibration of element concentration necessitates numerous additional analyses of discrete samples. Because XRF core scanner measurements are often conducted before discrete sample analysis, if approximate element concentrations in the sediment can be estimated from XRF core scanner data just after measurement, sampling intervals should be determined for further analysis.In this study, we developed a new database on the aging of X-ray tubes, and propose a method for estimating element concentrations in the wet sediments from XRF core scanner measurement without any additional measurement. The ITRAX XRF core scanner at Kochi University, the first ITRAX installed in Japan in 2014, was used in this study. Mo X-ray tubes were used at 30 kV, 55 mA with XRF exposure time of 10 s or 32 s. First, the aging effect of the X-ray tube was monitored and its correction method was proposed. Subsequently, the precision of the ITRAX XRF measurements was evaluated and the relationship between the element peak area count by ITRAX and element concentration was analyzed. Finally, we propose a new method for applying these databases constructed in this study to any results measured by ITRAX XRF core scanner at Kochi University. Using this method, anyone can estimate approximate element composition of the sediment from their XRF core scanner data obtained at Kochi University without any additional measurement. This method even enables estimation of element concentrations of sediments just after the ITRAX measurement.