Nitrogen‐15 and Oxygen‐18 Natural Abundance of Potassium Chloride Extractable Soil Nitrate Using the Denitrifier Method

Abstract

In agroecosystems, most isotopic investigations of NO3− involve the use of tracers that are artificially enriched in 15N. Although the dual isotope composition of NO3−— δ15N and δ18O is especially beneficial for understanding the origin and fate of NO3−, its use for KCl‐extractable soil NO3− has been hampered by the lack of a suitable analytical technique. Our objective was to test whether the denitrifier method, whereby NO3− is reduced to N2O before mass spectrometric analysis, can be used to determine the N and O isotopic composition of NO3− from 2 M KCl soil extracts. Several internationally accepted NO3−standards were dissolved in 2 M KCl, the conventional extractant for soil inorganic N, and inoculated with the bacterial strain Pseudomonas aureofaciens (ATCC no. 13985). The standard deviation of the NO3− standards analyzed did not exceed 0.2‰ for δ15N and 0.3‰ for δ18O values. After appropriate corrections, differences between our measured and consensus δ15N and δ18O values of standard NO3− generally were within the standard deviations given for the consensus values. Both δ15N and δ18O values were reproducible among separate analytical runs. The method was also tested on genuine 2 M KCl extracts from unfertilized and fertilized soils. Depending on N fertilization, the soils had distinct δ15N and δ18O values, which were attributed to amendment with NH4NO3 fertilizer. Hence, our data indicate that the denitrifier method provides a fast, reliable, precise, and accurate way of simultaneously analyzing the natural abundances of 15N and 18O in KCl‐extractable soil NO3−

Coupled molecular and isotopic evidence for denitrifier controls over terrestrial nitrogen availability

“…A Griess reagent was added to samples in semi-micro cuvettes, with a 1:1 ratio in most cases, adjusted for greater sensitivity with lower concentrations (Doane and Horwáth, 2003). Vanadium(III) chloride reagent produced a red color indicating NO 3 − , which was measured after 8 h at 540 nm in a spectrophotometer (Miranda et al, 2001;Doane and Horwáth, 2003 − extracts were analyzed via a ThermoFinnigan PreCon-GasBench interfacing a Delta V Plus isotoperatio mass spectrometer (Rock and Ellert, 2007) at UC Davis' Stable Isotope Facility. Samples were stored at À 20°C and thawed and mixed thoroughly before analysis.…”

Performance of different assessment methods to evaluate contaminant sources and fate in a coastal aquifer

“…Samples having high salinity were processed accordingly using the 'denitrifier method' (Rock and Ellert 2007) and analyzed using a HP 6890 gas chromatogram PreCon® device interfaced to a Finnigan Mat Delta Plus XL mass spectrometer that enables the simultaneous determination of δ 15 N and δ 18 O in NO 3 − . Accuracy and precision of δ 15 N and δ 18 O determinations based on a long-term record of analyses of ISL in-house reference (ISL-KNO 3 ) are 0.3 and 0.7‰, respectively.…”

Correcting for background nitrate contamination in KCl-extracted samples during isotopic analysis of oxygen and nitrogen by the denitrifier method

“…[4,6,7] Recent studies have analyzed potassium chloride (KCl) solutions containing NO 3 -extracted from soil samples using the denitrifying bacteria Pseudomonas chlororaphis ssp. aureofaciens (P. aureofaciens) without identifying any isotopic artifacts, [7][8][9] although earlier work by Stark and Hart [10] measured NO 3 -contamination by 2 M KCl while performing diffusion digests. While conducting similar analyses on KCl-extracted soils in southern California, we noted an increase in N 2 O production in blanks prepared in KCl and worrying shifts in the expected isotopic values of reference standards prepared in KCl relative to blanks and standards prepared in deionized water (DIW).…”