Large variability in CO 2 and N 2 O emissions and in 15 N site preference of N 2 O from reactions of nitrite with lignin and its derivatives at different pH

Rationale Chemodenitrification is an important N 2 O source in soil; however, knowledge about the production of CO 2 and N 2 O from abiotic nitrite‐SOM reactions, especially the N 2 O isotopic signatures (intramolecular 15 N site preference (SP), and δ 15 N bulk and δ 18 O values), is quite limited at present. Methods N 2 O and CO 2 emissions from chemical reactions of nitrite with lignin products were determined with gas chromatography, and their response surfaces as a function of pH from 3 to 6 and nitrite concentration from 0.1 to 0.5 mM were explored with polynomial regression. The intramolecular 15 N distribution of N 2 O, as well as δ 15 N bulk and δ 18 O values, were measured with an isotope ratio mass spectrometer coupled to an online pre‐concentration unit. The variability in N 2 O SP values was tested from pH 3 to 5, and for nitrite concentrations from 0.3 to 0.5 mM. Results Both CO 2 and N 2 O emissions varied largely with pH and the structure of lignin products. The highest N 2 O emission occurred at pH 4–5 in 4‐hydroxy‐3,5‐dimethoxybenzaldehyde and 4‐hydroxy‐3,5‐dimethoxybenzoic acid treatments, and at pH 3 in the treatments with lignin, 4‐hydroxy‐3‐methoxybenzaldehyde, 4‐hydroxy‐3‐methoxybenzoic acid, 4‐hydroxybenzaldehyde, and 4‐hydroxybenzoic acid. A wide range of N 2 O SP values (11.9–37.4‰), which was pH dependent and not distinguishable from microbial pathways, was observed at pH 3–5. The δ 15 N bulk and δ 18 O values of N 2 O were both in a similar range to that reported for fungal denitrification and bacterial denitrification. Conclusions These results present the first characterization of the isotopic composition of N 2 O from chemodenitrification in pure chemical assays. Chemical reactions of nitrite with lignin are pH‐dependent and associated with substantial CO 2 and N 2 O emissions. The SP values of N 2 O derived from chemodenitrification were neither distinguishable from the biotic pathways nor remained stable with varying pH. Therefore, the use of N 2 O isotopic signatures for source partitioning is restricted when chemodenitrification is contributing significantly to N 2 O emission.