Large variability in CO2 and N2O emissions and in 15N site preference of N2O from reactions of nitrite with lignin and its derivatives at different pH

Rationale: Chemodenitrification is an important N2O source in soil; however, knowledge about the production of CO2 and N2O from abiotic nitrite-SOM reactions, especially the N2O isotopic signatures (intramolecular N-15 site preference (SP), and delta N-15(bulk) and delta O-18 values), is quite limited at present. Methods: N2O and CO2 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 N-15 distribution of N2O, as well as delta N-15(bulk) and delta O-18 values, were measured with an isotope ratio mass spectrometer coupled to an online pre-concentration unit. The variability in N2O SP values was tested from pH 3 to 5, and for nitrite concentrations from 0.3 to 0.5 mM. Results: Both CO2 and N2O emissions varied largely with pH and the structure of lignin products. The highest N2O 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 N2O SP values (11.9-37.4%), which was pH dependent and not distinguishable from microbial pathways, was observed at pH 3-5. The delta N-15(bulk) and delta O-18 values of N2O 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 N2O from chemodenitrification in pure chemical assays. Chemical reactions of nitrite with lignin are pH-dependent and associated with substantial CO2 and N2O emissions. The SP values of N2O derived from chemodenitrification were neither distinguishable from the biotic pathways nor remained stable with varying pH. Therefore, the use of N2O isotopic signatures for source partitioning is restricted when chemodenitrification is contributing significantly to N2O emission.