Large variability in CO2 and N2 O emissions and in 15 N site preference of N2 O from reactions of nitrite with lignin and its derivatives at different pH.

Abstract

Chemodenitrification is an important N2 O source in soil; however, knowledge about the production of CO2 and N2 O from abiotic nitrite-SOM reactions, especially the N2 O isotopic signatures (intramolecular 15 N site preference (SP), and δ15 Nbulk and δ18 O values), is quite limited at present. N2 O 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 0.5mM were explored with polynomial regression. The intramolecular 15 N distribution of N2 O, as well as δ15 Nbulk and δ18 O values, were measured with an isotope ratio mass spectrometer coupled to an online pre-concentration unit. The variability in N2 O SP values was tested from pH3 to 5, and for nitrite concentrations from 0.3 to 0.5mM. Both CO2 and N2 O emissions varied largely with pH and the structure of lignin products. The highest N2 O emission occurred at pH4-5 in 4-hydroxy-3,5-dimethoxybenzaldehyde and 4-hydroxy-3,5-dimethoxybenzoic acid treatments, and at pH3 in the treatments with lignin, 4-hydroxy-3-methoxybenzaldehyde, 4-hydroxy-3-methoxybenzoic acid, 4-hydroxybenzaldehyde, and 4-hydroxybenzoic acid. A wide range of N2 O SP values (11.9-37.4‰), which was pH dependent and not distinguishable from microbial pathways, was observed at pH3-5. The δ15 Nbulk and δ18 O values of N2 O were both in a similar range to that reported for fungal denitrification and bacterial denitrification. These results present the first characterization of the isotopic composition of N2 O from chemodenitrification in pure chemical assays. Chemical reactions of nitrite with lignin are pH-dependent and associated with substantial CO2 and N2 O emissions. The SP values of N2 O derived from chemodenitrification were neither distinguishable from the biotic pathways nor remained stable with varying pH. Therefore, the use of N2 O isotopic signatures for source partitioning is restricted when chemodenitrification is contributing significantly to N2 O emission.