Influence of Electron Donors (Fe, C, S) on N2O Production during Nitrate Reduction in Lake Sediments: Evidence from Isotopes and Functional Genes

Nitrate reduction is a major source of nitrous oxide (N2O), which is a potent greenhouse gas and an ozone-depleting agent. Microbial nitrate reduction could be driven by organic carbon, sulfur, and iron oxidation. However, the biochemical mechanisms of N2O production driven by C, S, and Fe are poorly understood, and the contributions of these electron donors to N2O production are hard to discern. Here, we incubated lake sediments with glucose, sulfide, and iron(II) and measured N2O isotopes associated with each incubation. delta N-15(alpha)-N2O and delta N-15(beta)-N2O (N-alpha and N-beta, respectively, indicate central and terminal N atoms in the N2O molecule, N-beta-N-alpha-O) covaried with a ratio near 1 in the incubations with glucose and sulfide. Surprisingly, unchanged delta N-15(beta)-N2O values were observed during iron(II)-driven microbial denitrification in both short-term and long-term incubations. Such an unchanged delta N-15(beta)-N2O pattern did not show during chemodenitrification. Based on metagenomic analysis, the nitric oxide reductase receiving electrons from quinols (qNorB) had a higher contribution to N2O production in the incubations with iron than with other electron donors. The difference in the reductases might explain different delta N-15(beta)-N2O versus delta N-15(alpha)-N2O patterns. Our findings suggested that the unchanged delta N-15(beta)-N2O values could possibly be used as an indicator for iron(U)-driven microbial N2O production in field research and help to distinguish abiotic and biotic roles of iron in nitrogen cycling.