Chemodenitrification by Fe(II) and nitrite: Effects of temperature and dual N-O isotope fractionation

Chemodenitrification between Fe(II) and nitrite, an important nitrogen (N) cycling pathway, varies depending on the season, indicating the impact of ambient temperatures. However, the underlying mechanism for this occurrence has not been explained at a molecular scale. As an effective approach for studying N cycling, dual N-O isotope fractionation was analyzed to reveal the reactions of temperature-dependent chemodenitrification. The kinetics of chemodenitrification under different temperatures showed that nitrite reduction rates and N2O production increased with temperature (5-35 degrees C). Lepidocmcite was identified as the secondary mineral of Fe(II) oxidation at 5, 15, and 25 degrees C, and goethite was observed at 35 degrees C. Further, the value of the O isotope enrichment factor ((18)epsilon) decreased with increasing temperature while values of N isotope fractionation enrichment factor ((15)epsilon) were less sensitive to temperature. The isotope fractionation ratios of O to N ((18)epsilon:(15)epsilon) were 1.00 +/- 0.25, 0.75 +/- 0.05, 0.54 +/- 0.04, and 0.46 +/- 0.06 at 5, 15, 25, and 35 degrees C, respectively, indicating that the temperature effect should be considered when dual NO isotope data are applied to distinguish chemodenitrification with bio-denitrification. A kinetic model was established to correlate the rate constants of the elementary reactions during chemodenitrification with N-O isotope fractionation. The intrinsic isotope fractionation of N-O bond breakage during chemodenitrification was obtained. This study provides both qualitative and quantitative outcomes on temperature-dependent chemodenitrification. This study provides inputs for a better understanding of the role of chemodenitrification in effecting seasonal changes to the environment.

Automated summary

Chemodenitrification between Fe(II) and nitrite, a crucial nitrogen cycling pathway, is influenced by ambient temperatures. This study analyzed dual N-O isotope fractionation to investigate the temperature-dependent reactions and found that nitrite reduction rates and N2O production increased with temperature. Different secondary minerals were identified at varying temperatures, and the isotope fractionation ratios of O to N were affected by temperature change, stressing the importance of considering temperature effects in dual NO isotope data analysis.