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Comparative Study
. 2013 Apr 16;110(16):6328-33.
doi: 10.1073/pnas.1219993110. Epub 2013 Apr 1.

Ammonia Oxidation Pathways and Nitrifier Denitrification Are Significant Sources of N2O and NO Under Low Oxygen Availability

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Free PMC article
Comparative Study

Ammonia Oxidation Pathways and Nitrifier Denitrification Are Significant Sources of N2O and NO Under Low Oxygen Availability

Xia Zhu et al. Proc Natl Acad Sci U S A. .
Free PMC article

Abstract

The continuous increase of nitrous oxide (N2O) abundance in the atmosphere is a global concern. Multiple pathways of N2O production occur in soil, but their significance and dependence on oxygen (O2) availability and nitrogen (N) fertilizer source are poorly understood. We examined N2O and nitric oxide (NO) production under 21%, 3%, 1%, 0.5%, and 0% (vol/vol) O2 concentrations following urea or ammonium sulfate [(NH4)2SO4] additions in loam, clay loam, and sandy loam soils that also contained ample nitrate. The contribution of the ammonia (NH3) oxidation pathways (nitrifier nitrification, nitrifier denitrification, and nitrification-coupled denitrification) and heterotrophic denitrification (HD) to N2O production was determined in 36-h incubations in microcosms by (15)N-(18)O isotope and NH3 oxidation inhibition (by 0.01% acetylene) methods. Nitrous oxide and NO production via NH3 oxidation pathways increased as O2 concentrations decreased from 21% to 0.5%. At low (0.5% and 3%) O2 concentrations, nitrifier denitrification contributed between 34% and 66%, and HD between 34% and 50% of total N2O production. Heterotrophic denitrification was responsible for all N2O production at 0% O2. Nitrifier denitrification was the main source of N2O production from ammonical fertilizer under low O2 concentrations with urea producing more N2O than (NH4)2SO4 additions. These findings challenge established thought attributing N2O emissions from soils with high water content to HD due to presumably low O2 availability. Our results imply that management practices that increase soil aeration, e.g., reducing compaction and enhancing soil structure, together with careful selection of fertilizer sources and/or nitrification inhibitors, could decrease N2O production in agricultural soils.

Conflict of interest statement

The authors declare no conflict of interest.

Figures

Fig. 1.
Fig. 1.
The main pathways of nitrous oxide (N2O) and nitric oxide (NO) production in soil.
Fig. 2.
Fig. 2.
Relative contributions of nitrifier denitrification (ND), nitrifier nitrification (NN), nitrification-coupled denitrification (NCD), heterotrophic denitrification (HD) to N2O production in clay loam soil.
Fig. 3.
Fig. 3.
Total nitrous oxide (N2O) produced at different levels of headspace oxygen (percentage O2) over 36 h after application of fertilizers to soils. For each soil (A–C), different uppercase letters indicate a significant difference in total N2O emission across all treatments. Different lowercase letters indicate a significant difference in the proportion of N2O derived from ammonia oxidation.
Fig. 4.
Fig. 4.
Total nitric oxide (NO) at different levels of headspace oxygen (percentage O2) 4 h after application of fertilizers to soils. For each soil (A–C), different uppercase letters indicate a significant difference in total NO production across all treatments, and different lowercase letters indicate a significant difference in the proportion of NO derived from ammonia oxidation.

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