Ammonia oxidation pathways and nitrifier denitrification are significant sources of N₂O and NO under low oxygen availability

• Published in: Proceedings of the National Academy of Sciences - PNAS 2013-04, Vol.110 (16), p.6328-6333
• Main Authors: Zhu, Xia, Burger, Martin, Doane, Timothy A., Horwath, William R.
• Format: Article
• Language: English
• Subjects: Agricultural soils; Agronomy. Soil science and plant productions; Air Pollution - analysis; Air Pollution - prevention & control; Ammonium Sulfate; Analysis of Variance; Atmosphere - analysis; Biological and medical sciences; Biological Sciences; Chromatography, Gas; Clay loam soils; Colorimetry; Denitrification - physiology; Fertilizers; Fundamental and applied biological sciences. Psychology; General agronomy. Plant production; Nitric Oxide - metabolism; Nitrification; Nitrification - physiology; Nitrogen fertilization; Nitrogen, phosphorus, potassium fertilizations; Nitrous oxide; Nitrous Oxide - metabolism; Oxidation; Oxidation-Reduction; Oxygen - analysis; Oxygen - metabolism; Pollutant emissions; Sandy loam soils; Soil - analysis; Soil biochemistry; Soil water; Soil-plant relationships. Soil fertility. Fertilization. Amendments; Urea
• ISSN: 0027-8424; 1091-6490
• DOI: 10.1073/pnas.1219993110

The continuous increase of nitrous oxide (N ₂O) abundance in the atmosphere is a global concern. Multiple pathways of N ₂O production occur in soil, but their significance and dependence on oxygen (O ₂) availability and nitrogen (N) fertilizer source are poorly understood. We examined N ₂O and nitric oxide (NO) production under 21%, 3%, 1%, 0.5%, and 0% (vol/vol) O ₂ concentrations following urea or ammonium sulfate [(NH ₄) ₂SO ₄] additions in loam, clay loam, and sandy loam soils that also contained ample nitrate. The contribution of the ammonia (NH ₃) oxidation pathways (nitrifier nitrification, nitrifier denitrification, and nitrification-coupled denitrification) and heterotrophic denitrification (HD) to N ₂O production was determined in 36-h incubations in microcosms by ¹⁵N- ¹⁸O isotope and NH ₃ oxidation inhibition (by 0.01% acetylene) methods. Nitrous oxide and NO production via NH ₃ oxidation pathways increased as O ₂ concentrations decreased from 21% to 0.5%. At low (0.5% and 3%) O ₂ concentrations, nitrifier denitrification contributed between 34% and 66%, and HD between 34% and 50% of total N ₂O production. Heterotrophic denitrification was responsible for all N ₂O production at 0% O ₂. Nitrifier denitrification was the main source of N ₂O production from ammonical fertilizer under low O ₂ concentrations with urea producing more N ₂O than (NH ₄) ₂SO ₄ additions. These findings challenge established thought attributing N ₂O emissions from soils with high water content to HD due to presumably low O ₂ 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 N ₂O production in agricultural soils.