Loss of fixed nitrogen from soils by nitric oxide exhalation

Abstract

LABORATORY studies^1–8 have indicated that chemical de-nitrification processes in nitrogen rich soils and nitrite solutions can produce NO or NO₂ and perhaps methyl nitrite⁹. It has been suggested^1–6 that self decomposition of nitrite produces various gaseous nitrogen compounds including nitric oxide (NO) and it was thought that NO was unlikely to escape from the soil because in aerobic soils NO is readily oxidised by molecular oxygen to NO₂ and this in turn is rapidly adsorbed by soil materials and by soil water^1,2. However the half life of oxidation is dependent on the NO concentration because it is a termolecular reaction^10,11. At concentrations of 100 p.p.m. or greater (as in some studies) the half life for oxidation of NO to NO₂ by atmospheric oxygen is one hour or less respectively, whereas at low concentrations (∼0.01 p.p.m.) the half life for this oxidation is of the order of 10⁴ h. This variation in the oxidation rate by molecular oxygen explains why NO at low concentrations in the field can pass unoxidised from the soil to the atmosphere and perhaps explains why in some of the laboratory studies (at high concentrations) NO₂ and not NO is detected. This NO exhalation in the field could not be detected previously due to the lack of a sufficiently sensitive measurement technique. We report here the first measurements indicating that NO is continuously exhaled from soils including ungrazed, unfertilised grassland. The measured rates of exhalation from the soil to the atmosphere are 0.06 to 0.73 × 10⁻¹¹ kg nitrogen m⁻² s⁻¹ (equivalent to 0.2 to 2.3 kg N ha⁻¹ yr⁻¹). We show that this exhalation of NO is a significant source of nitrogen oxides (NO_x) in the lower atmosphere as has been previously suggested^12,13. These nitrogen oxides exert a direct influence on the ambient ozone and hydroxyl radical concentrations¹⁴ which in turn predominantly determine the oxidation rate of almost all substances in the troposphere.