Evolution of Dinitrogen and Nitrous Oxide from Nitrogen-15 Fertilized Soil Cores Subjected to Wetting and Drying Cycles

A laboratory study was conducted to establish trends in evolution of dinitrogen (N₂) and nitrous oxide (N₂O) associated with the wetting and drying of soils and to assess the relative significance of nitrification and denitrification as sources of the N₂O evolved from NH⁺₄-fertilized soils. Mass spectrometric procedures were used to analyze atmospheric samples collected each day during a 6-week period from intact soil cores treated with ¹⁵N-labeled KNO₃ (65.5 atom % ¹⁵N) or (NH₄)₂SO₄ (81.4 atom % ¹⁵N) at a rate equivalent to 100 kg N ha⁻¹ and periodically treated with water to simulate rainfall. Data reported show that N₂ and N₂O were evolved only if enough water was applied to increase the soil moisture content to a level above the 0.03-MPa moisture percentage, that these gases were evolved concurrently, that maximal evolution occurred 2 to 9 d after the water had been applied, and that more N₂ was usually evolved than N₂O. Calculations to determine the origin of the N₂ and N₂O evolved from the soil cores showed that most (usually 60–80%) of this N₂ and N₂O was derived from the fertilizer applied, that the percentage of fertilizer-derived N₂ and N₂O was greater for the NO⁻₃-treated cores than for the NH⁺₄-treated cores, and that the ¹⁵N enrichment of the N₂O often exceeded that of the N₂. The work reported provides support for the usual assumption that N₂ and N₂O evolved from NH⁺₄-, as well as NO⁻₃, fertilized soils originate largely, if not entirely, through denitrification of NO⁻₃ under anaerobic conditions, and it suggests that evolution of these gases follows an over-all cycle associated with wetting and drying of the soil. Very little, if any, N₂ and N₂O appears to be evolved when the soil moisture content is well below field capacity. Evolution virtually ceases when the soil is saturated by rainfall, and it subsequently increases as the soil dries, with maximal evolution occurring several days after the rainfall. Evolution then decreases and eventually ceases.