Gaseous losses of applied nitrogen from a corn field determined by ¹⁵N abundance of N₂ and N₂O

This study was carried out to investigate the magnitude of N loss through denitrification by ¹⁵N gas-flux method in a corn field where rape was incorporated with surface soils as green manure. To measure the in situ emissions of N₂ and N₂O, micro-plots with plastic barriers (0.70 m X 0.30 m) were setup in triplicate in a corn field, (¹⁵NH₄)₂SO₄ (70.7 atom% ¹⁵N) was mixed thoroughly with the 0-10cm topsoil inside the plots at a rate equivalent to 150 kg N ha^-1. The gas samples were taken periodically by using a static chamber method, ¹⁵N abundance in N₂O and N₂ was determined by mass spectrometry, and N₂O concentrations were determined by gas chromatography. The ¹⁵N mass balance in the micro-plots was estimated at harvest. The total amount of gaseous N losses was about 10.8% of the applied fertilizer N, and N₂O emissions accounted for only 2.9% of the gaseous N loss. The average emission rates of N₂ and N₂O during the corn-growing period were 13.4 and 0.041 mg N m^-2 d^-1, respectively. However, after 30-40 days from the sowing, both the fluxes of N₂ and N₂O were rather low. The ¹⁵N abundance of N₂O was as high as 34.3 atom% ¹⁵N during the first week after fertilization and then decreased drastically to the background level. These temporal changes in ¹⁵N abundance of N₂O coincided well with those in nitrate contained in the soil. At harvest time; the recovery of the applied fertilizer N calculated from ¹⁵N mass balance was 86%, and 55% of the applied N still remained in soil profile as nitrate.