Dual isotope and isotopomer ratios of N 2 O emitted from a temperate grassland soil after fertiliser application

The N 2 O and N 2 fluxes emitted from a temperate UK grassland soil after fertiliser application (equivalent to 25 and 75 kg N ha −1 ) were simultaneously measured, using a new automated soil incubation system, which replaces soil atmosphere (N 2 dominated) with a He+O 2 mixture. Dual isotope and isotopomer ratios of the emitted N 2 O were also determined. Total N 2 O and N 2 fluxes were significantly lower ( P  < 0.001) in the control (0 kg N) than in the 25 and 75 kg N treatments. The total N 2 O flux was significantly higher ( P  < 0.001) in the 75 kg N than in the 25 kg N treatment. The general patterns of N 2 O and N 2 fluxes were similar for both fertiliser treatments. The total gaseous N loss in the control treatment was nearly all N 2 , whereas in the fertiliser treatment more N 2 O than N 2 was emitted from the soil. The ratio N 2 O/N 2 fluxes as measured during the experiment suggested three phases in N 2 O production, in phase 1 nitrification > denitrification, in phase 2 denitrification > nitrification, and in phase 3 denitrification (and total denitrification) ≫ nitrification. Dual δ 15 N and δ 18 O isotope and isotopomer ( δ 15 N α and δ 15 N β ) value ratios of emitted N 2 O also pointed towards an increasing dominance of the production of N 2 O by denitrification and total denitrification. The site preference value from the soil‐emitted N 2 O was lower than the troposphere value. This confirmed that the enhanced troposphere N 2 O site preference could result from back injection of N 2 O from the stratosphere. The measurements of N 2 O/N 2 flux ratio and the isotopic content of emitted N 2 O pointed, independently, to similar temporal trends in N 2 O production processes after fertiliser application to grassland soil. This confirmed that both measurements are suitable diagnostic tools to study the N 2 O production process in soils. Copyright © 2003 John Wiley & Sons, Ltd.