Improved isotopic model based on 15 N tracing and Rayleigh-type isotope fractionation for simulating differential sources of N 2 O emissions in a clay grassland soil

• Published in: Rapid communications in mass spectrometry 2019-03, Vol.33 (5), p.449-460
• Main Authors: Castellano-Hinojosa, Antonio, Loick, Nadine, Dixon, Elizabeth, Matthews, G Peter, Lewicka-Szczebak, Dominika, Well, Reinhard, Bol, Roland, Charteris, Alice, Cardenas, Laura
• Format: Article
• Language: English
• ISSN: 0951-4198; 1097-0231
• DOI: 10.1002/rcm.8374

Isotopic signatures of N O can help distinguish between two sources (fertiliser N or endogenous soil N) of N O emissions. The contribution of each source to N O emissions after N-application is difficult to determine. Here, isotopologue signatures of emitted N O are used in an improved isotopic model based on Rayleigh-type equations. The effects of a partial (33% of surface area, treatment 1c) or total (100% of surface area, treatment 3c) dispersal of N and C on gaseous emissions from denitrification were measured in a laboratory incubation system (DENIS) allowing simultaneous measurements of NO, N O, N and CO over a 12-day incubation period. To determine the source of N O emissions those results were combined with both the isotope ratio mass spectrometry analysis of the isotopocules of emitted N O and those from the N-tracing technique. The spatial dispersal of N and C significantly affected the quantity, but not the timing, of gas fluxes. Cumulative emissions are larger for treatment 3c than treatment 1c. The N-enrichment analysis shows that initially ~70% of the emitted N O derived from the applied amendment followed by a constant decrease. The decrease in contribution of the fertiliser N-pool after an initial increase is sooner and larger for treatment 1c. The Rayleigh-type model applied to N O isotopocules data (δ N -N O values) shows poor agreement with the measurements for the original one-pool model for treatment 1c; the two-pool models gives better results when using a third-order polynomial equation. In contrast, in treatment 3c little difference is observed between the two modelling approaches. The importance of N O emissions from different N-pools in soil for the interpretation of N O isotopocules data was demonstrated using a Rayleigh-type model. Earlier statements concerning exponential increase in native soil nitrate pool activity highlighted in previous studies should be replaced with a polynomial increase with dependency on both N-pool sizes.