Field comparison of nitrous oxide emission measurements using micrometeorological and chamber methods

Abstract

In order to check the reliability of micrometeorological and chamber methods for the measurement of N 2 O fluxes at the 'soil-atmosphere' interface, an experiment using these two methodologies was performed in autumn on a bare soil fertilized with 150 kg N-NH 4 NO 3 ha -1 . Micrometeorological measurements were continuously taken using either the gradient or eddy correlation methods, the N 2 O gas mixing ratio being measured with a tunable diode laser absorption spectrometer. N 2 O emission fluxes were also monitored in two automatic chambers during two 3-h periods every day, ie, around midday and midnight, whereas 16 manual chambers were monitored only around midday. Areas over which the fluxes were measured ranged from 0.18 and 0.49 m 2 for the manual and automatic chambers, respec- tively, to more than 10 4 m 2 for the micrometeorological methods. Additional soil variables were obtained, including the soil bulk density profile, water content, temperature and N-NO 3 -and N-NH 4 + concentrations. The different meth- ods provided similar estimates of N 2 O fluxes with an average over the whole of the common measurement period of around 6 ng N-N 2 O m -2 s -1 . The 16 manual chambers showed coefficients of variation comprised between 30 and 150 % during the 2 weeks of common observations. This variability may explain the unusual shape of the N 2 O concentration gradient in the atmosphere observed sometimes, which led to the conclusion that the gradient data set contained infor- mation allowing the spatial variability in N 2 O emission to be characterized. Hourly micrometeorological fluxes indi- cated a temporal variability of about 100 % over daily periods. Microbial processes may be assumed to vary little over such a period and this temporal variability is probably a consequence of physical processes, such as air pressure fluc- tuations. Slight daily cycle tendencies were observed. The weak levels of N 2 O emissions were caused by rainfall deficit, which maintained the soil at a low water content, preventing strongly anaerobic conditions and the N fertilizer from leaching from the dry surface to wet soil layers. ( © Inra/Elsevier) nitrous oxide flux / greenhouse effect / denitrification / soil / micrometeorological methods / chamber methods Rsum -Comparaison au champ des missions de protoxyde d'azote mesures par mthodes micromtorolo- giques et par mthodes des chambres. Afin de tester la fiabilit des mthodes micromtorologiques et des mthodes par chambres au sol pour la mesure des flux de N 2 O l'interface sol-atmosphre , une exprimentation utilisant ces deux mthodologies a t conduite l'automne sur un sol nu fertilis pralablement par 150 kg N-NH 4 NO 3 ha -1 . Les mesures micromtorologiques ont permis un suivi continu des missions par la mthode des gradients ou par la mthode des corrlations turbulentes, les rapports de mlange en N 2 O tant mesurs par un spectromtre d'absorption Article communicated by Steve Jarvis (North Wyke Okehampton