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Direct In-field Measurement of Nitrous Oxide Flux from Soils¹

ABSTRACT

A method was developed whereby nitrous oxide (N₂O) effusing from a soil surface could be contained and selectively trapped for subsequent analysis. Containment of N₂O was achieved by inserting a steel cover box with inlet and outlet ports into the soil. The enclosed air space above the soil surface was continuously swept by drawing external air through the cover at a flow rate of 20 liters/hour. N₂O in the air swept from the enclosed airspace was adsorbed on 5Å molecular sieve. Adsorbed N₂O was displaced for gas chromatographic analysis by addition of water to the molecular sieve in a closed system. N₂O evolved from the soil surface was distinguished from that drawn into the cover during operation by concurrent measurement of the amount of N₂O adsorbed from an equivalent flow of the external atmosphere. The adsorption and recovery of N₂O by molecular sieve was affected by the N₂O concentration in the air flow and the amount of N₂O passed through the molecular sieve. Preliminary experiments in which N₂O-air mixtures were passed through 20g molecular sieve demonstrated that for N₂O concentrations up to 34 µg N/liter, a flow rate of 20 liters/hour and sampling periods of 4 hours or less, complete recovery of N₂O passed into the molecular sieve was achieved upon displacement with water. N₂O released into the enclosed air space from an unsealed syringe was also quantitatively recovered providing a flow rate of at least 20 liters/hour was used to sweep the cover box. Flow rates of 10 to 40 liters/hour through covers inserted into moist soil had no effect on the measured rate of N₂O evolution from the soil surface. Preliminary measurements at an irrigated, fertilized (120 kg N/ha) celery-production block on a Haploxeroll indicated an overall mean N₂O flux of 9.42 g N/ha per hour over a period of 76 hours. There was a diurnal variation in the N₂O flux with mean peak fluxes of up to 15.8 g N/ha per hour occuring during the early afternoon. The method provides a basis for monitoring N₂O flux from field soils under on-going agricultural practice.

¹ Contribution from the Dep. of Soil and Environmental Sci., Univ. of Calif., Riverside, CA 92521. Research supported by R.A.N.N. Div. of the Nat. Sci. Found., Project AEN74-11136A01.

² Research Soil Scientist, and Associate Professors, respectively.

Received for publication October 17, 1977. Accepted for publication June 15, 1978.

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