Direct In‐field Measurement of Nitrous Oxide Flux from Soils

Abstract

Abstract A method was developed whereby nitrous oxide (N 2 O) effusing from a soil surface could be contained and selectively trapped for subsequent analysis. Containment of N 2 O 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. N 2 O in the air swept from the enclosed airspace was adsorbed on 5Å molecular sieve. Adsorbed N 2 O was displaced for gas chromatographic analysis by addition of water to the molecular sieve in a closed system. N 2 O evolved from the soil surface was distinguished from that drawn into the cover during operation by concurrent measurement of the amount of N 2 O adsorbed from an equivalent flow of the external atmosphere. The adsorption and recovery of N 2 O by molecular sieve was affected by the N 2 O concentration in the air flow and the amount of N 2 O passed through the molecular sieve. Preliminary experiments in which N 2 O‐air mixtures were passed through 20g molecular sieve demonstrated that for N 2 O 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 N 2 O passed into the molecular sieve was achieved upon displacement with water. N 2 O 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 N 2 O 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 N 2 O flux of 9.42 g N/ha per hour over a period of 76 hours. There was a diurnal variation in the N 2 O 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 N 2 O flux from field soils under on‐going agricultural practice.