Direct Measurement of Denitrification Loss from Soils: II. Development and Application of Field Methods

A method was developed for the direct measurement of denitrification N loss from field soils. The method was based on the observation that in the presence of acetylene (C₂H₂) concentrations of 0.1 to 5.0% (vol/vol) the sole product of denitrification in soils was nitrous oxide (N₂O). Sustained C₂H₂ concentrations of approximately 0.4 to 1.6% were induced in the atmosphere of a field soil by supplying C₂H₂ through probes inserted into the soil to a depth of 1 m. The time required to establish these C₂H₂ concentrations ranged from 15 to 30 min. The concentrations could be maintained for at least 17 hours during which C₂H₂ was supplied at 2.8 liters/hour. Essentially, no increase in ³⁰N₂ concentration was detected under closed soil covers installed at C₂H₂-treated field locations which had received ¹⁵N-NO₃. At locations without C₂H₂ treatment an average ³⁰N₂ enrichment of 14.2% relative to air was observed. Production of ¹⁵N-labeled N₂O and N₂ at locations with and without C₂H₂ treatment was very similar, and essentially 100% of the ¹⁵N evolved at C₂H₂-treated locations appeared as N₂O. Direct measurements of N₂O fluxes from an irrigated, fertilized Haploxeroll at locations with and without C₂H₂ treatment were made using a previously established method. Net denitrification N fluxes, measured as N₂O flux from C₂H₂-treated locations ranged from 0.5 to 80 g N/ha per hour. Peak N fluxes were between 30 and 80 g N/ha per hour and usually occurred 30 to 60 hours after irrigation had ceased. Total denitrification loss during 123 days was 51.2 kg N/ha with the total fertilizer application being 335 kg N/ha. Dinitrogen/nitrous oxide ratios, determined from the difference in N₂O evolution at sites with and without C₂H₂ treatment, ranged from 0 to 1 during the first 12 to 24 hours of each denitrification cycle after which the ratio gradually increased to values of 10 to 20 towards the end of each cycle. The average value was between 5.6 and 7.4. The methodology provides a technique for direct monitoring of denitrification N loss in situ.