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Denitrification Measured Directly from Nitrogen and Nitrous Oxide Gas Fluxes¹

ABSTRACT

The amount of denitrification is generally the unknown in attempts to evaluate the fate of N fertilizers applied to soils. Substantial error can result when denitrification is determined from the difference between measurements of plant uptake, residual soil N, leaching, and the amount of N applied. An independent, direct measure of denitrification potentially has merit by containing only the error from direct measurement and not a cumulative error from several measurements. Nitrate at a rate of 300 kg of N/ha and enriched with 20 and 10 atom percent excess ¹⁵N was applied to laboratory columns and a field plot, respectively. The columns were maintained at soil-water pressure heads of –22 and –70 cm. The field plot was maintained at a soil-water pressure head of approximately –10 cm in the upper 10 cm of soil. The field plot was cropped with perennial ryegrass (Lolium perenne). The columns and field plot were instrumented with tensiometers, soil solution samplers, and gas samplers. The concentration and isotopic ratio of NO₃, N₂, and N₂O were measured as a function of soil depth and time. The gaseous concentration gradients and measured apparent diffusion coefficients were used to calculate the fluxes of ¹⁵N₂ and ¹⁵N₂O gas from the soil. Residual soil N, plant uptake, and leaching were measured in order to calculate denitrification by difference. For the laboratory columns, the amount of denitrification determined directly compared favorably with that determined by difference. For the field plot, the disparity between denitrification calculated directly and by difference was attributable to the uncertainty in the gaseous concentration gradient near the soil surface and the spatial variability of the apparent diffusion coefficient near water saturation.

¹ Contribution from the Dep. of Land, Air, and Water Resources, Univ. of California, Davis.

² Assistant Prof. of Soil Science, Visiting Research Soil Scientist, and Staff Research Associate, respectively, Dep. of Land, Air, and Water Resources, Univ. of California, Davis, CA 95616.

Received for publication August 28, 1975. Accepted for publication November 14, 1975.

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