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Field Measurement of Denitrification: I. Flux of N₂ and N₂O¹

ABSTRACT

Absolute amounts and rates of denitrification from a Yolo loam (Typic Xerorthents) field profile were studied in relation to the influence of soil-water content, organic C source, and soil temperature. Field plots were intensely instrumented with soil atmosphere samplers, tensiometers, and thermocouples. Soil-water contents were maintained constant with a spray irrigation system activated at frequent time intervals. The upper 15 cm of soil was maintained at soil-water pressure heads (h) of –15 and –70 cm of water for the two different water regimes of the experiment conducted at a soil temperature (5 cm depth) of 23°C. Soil-water pressure heads were maintained at –8 and –50 cm for the two water regimes conducted at 8°C. Plots cropped with ryegrass, uncropped plots, and plots to which manure was mixed in the top 10 cm of soil were used to establish three different C levels. Fertilizer was applied at the rate of 300 kg N ha^–1 as KNO₃ enriched with 20 and 40% ¹⁵N for the h = –15 or –8 and h = –70 or –50 cm treatments, respectively. The flux of gases at the soil surface was measured from the accumulation of N₂O and ¹⁵N₂ beneath an air-tight cover placed over the soil surface for 1 or 2 hours per day. Denitrification from gas fluxes occurred for the 23°C experiment in order of decreasing magnitude in manure (h = –15 cm), manure (h = –70 cm), cropped (h = –15 cm), cropped (h = –70 cm), uncropped (h = –15 cm), and uncropped (h = –70 cm) plots. Approximately 70% of the fertilizer N was denitrified for the manure (h = –15 cm) treatment. Approximately 1% of the added fertilizer was denitrified in the uncropped (h = –70 cm) treatment. Denitrification from gas fluxes for the 8°C experiment occurred in the same order as that of the 23°C experiment except that rates and absolute magnitudes were much smaller. Approximately 11% of the fertilizer N was denitrified for the manure (h = –8 cm), and no measurable denitrification occurred in the uncropped (h = –50 cm) treatment. The amount of N₂ produced was much greater than N₂O for all plots. The N₂O flux at the soil surface was dependent upon time and degree of anoxic development and varied between 5 and 26% of total denitrification.

¹ Contribution from the Department of Land, Air, and Water Resources, Univ. of Calif., Davis, Calif. This research was supported by Grant R804259 of the U.S. Environmental Protection Agency and Grant GI34733X of the National Science Foundation.

² Associate Professor of Soil Science and Staff Research Associates, respectively, Dept. of Land, Air, and Water Resources, Univ. of Calif., Davis, CA 95616.

Received for publication June 9, 1978. Accepted for publication July 25, 1978.

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