Field Measurement of Denitrification: I. Flux of N2 and N2O

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.