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Oxygen Consumption and Denitrification Activity of a Conifer Forest Soil Profile¹

ABSTRACT

Forest soil profile samples (L-H, 0–5, 5–15, 15–30, 30–60 cm depths) were collected from a Douglas-fir [Pseudotsuga menziesii (Mirb.) Franco] plantation in British Columbia. These soil samples were incubated at 20°C in the presence of (¹⁵NH₂)₂CO, Ca(¹⁵NO₃)₂, and Na¹⁵NO₂ to investigate the biological activity, expressed as oxygen consumption rate (O₂CR), carbon dioxide production rate (CO₂PR), and potential denitrification (both biological and chemical) activity under controlled conditions in the laboratory. The gaseous samples were analyzed mass spectrometrically. The O₂CR of the L-H sample was high (29.0 µg g^–1 h^–1) and decreased with depth in the mineral soil (2.9 µg g^–1 h^–1 at 30–60-cm depth). The CO₂PR followed the pattern of O₂CR, indicating they are related. Denitrification depended on the N-source. When Ca(¹⁵NO₃)₂ (100 mg N L^–1) was applied to the L-H samples, the NO^-₃ was denitrified to N₂O and N₂ under anaerobic conditions. The quantity and percentage of applied-N from Ca(¹⁵NO₃)₂ denitrified over the 168-hour period decreased with increasing depth. When Na¹⁵NO₂ (400 mg N L^–1) was added to the L-H samples, N₂O and N₂ were detected under aerobic conditions. The N₂ production appeared to be the result of both chemical and biological denitrification. Denitrification was not detected when (¹⁵NH₂)₂CO (500 mg N L^–1) was incubated with L-H and 0 to 5-cm soil profile samples under aerobic condition.

¹ Contribution from Environment Canada, Canadian Forestry Service, Pacific Forest Research Centre, Victoria, British Columbia, V8Z 1M5 and the Dep. of Soil Sci., Univ. of Manitoba, Winnipeg, Manitoba; R3T 2N2, Canada.

² Research Scientist, Canadian Forestry Service, Pacific Forest Research Centre, and Professor, Dep. of Soil Sci., Univ. of Manitoba, respectively.

Received for publication January 24, 1983. Accepted for publication August 12, 1983.