Oxygen Consumption and Denitrification Activity of a Conifer Forest Soil Profile

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 ( 15 NH 2 ) 2 CO, Ca( 15 NO 3 ) 2 , and Na 15 NO 2 to investigate the biological activity, expressed as oxygen consumption rate (O 2 CR), carbon dioxide production rate (CO 2 PR), and potential denitrification (both biological and chemical) activity under controlled conditions in the laboratory. The gaseous samples were analyzed mass spectrometrically. The O 2 CR 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 CO 2 PR followed the pattern of O 2 CR, indicating they are related. Denitrification depended on the N‐source. When Ca( 15 NO 3 ) 2 (100 mg N L −1 ) was applied to the L‐H samples, the NO ‐ 3 was denitrified to N 2 O and N 2 under anaerobic conditions. The quantity and percentage of applied‐N from Ca( 15 NO 3 ) 2 denitrified over the 168‐hour period decreased with increasing depth. When Na 15 NO 2 (400 mg N L −1 ) was added to the L‐H samples, N 2 O and N 2 were detected under aerobic conditions. The N 2 production appeared to be the result of both chemical and biological denitrification. Denitrification was not detected when ( 15 NH 2 ) 2 CO (500 mg N L −1 ) was incubated with L‐H and 0 to 5‐cm soil profile samples under aerobic condition.