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Abstract

Total belowground respiration (R[sub s]) was partitioned into heterotrophic (R[sub h]) and rhizospheric (R[sub r]) respiration to determine the amount of CO₂ originating from each component. The 15-month experiment took place in a 55-year-old coastal Douglas-fir (Pseudotsuga menziesii (Mirbel) France) forest on Vancouver Island, Canada (49°51'N, 125°19'W). R[sub s] was measured within cylinders (10 cm in diameter and 7 cm long) installed 2 cm into the soil. R[sub h] was measured within longer cylinders (10 cm in diameter and 55 cm long) from which roots, hyphae, and associated rhizosphere organisms where excluded by a 0.5-micron nylon mesh. These cylinders were installed 50 cm into the soil. R[sub r] was calculated as the difference between the two measured respiration rates (R[sub s] and R[sub h]) R[sub s] was 12 Mg C ha⁻¹ yr⁻¹ and ranged from 0.71 to 6.57 g C m⁻²day⁻¹ over the 15- month experiment. R[sub h] was 7.8 Mg C ha⁻¹ yr⁻¹, which contributed 65% of R[sub s] mostly between May and August. R[sub r] was 4.2 Mg C ha⁻¹ yr⁻¹ (35% of R[sub s]) and peaked in spring and fall. Soil temperature could describe the variability in R[sub s] (p=0.0004) better than soil moisture (p=0.6156) and Q₁₀ values for R[sub s] and R[sub h] were 1.7 and 2.2, respectively. Also measured were potential sources of error associated with using this sampling technique such as: respiration resulting from decaying severed roots inside meshed cylinders, disturbance of cylinder installation, and lateral diffusion of CO₂ through the mesh.