Partitioning heterotrophic and rhizospheric soil respiration in a mature Douglas-fir (Pseudotsuga menziesii) forest

Total belowground respiration (Rs) was partitioned into heterotrophic (Rh) and rhizospheric (Rr) respiration to determine the amount of CO2 originating from each component in a coastal Douglas-fir ( Pseudotsuga menziesii (Mirb.) Franco) forest. Rh was measured within cylinders from which roots, hyphae, and associated rhizosphere organisms were excluded by a 0.5 μm nylon mesh and installed 50 cm into the soil. Rs was 12 Mg C·ha–1·year–1 and ranged from 0.71 to 6.57 g C·m–2·day–1 during the 15 month experiment. Rh was 7.8 Mg C·ha–1·year–1, which contributed 65% of Rs, mostly between May and August. Rr was 4.2 Mg C·ha–1·year–1 (35% of Rs) and peaked in spring and fall. Soil temperature described the variability in Rs (p = 0.0004) better than soil moisture (p = 0.6156) and Rh was more closely related to temperature than was Rr. Values of Q10 were 1.7 for Rs and 2.2 for Rh. We also assessed three potential sources of error associated with this root-exclusion technique: respiration from decaying severed roots, stimulated respiration as a result of cylinder installation, and lateral diffusion of CO2 into cylinders. None of these artifacts were found to be significant sources of error in this experiment.

The growing season carbon balance of a sub-boreal clearcut 5 years after harvesting using two independent approaches to measure ecosystem CO2 flux

From 27 June to 3 September 1999, CO2 fluxes from a 5-year-old, 84.15-ha vegetated clearcut in sub-boreal British Columbia were measured using a Bowen ratio energy balance (BREB) system and a second approach (the component model) that was based on scaled up CO2-flux measurements from belowground and plants (spruce seedlings and representative deciduous species). Over the 69-day study period both methods estimated the site to be a small sink for CO2 (–22.4 and –85 g C·m–2, respectively). Differences between the sink size of the two approaches largely resulted from a divergence in the data after 7 August when the BREB data indicated a switch from sink to source approximately 14 days in advance of the same change from sink to source seen in the component model data. The main components of the CO2 flux within the clearcut were belowground respiration (338 g C·m–2) and deciduous plant photosynthesis (–375 g C·m–2). The conifer seedlings were only a minor component in overall CO2 flux over the growing season (–48 g C·m–2). The small overall sink estimated for the site for the approximately 2.5-month growing period would likely have been surmounted by the belowground respiration if the yearly CO2 fluxes had been taken into account. For example, an additional 68 g C·m–2 was added to the atmosphere from 3 to 23 September (based on belowground respiration data only), after deciduous plants senesced. This source alone was enough to push the site from a sink to a source for CO2.