Soil respiration fluxes in relation to photosynthetic activity in broad-leaf and needle-leaf forest stands

Soil respiration is a combination of CO2 fluxes derived from a diversity of belowground sources, many depending directly on the input of carbon from living plants. Here we present data from two different forest ecosystems, a beech and a spruce forest, where a partitioning of soil respiration was carried out. We used soil cores inside micro-pore meshes together with periodic chamber-based measurements to estimate rhizosphere, mycorrhizal fungal and microbial heterotrophic respiration. Calculated mycorrhizal mycelium respiration was 8% at the spruce forest and 3% at the beech forest. Given the nature of the partitioning method these values represent minimum estimates. The ratio of root-derived carbon respiration to heterotrophic respiration was about 1:1 at both forest types. The relationship of each source with temperature and photosynthesis, measured as gross primary productivity derived from eddy covariance measurements, was subsequently explored. Both factors revealed effects specific to the respiration source and the forest type. A response to temperature was evident in all cases except for mycorrhizal mycelium respiration at the spruce forest (R2 = 0.06, p = 0.41). Significant correlations of photosynthesis with rhizosphere and mycorrhizal fungal respiration were found in all cases. Peaks in correlation values showed time lags between photosynthetic activity and a respiration response ranging from 1 day for the fungal component and 4 days for the rhizosphere component at the beech forest (R2 = 0.70, p < 0.01 and R2 = 0.42, p < 0.05, respectively) to 5 days for both fluxes at the spruce forest (R2 = 0.44, p < 0.01 and R2 = 0.72, p < 0.01, respectively). Results show that respiration of the mycorrhizal component cannot be predicted by common temperature driven models in some ecosystems. They also indicate a strong influence of forest canopy processes on the activity of roots and associated organisms. The specific response in each vegetation type should be ideally explained by physiological mechanisms inherent to different species as a next step towards understanding belowground carbon dynamics.