Soil respiration in a mixed temperate forest and its contribution to total ecosystem respiration

Soil respiration (SR) was measured with an infrared gas analyzer in nine plots representative of the heterogeneous vegetation in a mixed coniferous-deciduous forest in the Belgian Campine region. Selected plots included the two most representative overstory species (Pinus sylvestris L. and Quercus robur L.) in combination with the most representative understory species of the forest. A model that includes temperature and water as the main controlling variables was fitted to the data. We found large spatial variability in SR among plots, with typically lower fluxes under the coniferous overstory than under the deciduous overstory (means of 4.8 +/- 0.4 and 8.8 +/- 0.5 Mg C ha(-1) year(-1), respectively). Total annual soil carbon (C) emissions were estimated by weighting fluxes from different types of vegetation according to their relative contribution to the footprint area of the eddy covariance flux measurement. The relative contribution of the two main tree species to the footprint-weighted total SR varied among seasons with the more abundant coniferous overstory contributing the most to total SR during most of the year. Nonetheless, during summer, the contribution of deciduous plots to total SR was disproportionally high because of the more pronounced seasonality of belowground metabolic activity. Net ecosystem carbon dioxide exchange was measured by eddy covariance, and we estimated total ecosystem respiration (TER) with footprint-constrained nighttime fluxes. Mean total annual SR and TER were 6.1 +/- 0.11 and 9.1 +/- 1.15 Mg C ha(-1) year(-1), respectively. The 95% confidence interval of the ratio of annual SR:TER ranged from 0.58 to 0.76, with a mean of 0.67. The contribution of SR to TER tended to vary seasonally, with minimum contributions during summer (less than 50% of TER) and maximum contributions during winter (about 94% of TER).