Source Contributions to the Carbon Isotopic Signature of Ecosystem Respiration in a Northern Deciduous Forest Ecosystem

Respiratory carbon (R_c) losses from forests are large and may account for regional differences in annual carbon sequestration. In addition, different sources of R_c respond differently to temperature, soil moisture, and current photosynthesis. One of our objectives at the UMBS Forest Carbon Cycle Research Program in northern lower Michigan USA (45.6° N, 84.7° W) is to partition ecosystem respiration (R_e) among major contributing fluxes and to account mechanistically for annual and interannual variation in R_e. Previous work based on analyses of the temperature and moisture dependencies of soil, leaf, and bole respiration suggested that over three years (1999-2001), soil respiration contributed ∼ 70% of R_e, with leaves and boles contributing 11% and 19% respectively. With the installation of a ThermoFinnigan Delta^Plus XL isotope ratio mass spectrometer at UMBS in early 2003 we have expanded our analyses of R_e to include partitioning the isotopic signature of R_e into that contributed by the autotrophic and heterotrophic components of soil respiration, and by leaves, branches, and coarse woody debris. Results from Keeling plots taken mid-summer 2003 at 2.5 m above the soil surface showed an ecosystem δ ¹³C of -24.5‰ . Soil respiratory δ ¹³C taken from soil cuvettes was very similar, at -24.7‰ . Corticular photosynthesis in Populus, the dominant canopy species, caused a shift in branch respiratory δ ¹³C from -21.5‰ in the dark to -15.2‰ in the light. Respiratory δ ¹³C from coarse woody debris varied with decay class, but averaged ∼ -26‰ . Leaf organic matter δ ¹³C varied with canopy position and species, from ∼ -27‰ at the top of the canopy to ∼ -30‰ in the shrub layer. We will use these observations as an independent check against our partitioning of R_e based on measured component fluxes and to facilitate interpretation of the responses of these ecosystem components to changing environmental conditions.