Partitioning the net CO 2 flux of a deciduous forest into respiration and assimilation using stable carbon isotopes

Partitioning net ecosystem CO 2 fluxes measured by the eddy covariance technique into their components assimilation and respiration is crucial for predicting future responses and feedbacks of ecosystems to a changing climate. On the basis of an isotopic approach with 13 C, we partitioned the daytime net CO 2 fluxes of a deciduous forest in central Germany into assimilation and respiration fluxes over a period of 3 weeks. This is the first attempt so far to quantify component fluxes with stable isotopes over the period of 3 weeks, enabling us to investigate the impact of environmental factors on the partitioned fluxes. Large variability in environmental conditions during the 3‐week measurement campaign led to strong changes in isotopic disequilibrium between assimilation and respiration, ranging from 1 to 5‰. Although this approach is still associated with large uncertainty, we found reasonable patterns in ecosystem respiration and assimilation, and a significant correlation of daytime respiration with soil temperature (R 2 = 0.48). The ratio of respiration to assimilation was highly variable on a day‐to‐day basis, ranging from 10% to more than 25%. This ratio was mainly controlled by soil temperature (R 2 = 0.61), indicating a strong sensitivity of ecosystem carbon dynamics to temperature changes and higher carbon uptake efficiency during cooler days.