Partitioning the net CO2 flux of a deciduous forest into respiration and assimilation using stable carbon isotopes

Partitioning net ecosystem CO₂ fluxes into their components assimilation and respiration is essential to be able to predict future responses and feedbacks of ecosystems to a changing climate. We partitioned daytime net CO₂ fluxes of an old unmanaged deciduous forest in Central Germany into assimilation and respiration with an isotopic approach based on ¹³C in order to understand the link between assimilation and respiration, its dynamics over a three week period and its environmental controls. The partitioning worked well on days with large differences in isotopic signature of assimilation and respiration showing clear diurnal cycles of respiration and a strong correlation of respiration with soil temperature. Although assimilation and respiration were isotopically linked, they did not show a link in their flux rates. The contribution of respiration to assimilation was highly variable on a day-to-day basis, ranging from 15% to more than 35%. The contribution was mainly controlled by soil temperature (R² = 0.72), indicating a strong sensitivity of carbon dynamic to temperature changes and a higher carbon uptake efficiency during cold days.