Extensive green roof CO exchange and its seasonal variation quantified by eddy covariance measurements.

The CO surface-atmosphere exchange of an unirrigated, extensive green roof in Berlin, Germany was measured by means of the eddy covariance method over a full annual cycle. The present analysis focusses on the cumulative green roof net ecosystem exchange of CO (NEE), on its seasonal variation and on green roof physiological characteristics by applying a canopy (A-g) model. The green roof was a carbon sink with an annual cumulative NEE of -313gCOmyear, equivalent to -85gCmyear. Three established CO flux gap-filling methods were applied to estimate NEE and to study the performance during different meteorological situations. A best estimate NEE time series was established, which chooses the gap filling method with the highest performance. During dry periods daytime carbon uptake was shown to decline linearly with substrate moisture below a threshold of 0.05mm, whereas night-time respiration was unaffected by substrate moisture variation. The roof turned into a temporary C source during dry conditions in summer 2015. We conclude that the carbon uptake of the present green roof can be optimized when substrate moisture is kept above 0.05mm.