Rapid changes in δ¹³C of ecosystem-respired CO₂ after sunset are consistent with transient ¹³C enrichment of leaf respired CO₂.

The CO₂ respired by darkened, light-adapted, leaves is enriched in ¹³C during the first minutes, and this effect may be related to rapid changes in leaf respiratory biochemistry upon darkening. We hypothesized that this effect would be evident at the ecosystem scale. High temporal resolution measurements of the carbon isotope composition of ecosystem respiration were made over 28 diel periods in an abandoned temperate pasture, and were compared with leaf-level measurements at differing levels of pre-illumination. At the leaf level, CO₂ respired by darkened leaves that had been preadapted to high light was strongly enriched in ¹³C, but such a ¹³C-enrichment rapidly declined over 60-100 min. The ¹³C-enrichment was less pronounced when leaves were preadapted to low light. These leaf-level responses were mirrored at the ecosystem scale; after sunset following clear, sunny days respired CO₂ was first ¹³C enriched, but the ¹³C-enrichment rapidly declined over 60-100 min. Further, this response was less pronounced following cloudy days. We conclude that the dynamics of leaf respiratory isotopic signal caused variations in ecosystem-scale ¹²CO₂/¹³) CO₂ exchange. Such rapid isotope kinetics should be considered when applying ¹³C-based techniques to elucidate ecosystem carbon cycling.