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

Summary • 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 12CO2/13CO2 exchange. Such rapid isotope kinetics should be considered when applying 13C‐based techniques to elucidate ecosystem carbon cycling.