Disentangling CO₂ fluxes: direct measurements of mesocosm‐scale natural abundance ¹³CO₂/¹²CO₂ gas exchange, ¹³C discrimination, and labelling of CO₂ exchange flux components in controlled environments

A 13 C/ 12 C mass spectrometer was interfaced with a open gas exchange system including four growth chambers to investigate CO 2 exchange components of perennial ryegrass ( Lolium perenne L.) stands. Chambers were fed with air containing CO 2 with known δ 13 C ( δ CΟ2 −2.6 or −46.8‰). The system did not fractionate C isotopes and no extraneous CO 2 leaked into chambers. The on‐line 13 C discrimination (Δ) of ryegrass stands in light was independent of δ CΟ2 when δ CΟ2 was constant. The δ of CO 2 exchanged by the stands in light ( δ Nd ) and darkness ( δ Rn ) differed by 0.7‰, suggesting some Δ in dark respiration at the stand‐level. However, Δ decreased by ∼ 10‰ when δ CΟ2 was switched from −46.8 to −2.5‰, and increased by ∼ 10‰ following a shift from −2.6 to −46.7‰ due to isotopic disequilibria between photosynthetic and respiratory fluxes. Isotopic imbalances were used to assess (non‐photorespiratory) respiration in light and the replacement of the respiratory substrate pool(s) by new photosynthate. Respiration was partially inhibited by light, but increased during the light period and decreased in darkness, in association with temperature changes. The labelling kinetics of respiratory CO 2 indicated the existence of two major respiratory substrate pools: a fast pool which was exchanged within hours, and a slow pool accounting for ∼ 60% of total respiration and having a mean residence time of 3.6 d.