Temporal dynamics of the carbon isotope composition in a Pinus sylvestris stand:: from newly assimilated organic carbon to respired carbon dioxide

The C-13 isotopic signature (C stable isotope ratio; delta C-13) of CO2 respired from forest ecosystems and their particular compartments are known to be influenced by temporal changes in environmental conditions affecting C isotope fractionation during photosynthesis. Whereas most studies have assessed temporal variation in delta C-13 of ecosystem-respired CO2 on a day-to-day scale, not much information is available on its diel dynamics. We investigated environmental and physiological controls over potential temporal changes in delta C-13 of respired CO2 by following the short-term dynamics of the C-13 signature from newly assimilated organic matter pools in the needles, via phloem-transported organic matter in twigs and trunks, to trunk-, soil- and ecosystem-respired CO2. We found a strong 24-h periodicity in delta C-13 of organic matter in leaf and twig phloem sap, which was strongly dampened as carbohydrates were transported down the trunk. Periodicity reappeared in the delta C-13 of trunk-respired CO2, which seemed to originate from apparent respiratory fractionation rather than from changes in delta C-13 of the organic substrate. The diel patterns of delta C-13 in soil-respired CO2 are partly explained by soil temperature and moisture and are probably due to changes in the relative contribution of heterotrophic and autotrophic CO2 fluxes to total soil efflux in response to environmental conditions. Our study shows that direct relations between delta C-13 of recent assimilates and respired CO2 may not be present on a diel time scale, and other factors lead to short-term variations in delta C-13 of ecosystem-emitted CO2. On the one hand, these variations complicate ecosystem CO2 flux partitioning, but on the other hand they provide new insights into metabolic processes underlying respiratory CO2 emission.