Field measurements of carbon isotopic signature of bulk leaf material (δ¹³C) and CO₂ respired by plants or soils (δ¹³C_R) are often used to constrain carbon global circulation models, based on well-established relationships among environmental parameters, plant ecophysiology and photosynthetic carbon discrimination (Δ). However, recent studies have shown that fractionation associated to mesophyll conductance and post-carboxylation discrimination (grouped under the acronym MPCD) can affect the isotopic signature of plant organs and organic compounds. In addition, leaf respiratory discrimination (RD) has been shown. Nevertheless, little is known about the variability of MPCD and RD and their controls as different plant species grow older.

The present study addressed the effects of ontogeny and species identity on Δ, MPCD and RD, and studied the ecophysiological mechanisms through which ontogeny might affect the δ¹³C of organic and respired carbon. Carbon isotopic signatures in leaf, phloem and root organic matter, bulk soil and microbial biomass as well as in leaf- and soil-respired CO₂ of seven temperate grass and legume species at three ontogenetic stages covering their life cycle (young, mature, old) were investigated under controlled conditions. In addition, selected ecophysiological parameters (including assimilation rate, stomatal conductance and water use efficiency) were quantified.

Results/Conclusions

Over the different ontogenetic stages and species, Δ ranged from 19‰ to 25‰, MPCD varied by up to 10‰, and RD by up to 7‰. These three discrimination steps were strongly affected by species/functional groups and ontogeny. While changes in Δ were not explained by changes in assimilation, c_i/c_a or stomatal conductance, MPCD scaled tightly with stomatal conductance (r²=0.72) and assimilation (r²=0.43), suggesting that MPCD may be driven by carbon sink regulation. The changes in RD were not explained by the measured plant physiological parameters.

First, our study showed unexpectedly large ontogenetic effects on three major carbon isotope discrimination steps of temperate herbaceous plants, which were also species/functional group dependent,. The changes in carbon isotopic signature of plant tissues due to ontogeny were at least of similar magnitude as those caused by environmental factors reported in other studies, i.e, VPD and PAR. Ontogeny should therefore be considered in ecosystem studies, as well as integrated in larger-scale ecosystem models that are based on carbon isotopic signatures. Second, the magnitude of MPCD supports recent studies that limit the applicability of bulk leaf δ¹³C as a proxy for water use efficiency studies in herbaceous plants.