Seasonal variations of belowground carbon transfer assessed by in situ ¹³CO₂ pulse labelling of trees

Abstract. Soil CO₂ efflux is the main source of CO₂ from forest ecosystems and it is tightly coupled to the transfer of recent photosynthetic assimilates belowground and their metabolism in roots, mycorrhiza and rhizosphere microorganisms feeding on root-derived exudates. The objective of our study was to assess patterns of belowground carbon allocation among tree species and along seasons. Pure ¹³CO₂ pulse labelling of the entire crown of three different tree species (beech, oak and pine) was carried out at distinct phenological stages. Excess ¹³C in soil CO₂ efflux was tracked using tuneable diode laser absorption spectrometry to determine time lags between the start of the labelling and the appearance of ¹³C in soil CO₂ efflux and the amount of ¹³C allocated to soil CO₂ efflux. Isotope composition (δ¹³C) of CO₂ respired by fine roots and soil microbes was measured at several occasions after labelling, together with δ¹³C of bulk root tissue and microbial carbon. Time lags ranged from 0.5 to 1.3 days in beech and oak and were longer in pine (1.6–2.7 days during the active growing season, more than 4 days during the resting season), and the transfer of C to the microbial biomass was as fast as to the fine roots. The amount of ¹³C allocated to soil CO₂ efflux was estimated from a compartment model. It varied between 1 and 21 % of the amount of ¹³CO₂ taken up by the crown, depending on the species and the season. While rainfall exclusion that moderately decreased soil water content did not affect the pattern of carbon allocation to soil CO₂ efflux in beech, seasonal patterns of carbon allocation belowground differed markedly between species, with pronounced seasonal variations in pine and beech. In beech, it may reflect competition with the strength of other sinks (aboveground growth in late spring and storage in late summer) that were not observed in oak. We report a fast transfer of recent photosynthates to the mycorhizosphere and we conclude that the patterns of carbon allocation belowground are species specific and change seasonally according to the phenology of the species.