Abstract
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.
Highlights
Soil CO2 efflux is the major biospheric source of carbon from terrestrial ecosystems to the atmosphere and it accounts for a large fraction (40–70 %) of total ecosystem respiration (Janssens et al, 2001)
We demonstrated that pulse labelling of field-grown trees with 13CO2 and high frequency of measurement of 13C in respiratory efflux during the chase period using laser-based infrared gas analysers is a powerful approach to study seasonal variations of carbon transfer
The aim of this study was to assess the belowground transfer of recently assimilated carbon in two deciduous broadleaved species and one coniferous evergreen species at distinct phenological stages during the growing season. 13CO2 recovery in soil CO2 efflux was recorded for 3 weeks by tuneable diode laser absorption spectrometers (TDLAS) after pulse labelling of 8–10 m tall trees and compared to 13C recovery in both root and microbial biomass and their respective respiratory CO2
Summary
Soil CO2 efflux is the major biospheric source of carbon from terrestrial ecosystems to the atmosphere and it accounts for a large fraction (40–70 %) of total ecosystem respiration (Janssens et al, 2001). It includes the respiration of microorganisms and soil meso- or macroorganisms involved in the mineralisation of soil organic matter (“heterotrophic respiration”), and the use of recently assimilated substrates that fuels root metabolism and rhizospheric microorganisms that. The total flux of soil CO2 efflux and its 13C composition was correlated to short-term fluctuations in microclimate confirming a close coupling between photosynthesis and soil CO2 efflux (Bowling et al, 2008; Ekblad et al, 2005; Marron et al, 2009; Mortazavi et al, 2005)
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