Seasonal variations of belowground carbon transfer assessed by in situ <sup>13</sup>CO<sub>2</sub> pulse labelling of trees

Abstract

Abstract. Soil CO 2 efflux is the main source of CO 2 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 13 CO 2 pulse labelling of the entire crown of three different tree species (beech, oak and pine) was carried out at distinct phenological stages. Excess 13 C in soil CO 2 efflux was tracked using tuneable diode laser absorption spectrometry to determine time lags between the start of the labelling and the appearance of 13 C in soil CO 2 efflux and the amount of 13 C allocated to soil CO 2 efflux. Isotope composition (δ 13 C) of CO 2 respired by fine roots and soil microbes was measured at several occasions after labelling, together with δ 13 C 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 13 C allocated to soil CO 2 efflux was estimated from a compartmentCorrespondence to: D. Epron (daniel.epron@scbiol.uhp-nancy.fr) model. It varied between 1 and 21 % of the amount of 13 CO 2 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 CO 2 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.

Carbon allocation and carbon isotope fluxes in the plant-soil-atmosphere continuum: a review

“…In certain studies, seasonal changes in belowground C allocation had no effect on the time lag between assimilation and use of assimilates in belowground respiration (Horwath et al, 1994;Högberg et al, 2010), suggesting that phloem path length and structural differences were the main determinants of C transfer velocity. In contrast, other studies reported considerable variation of the time lag during the growing season in the same trees Wingate et al 2010;Dannoura et al, 2011;Epron et al, 2011;Kuptz et al, 2011a) (Fig. 2).…”

Carbon flux to woody tissues in a beech/spruce forest during summer and in response to chronic O<sub>3</sub> exposure

“…However, soil-respired CO 2 , which includes large contributions by root-respired CO 2 of unlabeled neighboring trees and heterotrophic soil respiration (Högberg et al, 2001;Andersen et al, 2005Andersen et al, , 2010, was reduced in δ 13 C by 1.5 to 3 ‰. Hence, beech fine roots and associated micro organisms appear to be a relatively strong sink for recently fixed C during summer (Högberg et al, 2001;Plain et al, 2009;Epron et al, 2011). Slightly pronounced shifts in soil-respired CO 2 under 2 × O 3 fit well with previously reported increases in fine-root turn-over of beech under long-term O 3 exposure (Nikolova et al, 2010).…”

Progress and challenges in using stable isotopes to trace plant carbon and water relations across scales

“…This is especially important to test hypotheses about temporal δ 13 C R patterns, for example, if δ 13 C R dynamics are heavily influenced by a sole component flux, resulting in a poorly mixed ecosystem source signal. Similarly, species-specific transport times of recent assimilates (Epron et al, 2011) can potentially delay the photosynthetic response signal in δ 13 C R , or abiotic phenomena (following section) can obscure component iso-fluxes (e.g. Ekblad et al, 2005;Knohl et al, 2005).…”

“…Advances in our understanding of ecosystem processes through canopy labelling include assessing photosyntheticsoil-respiration coupling strength (Steinmann et al, 2004;Högberg et al, 2008;Bahn et al, 2009;Gamnitzer et al, 2009Gamnitzer et al, , 2011, carbon allocation patterns Epron et al, 2011), and shading impacts (Warren et al, 2012), to name a few. Quantitative methods of canopy labelling in connection with on-line tracer measurement techniques (Sect.…”

“…The challenge to advance our understanding of δ 13 C R lies in identifying and quantifying these fluxes and isotopic signatures of important ecosystem components (e.g. Unger et al, 2010a;Epron et al, 2011;Barbour et al, 2011a). This is especially important to test hypotheses about temporal δ 13 C R patterns, for example, if δ 13 C R dynamics are heavily influenced by a sole component flux, resulting in a poorly mixed ecosystem source signal.…”