Short-term natural δ<sup>13</sup>C and δ<sup>18</sup>O variations in pools and fluxes in a beech forest: the transfer of isotopic signal from recent photosynthates to soil respired CO<sub>2</sub>

Abstract

Abstract. The fate of photosynthetic products within the plant-soil continuum determines how long the reduced carbon resides within the ecosystem and when it returns back to the atmosphere in the form of respiratory CO 2 . We have tested the possibility of measuring natural variation in δ 13 C and δ 18 O to disentangle the potential times needed to transfer carbohydrates produced by photosynthesis down to trunk, roots and, in general, to belowground up to its further release in the form of soil respiration into the atmosphere in a beech (Fagus sylvatica) forest. We have measured the variation in stable carbon and oxygen isotope compositions in plant material and in soil respired CO 2 every three hours for three consecutive days. Possible steps and different signs of post-photosynthetic fractionation during carbon translocation were also identified. A 12 h-periodicity was observed for variation in δ 13 C in soluble sugars in the top crown leaves and it can be explained by starch day/night dynamics in synthesis and breakdown and by stomatal limitations under elevated vapour pressure deficits. Photosynthetic products were transported down the trunk and mixed with older carbon pools, therefore causing the dampening of the δ 13 C signal variation. The strongest periodicity of 24 h was found in δ 13 C in soil respiration indicating changes in root contribution to the total CO 2 efflux. Other non-biological causes like diffusion fractionation and advection induced by gas withdrawn from the measurement chamber complicate data interpretation on this step of C transfer path. Nevertheless, it was possible to identify the speed of carbohydrates' translocation from the point of assimilation to the trunk breast height because leaf-imprinted enrichment of δ 18 O in soluble sugarsCorrespondence to: O. Gavrichkova (olga.gavrichkova@ibaf.cnr.it) was less modified along the downward transport and was well related to environmental parameters potentially linked to stomatal conductance. The speed of carbohydrates translocation from the site of assimilation to the trunk at breast height was estimated to be in the range of 0.3-0.4 m h −1 .

Allocation dynamics of recently fixed carbon in beech saplings in response to increased temperatures and drought

“…The 13 Clabelfirst appeared in roots (bulk, sugars, organic acids and amino acids) between 3 and 27 h after labelling (i.e., minimum transfer time), implying a velocity of minimum 0.02 m h −1 to a maximum of 0.3 m h −1 (due to the temporal resolution of our sampling, no exact velocity can be given). Such fast transport velocities were also observed in other pulse-labelling experiments with F. sylvatica [for overview see Epron et al (2012)]: 1 m h −1 in Plain et al ( 2009) and Gavrichkova et al (2011), 0.15-0.62 m h −1 in Kuptz et al (2011), 0.22-1.21 m h −1 in Dannoura et al (2011). At the same time, the minimum velocity of 0.02 m h −1 is in the range as found for beech saplings under drought by Ruehr et al (2009).…”

Differences in carbon isotope leaf‐to‐phloem fractionation and mixing patterns along a vertical gradient in mature European beech and Douglas fir

“…Gavrichkova et al . () observed midday and night‐time 13 C‐enrichment in beech leaf sugars from the crown top at a Mountain‐Mediterranean site with high VPD. The absence of night‐time 13 C‐enrichment in our study might be related to the moderate climatic situation, as rainfall events can cause the cessation of the diel oscillation (Brandes et al ., ).…”

“…Several studies compared d 13 C in leaf and trunk phloem WSOM or sugars at breast height in forest trees (Gessler et al, 2009a;Gavrichkova et al, 2011;Offermann et al, 2011). However, a more detailed sampling pattern of higher resolution in the canopy, including the twigs directly adjacent to the sampled leaves, and along the stem is needed to clarify the influence of assimilate export, long-distance transport and canopy mixing on d 13 C WSOM in the trunk phloem.…”

Strong Coupling of Shoot Assimilation and Soil Respiration during Drought and Recovery Periods in Beech As Indicated by Natural Abundance δ13C Measurements

“…This causal relationship between above- and belowground physiological processes – often termed aboveground to belowground coupling – has been shown by girdling and stable isotope tracer studies (Högberg et al, 2001; Kuzyakov and Gavrichkova, 2010). For beech trees, transport velocities of up to 1 m h -1 have been reported (Plain et al, 2009; Gavrichkova et al, 2011; Kuptz et al, 2011), depending on seasonality (Kuptz et al, 2011), tree height (Kuzyakov and Gavrichkova, 2010), and environmental conditions like drought (Barthel et al, 2011). In a forest ecosystem, complexity is added to the straightforward link between assimilatory and respiratory CO 2 fluxes from the soil, e.g., due to different sources contributing to soil CO 2 fluxes (microbial vs. plant-related), different plant species and plant ages as well as changing environmental conditions on diurnal and seasonal scales.…”