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Coupling aboveground and belowground activities using short term fluctuations in 13C composition of soil respiration

Abstract : There is a growing amount of evidence that belowground processes in forest ecosystems are tightly coupled to aboveground activities. Soil CO2 efflux, the largest flux of CO2 to the atmosphere, is dominated by root respiration and by respiration of microorganisms that find the carbohydrates required to fulfil their energetic costs in the rhizosphere. A close coupling between aboveground photosynthetic activity and soil CO2 efflux is therefore expected. The isotopic signature of photosynthates varies with time because photosynthetic carbon isotope discrimination is dynamically controlled by environmental factors. This temporal variation of δ13C of photosynthate is thought to be transferred along the tree-soil continuum and it will be retrieved in soil CO2 efflux after a time lag that reflects the velocity of carbon transport from canopy to belowground. However, isotopic signature of soil CO2 efflux is not solely affected by photosynthetic carbon discrimination, bur also by post photosynthetic fractionation, and especially by fractionation processes affecting CO2 during the transport from soil layers to surface. Tunable diode laser spectrometry is a useful tool to quantify short-term variation in δ13C of soil CO2 efflux and of CO2 in the soil atmosphere. We set up hydrophobic tubes to measure the vertical profile of soil CO2 concentration and its δ13C composition in a temperate beech forest, and we monitored simultaneously δ13C of trunk and soil CO2 efflux, δ13C of phloem exudate and δ13C of leaf sugars. We evidenced that temporal changes in δ13C of soil CO2 and soil CO2 efflux reflected changes in environmental conditions that affect photosynthetic discrimination and that soil CO2 was 4.4‰ enriched compared to soil CO2 efflux according to diffusion fractionation. However, this close coupling can be disrupted when advective transport of CO2 took place. We also reported evidences that temporal variations in the isotopic composition of soil CO2 efflux reflect changes in partitioning between the respiration sources. This was particularly clear for eucalypt trees planted on a C4 soil in coastal Congo where soil CO2 efflux were less 13C depleted between 11am and 3pm when the xylem sap flux was maximum. This last result outlines that not only canopy photosynthesis but also canopy transpiration may affect the isotopic composition of soil CO2 efflux and should be considered in mechanistic models that account for the dynamics of soil CO2 efflux and its partitioning.
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Submitted on : Saturday, June 6, 2020 - 5:20:52 AM
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  • HAL Id : hal-02808464, version 1
  • PRODINRA : 180848

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Daniel Epron, Florian Parent, Charlotte Grossiord, Caroline Plain, Bernard Longdoz, et al.. Coupling aboveground and belowground activities using short term fluctuations in 13C composition of soil respiration. AGU Fall Meeting, Dec 2011, San Francisco, United States. ⟨hal-02808464⟩

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