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

Drought down-regulates above- and belowground carbon fluxes, however, the resilience of trees to drought will also depend on the speed and magnitude of recovery of these above- and belowground fluxes after re-wetting. Carbon isotope composition of above- and belowground carbon fluxes at natural abundance provides a methodological approach to study the coupling between photosynthesis and soil respiration (SR) under conditions (such as drought) that influence photosynthetic carbon isotope discrimination. In turn, the direct supply of root respiration with recent photoassimilates will impact on the carbon isotope composition of soil-respired CO2. We independently measured shoot and soil CO2 fluxes of beech saplings (Fagus sylvatica L.) and their respective δ13C continuously with laser spectroscopy at natural abundance. We quantified the speed of recovery of drought stressed trees after re-watering and traced photosynthetic carbon isotope signal in the carbon isotope composition of soil-respired CO2. Stomatal conductance responded strongly to the moderate drought (-65%), induced by reduced soil moisture content as well as increased vapor pressure deficit. Simultaneously, carbon isotope discrimination decreased by 8‰, which in turn caused a significant increase in δ13C of recent metabolites (1.5-2.5‰) and in δ13C of SR (1-1.5‰). Generally, shoot and soil CO2 fluxes and their δ13C were in alignment during drought and subsequent stress release, clearly demonstrating a permanent dependence of root respiration on recently fixed photoassimilates, rather than on older reserves. After re-watering, the drought signal persisted longer in δ13C of the water soluble fraction that integrates multiple metabolites (soluble sugars, amino acids, organic acids) than in the neutral fraction which represents most recently assimilated sugars or in the δ13C of SR. Nevertheless, full recovery of all aboveground physiological variables was reached within 4 days - and within 7 days for SR - indicating high resilience of (young) beech against moderate drought.

Soil H2¹8O labelling reveals the effect of drought on C¹8OO fluxes to the atmosphere.

Above- and belowground processes in plants are tightly coupled via carbon and water fluxes through the soil-plant-atmosphere system. The oxygen isotopic composition of atmospheric CO2 and water vapour (H2Ov) provides a valuable tool for investigating the transport and cycling of carbon and water within this system. However, detailed studies on the coupling between ecosystem components and environmental drivers are sparse. Therefore, we conducted a H2 (18)O-labelling experiment to investigate the effect of drought on the speed of the link between below- and aboveground processes and its subsequent effect on C(18)OO released by leaves and soils. A custom-made chamber system, separating shoot from soil compartments, allowed separate measurements of shoot- and soil-related processes under controlled conditions. Gas exchange of oxygen stable isotopes in CO2 and H2Ov served as the main tool of investigation and was monitored in real time on Fagus sylvatica saplings using laser spectroscopy. H2(18)O-labelling showed that drought caused a slower transport of water molecules from soil to shoot, which was indicated by its direct derivation from independently measured concentrations and (18)O/(16)O ratios of CO2 and H2Ov, respectively. Furthermore, drought reduced the (18)O equilibrium between H2O and CO2 at the shoot level, resulting in less-enriched C(18)OO fluxes from leaf to atmosphere compared with control plants. Compared with the shoot, (18)O equilibrium was not instantaneous in the soil and no drought effect was apparent.

Carbon isotope discrimination during branch photosynthesis of Fagus sylvatica: field measurements using laser spectrometry.

On-line measurements of photosynthetic carbon isotope discrimination ((13)Δ) under field conditions are sparse. Hence, experimental verification of the natural variability of instantaneous (13)Δ is scarce, although (13)Δ is, explicitly and implicitly, used from leaf to global scales for inferring photosynthetic characteristics. This work presents the first on-line field measurements of (13)Δ of Fagus sylvatica branches, at hourly resolution, using three open branch bags and a laser spectrometer for CO2 isotopologue measurements (QCLAS-ISO). Data from two August/September field campaigns, in 2009 and 2010, in a temperate forest in Switzerland are shown. Diurnal variability of (13)Δ was substantial, with mean diurnal amplitudes of ~9‰ and maximum diurnal amplitudes of ~20‰. The highest (13)Δ were generally observed during early morning and late afternoon, and the lowest (13)Δ during midday. An assessment of propagated standard deviations of (13)Δ demonstrated that the observed diurnal variation of (13)Δ was not a measurement artefact. Day-to-day variations of (13)Δ were summarized with flux-weighted daily means of (13)Δ, which ranged from 15‰ to 23‰ in 2009 and from 18‰ to 29‰ in 2010, thus displaying a considerable range of 8-11‰. Generally, (13)Δ showed the expected negative relationship with intrinsic water use efficiency. Diurnal and day-to-day variability of (13)Δ was, however, always better predicted by that of net CO2 assimilation, especially in 2010 when soil moisture was high and vapour pressure deficit was low. Stomatal control of leaf gas exchange, and consequently (13)Δ, could only be identified under drier conditions in 2009.

Carbon isotope discrimination during branch photosynthesis of Fagus sylvatica: a Bayesian modelling approach.

Field measurements of photosynthetic carbon isotope discrimination ((13)Δ) of Fagus sylvatica, conducted with branch bags and laser spectrometry, revealed a high variability of (13)Δ, both on diurnal and day-to-day timescales. We tested the prediction capability of three versions of a commonly used model for (13)Δ [called here comprehensive ((13)(Δcomp)), simplified ((13) Δsimple) and revised ((13)(Δrevised)) versions]. A Bayesian approach was used to calibrate major model parameters. Constrained estimates were found for the fractionation during CO(2) fixation in (13)(Δcomp), but not in (13)(Δsimple), and partially for the mesophyll conductance for CO(2)(gi). No constrained estimates were found for fractionations during mitochondrial and photorespiration, and for a diurnally variable apparent fractionation between current assimilates and mitochondrial respiration, specific to (13)(Δrevised). A quantification of parameter estimation uncertainties and interdependencies further helped explore model structure and behaviour. We found that (13)(Δcomp) usually outperformed (13)(Δsimple) because of the explicit consideration of gi and the photorespiratory fractionation in (13)(Δcomp) that enabled a better description of the large observed diurnal variation (≈9‰) of (13)Δ. Flux-weighted daily means of (13)Δ were also better predicted with (13)(Δcomp) than with (13)(Δsimple).

The diel imprint of leaf metabolism on the δ13 C signal of soil respiration under control and drought conditions.

Recent (13) CO(2) canopy pulse chase labeling studies revealed that photosynthesis influences the carbon isotopic composition of soil respired CO(2) (δ(13) C(SR)) even on a diel timescale. However, the driving mechanisms underlying these short-term responses remain unclear, in particular under drought conditions. The gas exchange of CO(2) isotopes of canopy and soil was monitored in drought/nondrought-stressed beech (Fagus sylvatica) saplings after (13) CO(2) canopy pulse labeling. A combined canopy/soil chamber system with gas-tight separated soil and canopy compartments was coupled to a laser spectrometer measuring mixing ratios and isotopic composition of CO(2) in air at high temporal resolution. The measured δ(13) C(SR) signal was then explained and substantiated by a mechanistic carbon allocation model. Leaf metabolism had a strong imprint on diel cycles in control plants, as a result of an alternating substrate supply switching between sugar and transient starch. By contrast, diel cycles in drought-stressed plants were determined by the relative contributions of autotrophic and heterotrophic respiration throughout the day. Drought reduced the speed of the link between photosynthesis and soil respiration by a factor of c. 2.5, depending on the photosynthetic rate. Drought slows the coupling between photosynthesis and soil respiration and alters the underlying mechanism causing diel variations of δ(13) C(SR).