SummaryClimate change effects on seasonal activity in terrestrial ecosystems are significant and well documented, especially in the middle and higher latitudes. Temperature is a main driver of many plant developmental processes, and in many cases higher temperatures have been shown to speed up plant development and lead to earlier switching to the next ontogenetic stage. Qualitatively consistent advancement of vegetation activity in spring has been documented using three independent methods, based on ground observations, remote sensing, and analysis of the atmospheric CO 2 signal. However, estimates of the trends for advancement obtained using the same method differ substantially. We propose that a high fraction of this uncertainty is related to the time frame analysed and changes in trends at decadal time scales. Furthermore, the correlation between estimates of the initiation of spring activity derived from ground observations and remote sensing at interannual time scales is often weak. We propose that this is caused by qualitative differences in the traits observed using the two methods, as well as the mixture of different ecosystems and species within the satellite scenes.
The terrestrial carbon (C) cycle has received increasing interest over the past few decades, however, there is still a lack of understanding of the fate of newly assimilated C allocated within plants and to the soil, stored within ecosystems and lost to the atmosphere. Stable carbon isotope studies can give novel insights into these issues. In this review we provide an overview of an emerging picture of plant-soil-atmosphere C fluxes, as based on C isotope studies, and identify processes determining related C isotope signatures. The first part of the review focuses on isotopic fractionation processes within plants during and after photosynthesis. The second major part elaborates on plant-internal and plant-rhizosphere C allocation patterns at different time scales (diel, seasonal, interannual), including the speed of C transfer and time lags in the coupling of assimilation and respiration, as well as the magnitude and controls of plant-soil C allocation and respiratory fluxes. Plant responses to changing environmental conditions, the functional relationship between the physiological and phenological status of plants and C transfer, and interactions between C, water and nutrient dynamics are discussed. The role of the C counterflow from the rhizosphere to the aboveground parts of the plants, e.g. via CO<sub>2</sub> dissolved in the xylem water or as xylem-transported sugars, is highlighted. The third part is centered around belowground C turnover, focusing especially on above- and belowground litter inputs, soil organic matter formation and turnover, production and loss of dissolved organic C, soil respiration and CO<sub>2</sub> fixation by soil microbes. Furthermore, plant controls on microbial communities and activity via exudates and litter production as well as microbial community effects on C mineralization are reviewed. The last part of the paper is dedicated to physical interactions between soil CO<sub>2</sub> and the soil matrix, such as CO<sub>2</sub> diffusion and dissolution processes within the soil profile. From the presented evidence we conclude that there exists a tight coupling of physical, chemical and biological processes involved in C cycling and C isotope fluxes in the plant-soil-atmosphere system. Generally, research using information from C isotopes allows an integrated view of the different processes involved. However, complex interactions among the range of processes complicate or impede the interpretation of isotopic signals in CO<sub>2</sub> or organic compounds at the plant and ecosystem level. This is where new research approaches should be aimed at
The variations in δ 13 C in both leaf carbohydrates (starch and sucrose) and CO 2 respired in the dark from the cotyledonary leaves of Phaseolus vulgaris L. were investigated during a progressive drought. As expected, sucrose and starch became heavier (enriched in 13 C) with decreasing stomatal conductance and decreasing p i /p a during the first half (15 d) of the dehydration cycle. Thereafter, when stomata remained closed and leaf net photosynthesis was near zero, the tendency was reversed: the carbohydrates became lighter (depleted in 13 C). This may be explained by increased p i /p a but other possible explanations are also discussed. Interestingly, the variations in δ 13 C of CO 2 respired in the dark were correlated with those of sucrose for both well-watered and dehydrated plants. A linear relationship was obtained between δ 13 C of CO 2 respired in the dark and sucrose, respired CO 2 always being enriched in 13 C compared with sucrose by ≈ 6‰. The whole leaf organic matter was depleted in 13 C compared with leaf carbohydrates by at least 1‰. These results suggest that: (i) a discrimination by ≈ 6‰ occurs during dark respiration processes releasing 13 C-enriched CO 2 ; and that (ii) this leads to 13 C depletion in the remaining leaf material.Abbreviations: A, leaf net CO 2 assimilation; a, fractionation against 13 C for CO 2 diffusion through air; b, net fractionation against 13 C during CO 2 fixation by Rubisco and PEPc; δ 13 C, carbon isotopic composition; ∆, discrimination against 13 C during CO 2 assimilation; d, the term including the fractionation due to CO 2 dissolution, liquid phase diffusion and also discrimination during both respiration and photorespiration; DW, leaf dry weight; dδ 13 C, the difference between CO 2 respired in the dark and plant material in their carbon isotope composition; d∆, variation in modelled discrimination at a given p i /p a relative to a reference value at p i /p a = 0·7; FW, leaf fresh weight; g c , leaf conductance to CO 2 diffusion; HPLC, high-performance liquid chromatography; LMA, leaf mass per area; p a , ambient partial pressure of CO 2 ; p i , intercellular partial pressure of CO 2 ; PEPc, phosphoenolpyruvate carboxylase; PPFD, photosynthetic photon flux density; R PDB , 13 C/ 12 C ratio of standard PDB; R S , 13 C/ 12 C ratio of sample; Rubisco, ribulose 1,5 bisphosphate carboxylase-oxygenase; RWC, leaf relative water content; SW, leaf saturated weight; VPD, vapour pressure deficit.δ 13 C of CO 2 respired in the dark in relation to δ 13 C of leaf carbohydrates in P. vulgaris 523
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