Summary: Climate 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 CO₂ 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 CO₂ 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 CO₂ 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. A further part of the paper is dedicated to physical interactions between soil CO₂ and the soil matrix, such as CO₂ diffusion and dissolution processes within the soil profile. Finally, we highlight state-of-the-art stable isotope methodologies and their latest developments. 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 currently impede the interpretation of isotopic signals in CO₂ or organic compounds at the plant and ecosystem level. This review tries to identify present knowledge gaps in correctly interpreting carbon stable isotope signals in the plant-soil-atmosphere system and how future research approaches could contribute to closing these gaps.

The carbon isotope composition (δ¹³C) of CO₂ produced in darkness by intact French bean (Phaseolus vulgaris) leaves was investigated for different leaf temperatures and during dark periods of increasing length. The δ¹³C of CO₂ linearly decreased when temperature increased, from −19‰ at 10°C to −24‰ at 35°C. It also progressively decreased from −21‰ to −30‰ when leaves were maintained in continuous darkness for several days. Under normal conditions (temperature not exceeding 30°C and normal dark period), the evolved CO₂ was enriched in ¹³C compared with carbohydrates, the most ¹³C-enriched metabolites. However, at the end of a long dark period (carbohydrate starvation), CO₂ was depleted in ¹³C even when compared with the composition of total organic matter. In the two types of experiment, the variations of δ¹³C were linearly related to those of the respiratory quotient. This strongly suggests that the variation of δ¹³C is the direct consequence of a substrate switch that may occur to feed respiration; carbohydrate oxidation producing ¹³C-enriched CO₂ and β-oxidation of fatty acids producing ¹³C-depleted CO₂ when compared with total organic matter (−27.5‰). These results are consistent with the assumption that the δ¹³C of dark respired CO₂ is determined by the relative contributions of the two major decarboxylation processes that occur in darkness: pyruvate dehydrogenase activity and the Krebs cycle.