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Global decadal variability of plant carbon isotope discrimination and its link to gross primary production.

ABSTRACT: Carbon isotope discrimination (Δ¹³ C) in C₃ woody plants is a key variable for the study of photosynthesis. Yet how Δ¹³ C varies at decadal scales, and across regions, and how it is related to gross primary production (GPP), are still incompletely understood. Here we address these questions by implementing a new Δ¹³ C modelling capability in the land-surface model JULES incorporating both photorespiratory and mesophyll-conductance fractionations. We test the ability of four leaf-internal CO₂ concentration models embedded in JULES to reproduce leaf and tree-ring (TR) carbon isotopic data. We show that all the tested models tend to overestimate average Δ¹³ C values, and to underestimate interannual variability in Δ¹³ C. This is likely because they ignore the effects of soil water stress on stomatal behavior. Variations in post-photosynthetic isotopic fractionations across species, sites and years, may also partly explain the discrepancies between predicted and TR-derived Δ¹³ C values. Nonetheless, the "least-cost" (Prentice) model shows the lowest biases with the isotopic measurements, and lead to improved predictions of canopy-level carbon and water fluxes. Overall, modelled Δ¹³ C trends vary strongly between regions during the recent (1979-2016) historical period but stay nearly constant when averaged over the globe. Photorespiratory and mesophyll effects modulate the simulated global Δ¹³ C trend by 0.0015 ± 0.005‰ and -0.0006 ± 0.001‰ ppm^-1 , respectively. These predictions contrast with previous findings based on atmospheric carbon isotope measurements. Predicted Δ¹³ C and GPP tend to be negatively correlated in wet-humid and cold regions, and in tropical African forests, but positively related elsewhere. The negative correlation between Δ¹³ C and GPP is partly due to the strong dominant influences of temperature on GPP and vapor pressure deficit on Δ¹³ C in those forests. Our results demonstrate that the combined analysis of Δ¹³ C and GPP can help understand the drivers of photosynthesis changes in different climatic regions.

SUBMITTER: Lavergne A

PROVIDER: S-EPMC9298043 | biostudies-literature |

REPOSITORIES: biostudies-literature

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Project description:Microbial transformations of organic carbon (OC) generate a large flux of CO2 into the atmosphere and influence the C balance of terrestrial and aquatic ecosystems. Yet, inherent heterogeneity in natural environments precludes direct quantification of multiple microbial C fluxes that underlie CO2 production. Here we used a continuous flow bioreactor coupled with a stable C isotope analyzer to determine the effects of temperature and C availability (cellobiose concentration) on C fluxes and 13C discrimination of a microbial population growing at steady-state in a homogeneous, well-mixed environment. We estimated C uptake affinity and C use efficiency (CUE) to characterize the physiological responses of microbes to changing environmental conditions. Temperature increased biomass-C specific respiration rate and C uptake affinity at lower C availability, but did not influence those parameters at higher C availability. CUE decreased non-linearly with increasing temperature. The non-linear, negative relationship between CUE and temperature was more pronounced under lower C availability than under relatively high C availability. We observed stable isotope fractionation between C substrate and microbial biomass C (7~12‰ depletion), and between microbial biomass and respired CO2 (4~10‰ depletion). Microbial discrimination against 13C-containing cellobiose during C uptake was influenced by temperature and C availability, while discrimination during respiration was only influenced by C availability. Shifts in C uptake affinity with temperature and C availability may have modified uptake-induced 13C fractionation. By stressing the importance of C availability on temperature responses of microbial C fluxes, C uptake affinity, CUE, and isotopic fractionation, this study contributes to a fundamental understanding of C flow through microbes. This will help guide parameterization of microbial responses to varying temperature and C availability within Earth-system models.

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Global decadal variability of plant carbon isotope discrimination and its link to gross primary production.

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