Understanding inhibition of daytime ecosystem respiration: Insights into diurnal dynamics and carbohydrate physiology

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Inhibition of daytime ecosystem respiration (Re) has been widely discussed as a potential bias in current estimates of ecosystem fluxes. Recent models and isotopic flux partitioning (IFP) approaches revealed 5 to 25% inhibition in plant daytime respiration. However, the diurnal dynamics and physiological reasoning behind inhibited Re remain unclear. Here, we present alternative estimates of CO2 fluxes using IFP along with standard nighttime flux partitioning (SFP) methods on a diurnal scale, collected in a shortleaf pine forest in East Texas. The average Re-IFP indicated about 31% lower than Re-SFP. The intensity of inhibition increased over the day, reaching 77% during the afternoon (between 1 to 8 pm). The greater inhibition of Re-IFP in the late afternoon arose from changes in soil respiration (Rs-IFP), whereas aboveground plant respiration (Ragp-IFP) was consistently about 5-fold lower than Ragp-SFP. Rs-IFP also decreased in the afternoon, but the difference compared to Rs-SFP was smaller (approximately 2-fold) than in the Ragp. This implies that our current estimate may underestimate the carbon allocated in the aboveground compartment of the plant. The relationship between Ragp and GEP exhibited some counterclockwise hysteresis (higher Ragp in the afternoon than morning at the same GEP) in the SFP data and during days with high VPD in IFP data, whereas during low VPD days, the relationship was nearly linear (no difference between morning and afternoon Ragp). The non zero Ragp at zero GEP (the intercept of the Ragp-GEP regression) indicates the use of stored carbohydrate substrates. The decoupling of the Ragp-IFP from GEPIFP during high VPD days may indicate the activation of additional carbon dissipation pathways (lipid synthesis, secondary compounds or futile cycles) in order to protect chloroplasts against the adverse effect of soluble carbohydrate buildup. AGU OSPA iPoster Dohee Kim, Benju Baniya, Malik Nkrumah, Asko Noormets (2024). “Understanding inhibition of daytime ecosystem respiration: Effect of carbon availability in the Kok effect”, Abstract (B41B 06), 2024 AGU Fall Meeting, Washington, DC, 9-13 Dec.

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