Differentiating Structural and Non-Structural Carbon Using Ecosystem Carbon Flux Measurements

Le résumé fourni par la source

Plants allocate photosynthetically fixed carbon (C) between immediate metabolic needs, biomass production, and non-structural carbon (NSCs), which can be further partitioned between storage, reproduction and exudates. The stored NSC, comprising soluble sugars, starch, and lipids, is a buffer between carbon supply and demand. Currently, these pools are not well captured in ecosystem models because of recognized deficiencies in both conceptual representations and relevant observational data. Here we present an ecosystem-level estimate of C allocation to or from the NSC pool (FNSC) as the difference between gross primary productivity (GPP), plant respiration (Rp), and biomass production (BP) in a shortleaf pine forest using a mass balance approach as well as integrating analytical Temporally Integrated Mass Balance Carbon Allocation (TIMBCA) model to calculate above and belowground fractions.Over three years of study, annual GPP remained relatively stable (1827-2053 g C m⁻² yr⁻¹) but FNSC varied. FNSC ranged from 320-478 g C m⁻² yr⁻¹ in dry years (2022-2023) and 298 g C m⁻² yr⁻¹ in the wet year (2024), representing 15-24% of annual GPP. Moreover, annual FNSC exceeded biomass production (BP) in the dry years (BP=170-233 g C m-² yr-¹) but not in the wet year (331 g C m-² yr-¹). FNSC was inversely related to both BP and plant water status. While FNSC varied considerably among years, how much of it remains in plants as storage requires further investigation. Preliminary tissue NSC analyses in 2024 suggested that only exuded into the soil, consistent with observed tight coupling between GPP and soil heterotrophic respiration (Rh).The TIMBCA model showed consistent C translocation within years, with estimated structural growth was less than available assimilates from GPP (NSC surplus of up to 35 g C m⁻² month⁻¹), while in the wet year, growth exceeded C supply from GPP, presumably drawing on stored NSC. These findings are consistent with the surplus carbon hypothesis, which posits sink strength-driven C allocation, and can serve as the conceptual foundation on which to build next-generation carbon allocation algorithms.

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