Nonlinear and phenology-specific responses of tomato to bicarbonate inputs: identifying physiological thresholds within a Critical Zone framework

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Soil inorganic carbon (SIC), particularly HCO 3 - , represents a potentially overlooked component of carbon cycling within the Earth’s Critical Zone (CZ). Although previous isotopic studies have demonstrated that higher plants can utilize bicarbonate under specific physiological conditions, the developmental windows and threshold behaviors governing plant responses to SIC inputs remain poorly understood. Here, we established a full life-cycle experimental framework using tomato to quantify stage-dependent physiological responses to root-applied bicarbonate and to identify nonlinear thresholds across germination, vegetative growth, flowering, and fruiting. By integrating germination kinetics, root morphology, biomass allocation, chlorophyll formation, fruit quality, and tissue carbon content through a weighted membership function model, we identified distinct optimal HCO 3 - input ranges for each phenophase. Moderate bicarbonate levels enhanced germination performance (8–11 g NaHCO 3 kg -1 soil), vegetative biomass and chlorophyll formation (1–7 g kg -1 ), reproductive traits (1–4 g kg -1 ), and fruit yield and carbon-rich organic matter (6–12 g kg -1 ), whereas excessive concentrations induced strong inorganic-carbon stress and sharply reduced survival. Tissue carbon content increased consistently within viable ranges, indicating enhanced carbon accumulation associated with bicarbonate exposure. While the present study does not directly quantify bicarbonate uptake or SOC formation, the observed physiological patterns are consistent with previously established mechanisms and suggest that SIC availability may influence plant carbon pools in CZ-relevant environments. These findings provide a quantitative basis for identifying phenology-specific bicarbonate thresholds and offer new insights into how SIC dynamics may interact with plant development within the broader CZ carbon cycling framework.