Modeling the partitioning of assimilated C along the soil-plant-atmosphere continuum based on a 13C labeling experiment

Modeling and quantification of carbon (C) allocation through the soil-plant-atmosphere continuum (SPAC) has received increasing attention in recent years. Although advanced imaging and numerical methods have boosted our knowledge, we still lack an experimental and mechanistic understanding of C flux and its partitioning across the SPAC. Here we combined a ¹³C pulse labeling technique with modeling of C transport across the SPAC to describe the flow of newly assimilated C from shoots to roots, to soils and then respired back to the atmosphere. To do so, different plants - maize (Zea mays L.), soybean (Glycine max (L.) merr. cv. sohae), and wheat (Triticum aestivum L.) - were exposed to a ¹³CO₂ pulse for 2 hoursduring daytime. A CO₂ Isotope Analyzer (CCIA-38d-EP, Los Gatos Research) was used to continuously monitor ¹³CO₂ flux from the soils. Subsequently, after harvest the respective ¹³C contents of shoot and root biomass and of the soil was quantified.

To model the C fluxes, we developed a simple multi-compartment domain, representing the SPAC. The SPAC was envisioned as four interconnected horizontal compartments namely atmosphere, shoots, roots, and soil compartment. The shoots and roots compartment were further simplified in three vertical compartments (phloem, storage, and structural pool). The C transport between these pools was represented by constant rates. These rates were inversely estimated by adjusting the model parameters to best reproduce the measured C flux and C contents in the various compartments.   

Our model described the allocation and transport of ¹³C within the shoots, roots, and soil well. The best-fitted coefficients of the model were reproducible among different replications of the same plant species. We also checked the sensitivity of our model to its parameters and observed a good sensitivity to most of the model parameters. In particular, our model was very sensitive to C loading and unloading in the phloem and also root exudation rates. The results of our study show that the combination of tracing ¹³C and modeling of ¹³C transport across SPAC is a promising tool to study C flux and its partitioning across SPAC.