SoyFACE for 3D soybean canopy modelling under elevated carbon dioxide

Atmospheric carbon dioxide (CO₂) concentrations are expected to reach 550 ppm by 2050. The elevated CO₂ levels are forecasted to increase photosynthetic carbon fixation, reduce plant respiration and lead to more leaf growth and crop yields. How exactly yields are expected to change rely on numerous physiological, canopy architecture and environmental factors.

Drs Qingfeng Song and Xin-Guang Zhu with two colleagues from the Chinese Academy of Sciences, University of Illinois at Urbana-Champaign and Lancaster University integrated a 3D canopy photosynthesis model with previous experimental data of soybeans grown under ambient and elevated CO₂ for a whole growing season. The mathematical model allowed the researchers to dissect the contribution of different acclimation responses (e.g. canopy architecture, light, photosynthesis), found that soybean leaves grew 1.1–1.9 times larger under elevated CO₂ and 17.2% of total plant CO₂ uptake was due to canopy structure.

The researchers used previous measurements from the SoyFACE (free‐air CO2 enrichment) facility at the University of Illinois at Urbana-Champaign where soybeans were grown at ambient (370 ppm) and elevated (550 ppm) CO₂ conditions. Researchers reported a 15% increase in soybean yields at 550 ppm and the increase in crop yield and biomass was suggested to be attributed to an increase in total CO₂ uptake of the whole canopy (i.e. gross primary productivity; GPP).

Song and colleagues used a sunlit–shaded canopy photosynthesis model, dividing leaves in a canopy dynamically into sunlit and shaded groups, and simulated the light environment of soybean plants for 267 days (entire growing season) at 3 day intervals. The rather complex mathematical model detangled the environmental responses (e.g. temperature, light, CO₂ concentration) of C₃ leaf photosynthesis. To name a few parameters, the model included consisted of calculating the stomatal conductance, leaf temperature, the maximal rate of carboxylation under Ribulose-1,5-bisphosphate (RuBP) and CO₂ saturation, leaf area index, leaf nitrogen content in different layers of the canopy, leaf intercellular CO2 concentration, and potential photosynthetic electron transport rate.

The leaf size was 1.1–1.9 times larger, leaf lengths and leaf widths increased by 10–90 % for different leaves and the internode distances increased by 6% under elevated CO₂ of soybean plants. The CO₂ “fertilization effect” explained 76.7% of GPP (i.e. total CO₂ uptake of the whole canopy), followed by canopy architecture (17.2 %). The modelling showed that there was a synergetic effect of CO₂ and light on GPP that led to different GPP on cloudy and bright sunny days at different developmental stages. 

“[T]his study presents a new integrative framework that coupled an explicit 3-D soybean architecture model with a ray tracing algorithm and a leaf photosynthesis model to compute whole canopy photosynthetic response under different environments,” Song and colleagues wrote. 

“[A]ssuming a fixed root:shoot ratio and a fixed fraction of dark respiration, the model predicted a 21.4 % increase in above-ground biomass when increasing [CO₂] from 370 to 550 ppm.”

The most differences were found in early developmental stages and sunny or cloudy days greatly impact the plant’s total CO₂ uptake.

“LAI [leaf area index] increased by about 19 % under elevated [CO₂] which caused and increase in total canopy respiration; however, the leaf nitrogen content on a leaf area basis decreases by an average of 3.9 % in crops under elevated [CO₂] as result of Rubisco acclimation”, Song and colleagues added. 

This new model allows scientists to better understand and predict soybean growth in a climate change in a few decades. Whilst crops might grow quicker or larger under an elevated CO₂ environment, crop nutritional values might decrease.