Plants do not rely only on their leaves to feed on carbon dioxide. A new study in Carbon Research reveals that maize roots can act as an active "second mouth" for carbon, taking up CO2 from the soil and helping regulate the carbon cycle between soil, plants and the air.
A new look at plant carbon
For decades, biology textbooks have emphasized that plants absorb CO2 only through chlorophyll containing leaves. The new research challenges this simplified view by showing that roots can also absorb CO2 from the soil atmosphere under certain conditions. This underground carbon intake can contribute noticeably to plant biomass and may change how scientists think about carbon balances in croplands.
"Plants are far more flexible in their carbon nutrition than we tend to assume," said study author Zalim Islamovich Dudarov of Kabardino Balkarian State University. "Our experiments show that the root system is not just a passive support structure but an active regulator of carbon flows between soil, plants and the atmosphere." Coauthor Amiran Khabidovich Zanilov added that understanding this hidden pathway "opens up new possibilities for managing agroecosystems to store more carbon in soils while maintaining crop productivity."
A two chamber plant "climate lab"
To uncover root CO2 uptake, the team built a custom installation that physically separates the aboveground and belowground parts of maize plants into two airtight chambers. Transparent polycarbonate boxes house the leaves in an upper chamber and the roots in a lower chamber filled with sandy substrate, with sensors monitoring CO2, oxygen, temperature, humidity, light and soil moisture in each compartment. This vertically differentiated system allows continuous tracking of carbon dioxide fluxes in the soil plant atmosphere system without mixing the air between leaf and root zones.
During a 40 day model experiment with 19 maize plants, the researchers tested four environmental "modes" that mimic common field conditions. These included day night light cycles, the addition of ammonium nitrate fertilizer, changes in light intensity and stepwise increases in CO2 concentration around the leaves. Data were logged in near real time through a cloud connected monitoring platform, enabling detailed analysis of how the two plant compartments responded.
Roots as an alternative CO2 intake
In the basic light dark mode, the team found a strong inverse relationship between CO2 changes in the leaf and root chambers over each 24 hour cycle. At night, leaves released CO2 through respiration while CO2 levels in the root chamber declined, indicating net CO2 absorption by the roots. During the day, as photosynthesis in the leaves consumed atmospheric CO2 and concentrations around the foliage dropped, the root system switched on an alternative uptake mechanism that drew CO2 from the soil side.
The researchers identified a threshold range of about 417 to 367 parts per million of CO2 in the leaf chamber as a critical trigger. When CO2 around the leaves fell to this level, root CO2 absorption became active, suggesting a self regulating response that helps the plant maintain its carbon supply under low atmospheric CO2 conditions. When the team artificially boosted CO2 in the leaf chamber up to 1500 parts per million, root CO2 uptake stopped and the CO2 concentration in the root zone began to rise instead.
Nitrogen fertilizer shifts carbon pathways
The study also examined how a standard dose of ammonium nitrate fertilizer affected plant carbon behavior. In the first three days after fertilization, the leaves respired more at night and their capacity to absorb CO2 during the day declined, with the minimum CO2 level reached in the leaf chamber rising compared with unfertilized conditions. At the same time, the root zone showed enhanced CO2 absorption over the following days, lowering CO2 concentrations in the lower chamber.
These patterns suggest that mineral nitrogen fertilizer can redirect part of the plant's carbon intake from the leaves to the roots. The authors note that this shift may be linked to changes in soil microbial activity and the decomposition of organic matter, with implications for soil organic carbon stocks. They argue that to maintain or increase soil carbon under intensive fertilizer use, additional carbon rich inputs such as crop residues or manures may be needed.
Light intensity and "holding their breath"
Light intensity turned out to matter not only for photosynthesis rate but also for how quickly plants responded to the switch between day and night. When the illumination in the leaf chamber was doubled, the onset of nighttime respiration in leaves was delayed by about 80 minutes after lights off, and the return to daytime CO2 uptake after lights on was also postponed, though to a lesser extent. Under stronger light, plants absorbed CO2 more rapidly and reached their daytime minimum CO2 level earlier, while recovering a higher carbon absorption capacity comparable to the control conditions.
According to the authors, these delays may reflect how plants manage internally stored organic compounds produced under bright light before shifting fully into respiring mode. The findings indicate that light regimes, nitrogen inputs and atmospheric CO2 levels jointly shape how much carbon flows into plants through leaves versus roots, and how much remains in or returns to the soil.
Rethinking the soil plant atmosphere carbon loop
Drawing together the experimental results, the team proposes an updated conceptual model of the carbon cycle in agroecosystems. In this view, soil is not only a major source of CO2 from respiration but also a dynamic reservoir from which plants can directly recapture CO2 through their roots, especially when atmospheric CO2 is relatively low. Part of the carbon that enters plants via leaves or roots then returns to the soil as root exudates, residues and humus, helping to replenish soil carbon stocks and potentially supporting climate mitigation goals.
The researchers emphasize that their work was conducted in a controlled model system, but they see strong potential for applying these insights to real fields. Future studies could explore different crops, soils and management strategies to harness root carbon nutrition for boosting soil carbon sequestration while sustaining yields in the face of climate change.
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Journal reference: Dudarov, Z.I., Zanilov, A.K., Altudov, Y.K. et al. Influence of external factors on the behavior of CO2 in the root system of plants in a model experiment. Carbon Res. 4, 66 (2025).
https://doi.org/10.1007/s44246-025-00237-1
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About Carbon Research
The journal Carbon Research is an international multidisciplinary platform for communicating advances in fundamental and applied research on natural and engineered carbonaceous materials that are associated with ecological and environmental functions, energy generation, and global change. It is a fully Open Access (OA) journal and the Article Publishing Charges (APC) are waived until Dec 31, 2025. It is dedicated to serving as an innovative, efficient and professional platform for researchers in the field of carbon functions around the world to deliver findings from this rapidly expanding field of science. The journal is currently indexed by Scopus and Ei Compendex, and as of June 2025, the dynamic CiteScore value is 15.4.
