By Hervé Maumus-Hue
Carbon dioxide (CO2): The Basics
Introduction
1 – Different types of plants, different biological processes
2 – C3 plants: the importance of CO2 supplementation
3 – Supplying CO2 – Considerations
4 – Injection techniques
5 – Estimating the cost of CO2
6 – Injecting CO2 can be more cost effective than supplement lighting
7 – Environmental considerations
Conclusion
Introduction
CO2 supplementation is a common and highly beneficial practice in modern horticulture. This document will explain why, when, and how supplementation is important and what the potential outcomes can be.
1 – Different types of plants, different biological processes
Plants are characterized by how they acquire CO2 and what type of carbohydrates they create during photosynthesis. Plants’ photosynthesis pathways are divided into 3 categories: C3, C4, and CAM. C3 accounts for 95% of the species on earth, the rest are equally divided between C4 and CAM.
- C3 :
- Classified as such because they produce a compound with 3 carbon atoms as a product of photosynthesis.
- This type of plant wants CO2: it boosts the production of carbohydrates available for growth by opening the stomata.
- C4:
- Photosynthesis produces a compound/molecule with 4 carbon atoms.
- Includes mostly grass-like plants like corn and cereal grains.
- CO2 is stored indirectly in internal holding sites.
- Stomata open only when they need to restore CO2. This particular timing makes the CO2 supplementation almost useless.
- CAM:
- Stands for Crassulacean Acid Metabolism.
- Fixation of the carbon evolved to adapt to arid conditions.
- Open their stomata at night to collect CO2 instead of during the day.
- Typically Cactus and succulents.
Photosynthesis Reaction: The Importance of CO2
Photosynthesis is divided into two sub-processes: a light dependent reaction and a light independent reaction. The light independent reaction, also known as the Calvin cycle, doesn’t need light to occur, as it occurs during the day and night.
- Light Dependent reaction: H2O + light -> ATP + NADPH + O2
- Light independent reaction: ATP + NADPH + CO2 -> Carbohydrates
Combining those two give an overall reaction as follows:
H2O + CO2 + Light -> Carbohydrates + O2
Or
Water + Carbon dioxide + light -> Carbohydrates + oxygen
Carbon dioxide is therefore vital and necessary for plants to thrive, whether they are C3, C4 or CAM. The difference between the categories is how they assimilate the carbon dioxide to be used as a source of energy and build carbohydrates.
2 – C3 plants – Horticultural application: the importance of CO2 supplementation
Atmospheric air contains approximately the following proportions of gases:
- Nitrogen (N2): 78%
- Oxygen (O2): 21%
- Argon (Ar): 0.93 %
- Carbon dioxide (CO2) : 0.04% (400 ppm)
For human beings, there are a lot of oxygen molecules in the air to breath. Plants are limited to a very small amount of carbon dioxide. If you are sharing a room with your plant, you can think of it this way: one fifth (20%)of the volume of the room is usable for you to breathe (oxygen) when only 0.04% (400 ppm) is carbon dioxide available for the plant.
Figure 1 below summarizes data from 60 scientific greenhouse experiments worldwide and shows:
- lower levels of carbon dioxide than ambient can decrease plant growth 30-40% (at 150 ppm)
- Conversely, with a CO2 level about 500 ppm plant growth increased by 15-25%.
- Between 340 ppm – 700 ppm, CO2 can increase growth by 30-40%.
The wide band is due to variation between crops and to conditions. This graph is based on data from about 60 publications of experiments worldwide in many greenhouse crops. (Source: Nederhoff, 1994).
3 – Supplying CO2 – Considerations
As leaves exchange gases: water vapor leaving, carbon dioxide entering the stomata, the concentration of CO2 is slowly reduced and needs to be replaced with a new influx. If not replaced, CO2 levels inside the greenhouse will be lower than atmospheric, known as CO2 depletion. With a proper air exchange from ventilation, CO2 should be at the same level inside the greenhouse as outside.
There are times that it can be detrimental to bring air into a greenhouse from outside, like cold temperatures or possibly pollen in the air.. . Assuming a bright sunny day, therefore a high photosynthetic rate, CO2 depletion is common. Enrichment is the only way to solve that problem.
In the summer or on warm days, when ventilation is high in order to cool the greenhouse, enriching the air with CO2 is not economically feasible. In fact, CERES greenhouses are designed for having 1 air exchange per minute (60 air exchange per hour), and a minimum acceptable air exchange in the industry is around 1 air exchange every 3 minutes (20 air exchanges per hour), meaning CO2 supplementation during venting would exhaust the CO2 immediately, wasting CO2 and money.
CO2 molecules are heavier than other gases, therefore they tend to stay low in the greenhouse. Placing your injection system high above the canopy is a good practice. Consider the importance of both horizontal and vertical airflow, to evenly distribute CO2 throughout the growing environment. Some vertical flow fans can ensure that this precious gas is moved in a perpetual way, and is not stuck below the canopy, hence not optimally assimilated.
No matter the size of the greenhouse, if you plan on using a CO2 system, it is highly recommended (if not mandatory) to have sensors installed that detect dangerous concentrations, and ideally to install small exhaust fans that will clear the air with fresh air in the case of an emergency.
4 – Injection techniques
The most common way of introducing CO2 is from compressed liquid CO2. Manufactured CO2 has the advantage of being contaminant free. Many commercial grows use compressed liquid CO2 with a resupply contract, which usually includes a large interior or exterior storage tank that is resupplied monthly or bimonthly from a supplier..
The second method of CO2 supplementation is from fuel combustion.
Disclaimer: In many commercial operations, burning fuel inside a growing space for CO2 injection is not authorized due to fire codes. Make sure you are compliant with your local codes.
If you do decide to combust fuel to produce CO2, Propane and butane are the most common types of fuel used because they burn cleaner than other fuels.
There are two different types of devices used to inject the gas: either small wall mounted burners installed inside the greenhouse or larger boilers installed outside the greenhouse. The disadvantages of the smaller burners is that flue gas can cause extremely high CO2 levels inside the greenhouse due to incomplete combustion during the on/off times of the device. The other disadvantage is that they produce heat as a byproduct of the combustion. Heat is mostly needed at night, while CO2 is mostly needed during the daytime to promote growth. C3 plants don’t uptake CO2 at night.
The second option is an outside boiler. The heat produced by combustion is ducted and either distributed through a hot water pipe network or stored in a water tank (buffer) for later use. The CO2 injection and the heat produced as a result are controlled separately. This is a preferred method compared to burning fuel inside the greenhouse and widely used in large greenhouses in Europe.
Lastly, a less known solution is using a “Combined Heat and Power” (CHP) system also known as cogeneration. This consists of burning natural gas to generate electricity, heat, and CO2. CHP is mostly used in places where electricity is not available or where electric rates are very high but natural gas is much cheaper.. In conclusion, CHP produces a “all in one” (electricity, heat, CO2) solution. However the cost effectiveness needs to be considered relative to the scale of the facility, and CHP usually isn’t an option in facilities under 20,000 to 30,000 square feet.
5 – Estimating the cost of CO2
As mentioned before, the atmospheric level of CO2 in the atmosphere is 400 ppm. Here are things to consider when you are trying to predict the amount of CO2 supplementation needed:
- Air exchange or ventilation rate, which depends on the tightness of the greenhouse and ventilation strategy,
- Plant intake: depends on light level, and therefore a required CO2 concentration.. The more light, the more “active” the photosynthesis, hence the more CO2 needed
Once you have calculated the air exchange rate and the intake of CO2 by the plants, the amount of supplementation can be determined.
6 – Injecting CO2 can be more cost effective than supplementing lighting
Members of the Greenhouse Lighting and Systems Engineering (GLASE, https://glase.org/webinars/strawberry-and-tomato-responses-to-light-and-co2-control/) have shown that the photosynthetic rate and yields can be similar with different combinations of CO2 concentrations and light level (measured as Daily Light Integral).
All the different combinations gave a yield of 190g of fresh weight. This is an interesting result because it demonstrates that you can adapt your lighting strategy and CO2 supplementation to lower the operating cost depending on the environmental conditions. Table 1. Different experiments on combinations of controlled light and carbon dioxide for growing lettuce. [2]
7 – Environmental considerations
CO₂ is a gas that increases the greenhouse effect on our planet. If possible, try to take this into account and be as efficient as possible when supplementing CO₂. Instead of adding CO₂ to the atmosphere, you may want to consider recycling CO₂ from a neighboring commerce burning fuel, or from a brewery that actively rejects CO₂.
Conclusion
We have given an overview of the benefits and ways to use CO2 enrichment in a greenhouse environment. Yield increases are substantial and justify the massive use in commercial operations. The techniques in which enrichment is used depends on the size of your facility and your budget. Note that combining light strategy and CO2 enrichment through smart controls is a way of making the whole process more cost-effective and less resource intensive.
As presented in our previous “Thinking About “Plant Diet”?” blog, CO2 is a part of the 9 cardinals. The other eight being: light, root zone temperature, air temperature, relative humidity, wind speed, oxygen in the root zone, nutrients, and water. CO2 needs to work in symbiosis with the rest of the parameters.
Sources:
[1] Elly Nederhoff, Carbon Dioxide Enrichment, Practical Hydroponics & Greenhouses. (1994).
[2] GLASE Webinar Strawberry and tomato responses to light and CO2 control.
[3] John W. Bartok Jr. CO2 Enrichment for Cannabis. https://www.cannabisbusinesstimes.com/article/co2-enrichment-for-cannabis/
