Science Buddies: "Ask an Expert"

Hello,

I am - as a teacher - preparing a science project on the question 'Do roomplants reduce the carbon dioxide level in our class'?

For this purpose, I designed a very simple home made plant chamber: a big glass jar, covered with a polythen bag which makes it pretty much air-tight.
Inside the jar are a plant (Mimosa pudica and a tool that measures carbon dioxide, relative moisture and temperature (with data logging).
I have now ran my experiment for almost 48 hours and I notice the following evolution in carbon dioxide:
- the level drops significantly during the day (as expected, due to photosynthesis, approx. - 1500 ppm during a day).
- the level rises during the night (as expected, due to cell respiration)
BUT: the level rises much more during the night then it drops during the day, which is not what I had expected.

I have been thinking about possible reasons, and came with the idea that the polythen bag might emit carbon dioxide. From what I found in literature, this is possible but only to a very limited amount, so it cannot explain what I am observing.
So here is my question: what migth be the explanation for the rise in carbondioxide during the night exceeding so much the intake during the day?
Thank you in advance

Hi Jutta,

Sounds like an interesting experiment, and also not the results I would have expected! I'm not sure, but here are a few thoughts:

• Growing plants absorb CO2 in the long-term, although they may respirate more than they absorb in the short-term. The absorption-respiration balance changes with the seasons (light-dark balance and growing-vs-dormant periods). Perhaps the plant is relatively dormant, which you might be able to change with a longer light-cycle or a warmer environment.
• Microbes in the dirt may contribute to excess respiration. (I'm not sure how much of an effect this would have).
• Is it possible that the seal on the bag are not quite vacuum tight? One way to check would be to compare the CO2 fluctuations to the ambient atmosphere, if you have another data logger.

Hope that helps. Perhaps someone else on the forum will have a better answer as well.

Best,
Charles