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How Much CO2 do Plants Consume?

Discussion in 'CO2 Enrichment' started by VaughnH, Nov 7, 2008.

  1. VaughnH

    VaughnH Lifetime Charter Member
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    I have been thinking about how to calculate the amount of CO2 our aquatic plants consume in growing. One way to do this, in order to get an order of magnitude answer is to consider the elemental composition of plant tissue. Plant tissue is more than 90% carbon (discounting the water content), and about 1% nitrogen. To grow a gram of plant tissue must take about 100 times as much carbon as nitrogen. Assuming aquatic plants growing fast with non-limiting nutrients consume about 10 ppm of KNO3 per week, which is about 1.4 ppm per week of nitrogen, then they must consume about 1400 ppm per week of carbon, which is about 7000 ppm of CO2 per week. Ten gallons of water is about 40,000 grams of water. 10 ppm of KNO3 for that water is .4 grams of KNO3, about 1/8 tsp.

    But, 7000 ppm/week of CO2 for 40,000 grams of water is about 280 grams of CO2 per week, or .6 pounds of CO2 per week. A 5 pound tank of CO2 would last about 8 weeks for that 10 gallons of water.

    Of course this is only a crude estimate, but I find it useful to gain some insight into how much CO2 has to be added to an aquarium to support plant growth. And, knowing the order of magnitude of the consumption of that CO2 helps in understanding why it is so hard to avoid big gradients in CO2 concentration in the water. Plants are CO2 hogs!

    I don't know about others, but I have been thinking in terms of most of the CO2 we add being lost from the water surface, so most of what we add goes to make up that loss. It seems obvious that this isn't true. Just as we have to keep dumping KNO3 into the water to resupply what the plants are consuming, we have to keep dumping CO2 into the water, but at a much higher rate, to resupply what the plants are consuming.

    This also gives me a feel for why non-CO2 tanks have such slow plant growth.

    Is there a flaw in my reasoning?

    Based on Tom's comments below, I changed this to:
    I have been thinking about how to calculate the amount of CO2 our aquatic plants consume in growing. One way to do this, in order to get an order of magnitude answer is to consider the elemental composition of plant tissue. Plant tissue is about 40% carbon, and about 1% nitrogen. To grow a gram of plant tissue must take about 40 times as much carbon as nitrogen. Assuming aquatic plants growing fast with non-limiting nutrients consume about 10 ppm of KNO3 per week, which is about 1.4 ppm per week of nitrogen, then they must consume about 60 ppm per week of carbon, which is about 300 ppm of CO2 per week. One hundred gallons of water is about 400,000 grams of water. 10 ppm of KNO3 for that water is 4 grams of KNO3, about 1 tsp.

    But, 300 ppm/week of CO2 for 400,000 grams of water is about 120 grams of CO2 per week, or 0.3 pounds of CO2 per week. A 5 pound tank of CO2 would last about 4 months for that 100 gallons of water. My 45 gallon tank gets about 3 months on 5 pounds of CO2, which would be about 1.5 months for a 100 gallon tank. So, somewhere around 1/3 to 1/2 of the CO2 we add is consumed by the plants.

    Of course this is only a crude estimate, but I find it useful to gain some insight into how much CO2 has to be added to an aquarium to support plant growth. And, knowing the order of magnitude of the consumption of that CO2 helps in understanding why it is so hard to avoid big gradients in CO2 concentration in the water. Plants are CO2 hogs!

    I don't know about others, but I have been thinking in terms of most of the CO2 we add being lost from the water surface, so most of what we add goes to make up that loss. So, it looks like most of it is lost to the air, but a lot more than I expected is consumed by the plants. Just as we have to keep dumping KNO3 into the water to resupply what the plants are consuming, we have to keep dumping CO2 into the water, but at a much higher rate, to resupply what the plants are consuming.

    This also gives me a feel for why non-CO2 tanks have such slow plant growth.
     
  2. Tom Barr

    Tom Barr Founder
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    This is simple, measure the O2.

    O2 is related to growth/biomass production.

    From that, you can correlate the amount of Carbon fixed.
    You can verify this farther by measuring the dry weight growth and measure the Carbon content over time at various stages of growth.

    Dry weight biomass etc

    40%, not 90 % of the plant tissue is carbon, less so in general for aquatics than terrestrials(ask yourself why that might be so).

    We use dry weights, so water is not an issue.
    O2 is non destructive and can measure in situ.
    You need to log the readings daily at several points to get a nice graph for the time series.

    Yes, most of the CO2 is lost.
    This is true for FL springs also.

    Regards,
    Tom Barr
     
  3. VaughnH

    VaughnH Lifetime Charter Member
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    If 40% of the plant tissue is carbon, but only about 1% is nitrogen, which is the next most abundant in plant tissue, what makes up the over 50% left if it isn't water?
     
  4. Tom Barr

    Tom Barr Founder
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    O's, H's.
    then the rest.........

    Regards,
    Tom Barr
     
  5. JJP2

    JJP2 Lifetime Charter Member
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    Along you line of thinking, VaughH, I've had an interesting situation occur this week. Since changing my CO2 tank and going from a 5lb to 20lb, I've had to reset my regulator and needle valve. I thought I had it dialed in, but observed the 3-4 bps would drop my ph from 7.5 to 7.15 when my lights kick on. About 1 hr afte the lights were on, the ph would raise to 7.3 and my bps remained the same. My only explaination has to be the plants utilizing it. If it were being outgassed, I would n't have had the ph drop before the lights kicked on.


    John
     
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