Approximately what proportion of the mass of a typical tree is made up of carbon got from atmospheric CO2?

Question

It would be interesting to know how much a of a tree's mass comes from elements its taken from the atmosphere relative to what it obtains from soil/water.

Answer 1

You ask a very good question. For all practical purposes the dry mass of the tree is the structural component of wood, cellulose.

Cellulose is by weight 54.55% carbon, 9.09% hydrogen and 36.36% oxygen.

The carbon and the oxygen in cellulose come from the carbon dioxide molecule from the air. The hydrogen comes from the water molecule from the water the roots picked up in the soil..

So essentially the mass of the tree is 90.91% from the carbon dioxide from the air and 9.09% from the hydrogen in the water picked up by the roots in the soil..

Note: The oxygen the tree gives off to the atmosphere also comes from the water picked up by the roots..

The only part of the mass of the tree that it receives from the soil is some nitrogen from nitrates in the soil if the tree is not a legume, that is if the tree is a species that does not fix nitrogen from the air, and some minerals from the soil such as Potassium, Phosporous, Iron, Calcium, some sodium, and some Zinc. This is why you have to add fertilizers containing Nitrogen, Potassium, Phosphorous, Calcium, Iron and Zinc from time to time, because the plant uses them.

Fortunately the large part of the mass of a tree comes from water from the soil and carbon dioxide from the air, but not from the soil itself, with the exception of some nitrogen and trace minerals.



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Answer 2

I'd guess carbon comes almost entirely from the air, not the soil or water. Only some micronutrients come from the soil. Assuming that, about 25% of the wet weight of a tree is carbon. Or about 40% of the dry weight.

EDIT: Yep. "Around 400 years ago, a Dutch physician named van Helmont conducted an experiment using a willow tree. He put a willow with a given weight in a pot after weighing both tree and soil. He watered the tree for 5 years, took it out of the pot, removed and dried the soil, weighed both soil and tree and discovered that the tree had gained around 165 pounds while the soil had only lost 1 ounce in weight. He was able to demonstrate that there was nothing inherent in the soil itself that made the difference in the trees mass and that it must have come from some other source."

Answer 3

6 mol of CO2 produces 1 mol of cellulose
6 CO2 (g) + 5 H2O (l) -> C6H10O5 (s) + 6 O2 (g)
a 2 ton tree(12-20 years old) is approximately 2.0 X10^6g cellulose

That is roughly 1.24 X10^4 mol of cellulose.

multiplying that by 6 = 7.44 x 10^4 mol of CO2

the molecular weight of CO2 is 44 g/mol

so a 2 ton tree would remediate about 3.27 X10^6 g of Co2 from the atmosphere.

Answer 4

I believe it obtains most from the ground. The CO2 that is taken in from the air though allows the photosynthesis to occur.