Recent report of a study in Nature says that “Carbon dioxide is absorbed in the spring and summer as plants produce lots of solar-hungry leaves. Once the energy has been converted to food, it is released into the atmosphere as the leaves die off in the autumn and winter, producing big fluctuations in CO2 levels.” I don’t have direct access to the journal, but this statement seems to be at odds with at least my own understanding of the Carbon cycle. When leaves die they release CO2 into the atmosphere? CO2 that has been converted to food? Presumably they’re talking about the conversion of CO2 into cellulose, starch and sugars. How could that be released as CO2? Something’s missing. Studies don’t show up in Nature unless there’s some decent science going on. There’s a huge piece of this that I don’t understand. Can anyone explain what seems to be left out of this report?
What’s left of the report is difficult to say, since you’ve only quoted a couple of sentences, but there’s an annual global cycle dominated by the growing season in the Northern Hemisphere where CO[sub]2[/sub]-levels decrease during summer when photosynthesis dominates, as you say, the gas is bound into cellulose, starches and sugars, and increase during summer when respiration dominates, i.e. plants, fungi, bacteria and animals use that food. Both processes happen the year round, but with a net gain in summer and a net loss in winter.
Or to put it another way: Plants produce sugars as an energy source. They produce enough to keep them running even when there is little or no sun. Deciduous trees get rid of their energy producing leaves in fall, pulling most of the good stuff back into the trunk. The leaves start to decay (bacteria and fungi use the “food” left in the leaves and release CO[sub]2[/sub]) and the trees start burning through the stored food.
No, as far as i can tell, the claim in the article is NOT that agriculture produces more CO[sub]2[/sub].
Here is the link to the Nature article, but you need to be a subscriber to read anything more than the abstract. A few of the authors discuss the article here and here. The best summary for lay folks like me appears to be at the last link.
If i understand the argument correctly, they are saying that the past half-century or so has seen a significant overall increase in the carbon dioxide cycle, and that agricultural crops are a significant factor in this increase. That is, the huge crops of corn, rice, wheat and soy produced by agriculture take up massive amounts of carbon dioxide during the growing season, and then releases much of that CO[sub]2[/sub] again at the end of the season. They are not arguing that the crops produce more CO[sub]2[/sub]; they are simply arguing that the growth of the CO[sub]2[/sub] cycle—absorbing and releasing—can be attributed, at least in part, to agriculture.
Some relevant quotes:
Emphasis mine. It’s about capture and release, not just release.
But can agriculture be blamed for that? Wouldn’t natural vegetation have done the same thing, with the same periodicity, if man had not converted land usage from natural cover to controlled agriculture?
No, because according to the article, these crops produce a disproportionate amount of the carbon dioxide cycle:
Agricultural productivity, in terms of output per acre, has also increased considerably as a result of new techniques and technologies, meaning that an acre of wheat or corn inhales and exhales more CO[sub]2[/sub] than an acre did 50 years ago.
Ok, maybe I’m stuck on the “inhale” “exhale” terminology, but even if we say that plants inhale air and use the CO2, they don’t exhale CO2. Granted, they take in large amounts of CO2 during the growing season, and there’s a lot more stuff growing these days, and they don’t take in any in the off season. I can understand the seasonal swing in the amount of absorbed CO2 associated with agriculture. But dead plant leaves don’t exhale anything. I don’t get any of the assertions in this whole scenario.
Most dead plants are digested, which releases CO2.
Here’s a quote from the NSF site: “They found that production of these crops in the Northern Hemisphere has more than doubled since 1961 and translates to about a billion metric tons of carbon captured and released each year.” What are they talking about? It sounds like they’re saying that all the CO2 captured by these crops is then released in the off-season. WTF?
Through the citric acid cycle, which converts glucose into carbon dioxide, ATP and NADH. Much of the carbohydrate you eat comes out of your body as CO[sub]2[/sub] from your lungs.
Of course you also need to account for all the CO[sub]2[/sub] produced by the engines that plow the fields, sow, water and harvest the crops, provide post harvest transport and processing etc.
You (and other animals) eat a substantial portion of the plant – all the fruits and grains. You absorb its carbon and “burn” it to release CO2. The rest of the crop, the stems and leaves and roots, are also eventually consumed. Some of it is animal feed, going to ruminants that rely on bacteria to ferment tough plant materials. The rest will decay, as it is digested by bacteria and fungus which also release CO2.
Otherwise, where does the carbon end up? Some ends up as organic matter in the soil, but that will eventually be released as CO2 on a longer time scale.
ETA: Most ecosystems, whether natural or manmade, have zero net CO2 consumption/production. Mature forests in particular are not net producers of oxygen. All of the carbon that is absorbed by trees becomes part of leaves and the trunk. Once the leaves drop to the forest floor and the tree dies and falls, decomposers will consume and release all the carbon.
If that’s all they’re saying, of course I understand. But the way the articles are written is extremely misleading or, I’d say, even inaccurate. It seems they were saying the plants release CO2 back into the atmosphere.
Actually, plants produce CO2 all the time via normal respiration. During times of darkness, they cease producing O2 while continuing to respire CO2. Over the course of a normal day/night cycle, they produce more O2, but they still respire CO2.
They do, after they die.
What??? Can you explain what you mean?
Bacteria and fungi consume the plants as part of the decomposition process after death. The bacteria and fungi metabolize the cellulose and other plant material and release carbon dioxide into the atmosphere (just like we do after we metabolize what we eat). So all the carbon trapped in the plant cells (as part of sugars, fats, protein etc) get returned into the atmosphere after the plant dies.
Ok, this seems to be a matter of semantics. I would say that the plants don’t release anything. CO2 is released when organisms metabolize material that they live on. Sometimes, that includes dead leaves.
I would that carbon dioxide trapped in the plants is released when they die, rather than the plants release it when they die.
Ultimately it means the same thing. All that plant material is a carbon store that gets released upon the inevitable decomposition.
Are you ignoring the posts mentioning respiration by plants? It’s not just a question of CO2 being released when plants are consumed. During the winter months plants “breathe” out more CO2 than they take in. Vice versa during summer.
To be fair, he was responding to a quote about carbon dioxide being released after the plants dies.