Comment by dbingham | Hacker News Reader

Comment by dbingham | Hacker News Reader

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The matter is conserved: it becomes soil. Most of the carbon an individual plant absorbed when it was growing will become soil when it dies and decays. (Should note "absorbed" here refers to the carbon that became part of the structure of the plant - not the carbon the plant breathed in. Plants breath out most of the carbon they breath in.) Yes, it releases some of it during the decay process, but a lot of it (most of it) does get sequestered even after death. That's where humus comes from. And it's pretty easy to meassure that. And for deciduous trees, they are sequestering yearly as the leaves they drop decay.

It wouldn't surprise me if the amount that is being sequestered is being overestimated in many studies, but the mechanism for it happening is pretty clear and that it is happening is pretty incontrovertible - it falls right out of the chemical math - and I would be much more likely to question any study that questioned that than to question that mechanism itself.

It's really frustating to me that what is being posted here is basically just a headline and not even an abstract unless you have a subscription to Nature.

Yes, but bacteria action continues in that soil and continues releasing CO2 for a long time to come. Some of it will feed new plants (anything taken from the soil is not being taken from the air). The question is how much of that soil carbon truly becomes permanently sequestered? Some of it may be there for a long time, but the humus layer will not become infinitely thick. Once we are talking about an old growth forest, the forest will have reached a somewhat steady state. I am not saying it cannot continue to act as a carbon sink, just that I suspect it to become less effective over time, and that it will not work in all conditions. A number of research papers have pretty much shown this lately, not just this one.

Assuming rich soil accumulates over time in a long-running forest, what happens to the lower layers? Do they release carbon more quickly as they "die" or do they store it? If they store it, then that's sequestration, right?

It is sequestration, but it can take thousands of years for an inch of rich soil to accumulate in a 'steady state' system.

Most fossil fuel carbon was originally sequestered in a world where very few things could break down dead trees. That world is millions of years gone, current natural carbon sinks are 99% temporary.

There's a certain argument to be made for timber farming as a way to extend our temporary carbon sinks -- trees that fall over rot, releasing their carbon, trees that are cut down and turned into houses may be protected from decay for tens or hundreds of years.

You could really go whole-hog into attempting to maximize the amount of timber you can stuff into a house -- start building double-stud walls with 12 inch on-center spacing just to triple the number of 2x6's used, fall in love with giant cross-laminated timber panels, etc etc.

That is incredibly labour and energy-inefficient. You're better off burying raw timber, as opposed to processing it, cutting it up, transporting it to a worksite, and paying professionals to hammer it into place.

How do you feel about baking the timber in solar ovens first, before burying the charcoal, to go full terra preta?

It seems to me that the low-hanging fruit is to sequester the sawdust and scraps that result from sawing logs into boards. I assume that much of that ordinarily finds its way into other products (paper, particle board, etc...), but that it's economically less valuable than usable boards and thus burying it the ground has lower cost.

There does seem to be increasing interest though in making large structures out of wood, so maybe that's a pretty good route for carbon storage.

Just to precise, "tree-roting" is incredibly slow, and the tree mainly turns into other living forms (thus, also sequestered carbon) rather than in carbon dioxide (one part does, but far from 100%).

It's like when you eat, you do not turn your food in pure gas, do you? And then the excreted/remaining mass will be buried etc (and will partly be turned into earthworms, nematodes, bacteria, moss, fungi, etc).

Hmmm, that's an interesting question.

A person breaths out about 2.3 lbs of CO2 each day, which amounts to 0.6 lbs of carbon, all of which must have come from your food.

Meanwhile, a person defecates about 1 lb per day, about 75% of which is water. Of the residue, about 25-50% of it is bacteria, and the remainder the undigested portion of your food. Bacteria is typically accounted 53% carbon; the undigested fats, carbohydrates and proteins will vary a lot in percentage, but is 40% carbon for carbohydrates, 50% for proteins, and "more" for fats.

Which leaves a lot of variance, but something like 0.15 lbs of carbon exiting at as a solid seems like a reasonable estimate.

So the answer would be: you turn about 80% of the carbon in your food into CO2 gas.

That seems a lot, 80%.

But after reading several times your calculation, it seems legit. Thanks.

Then, as a conclusion, when you eat you delay 20% of your carbon footprint, and a fraction of these 20% may well be stored for a long duration (in sediments, or back into the food chain, etc).

Then again, similarly, the fallen leaves of trees may store a fraction of their carbon for a long duration.

You can basically look at that 80% as the efficiency of the body machine. To a first approximation, once we have all the matter needed to build a body, you are just taking in carbon to oxidize it for fuel.

https://phys.org/news/2021-04-trees-world-offset-society-car...

Here:

> While this process naturally releases CO2 through the respiration (or breathing) of microbes that break down dead organisms, some fraction of plant carbon can remain underground for decades or even centuries. Together, land plants and soils hold about 2,500 gigatonnes of carbon—about three times more than is held in the atmosphere.

This was the point I wanted to make, probably worded awkwardly.

> It is sequestration, but it can take thousands of years for an inch of rich soil to accumulate in a 'steady state' system.

And as anyone that's been to western England knows, it takes a single generation for it to all blow away and leave you with nothing but a barren heath that's good only for grazing sheep.

Some will be sequestered in this way, but how much and under what conditions? Tropical rain forests, for example, are known to have very poor soils despite the massive amount of biomass growing in them. Almost everything is recycled back into the forest very quickly. Clearly, no significant accumulation of sequestered carbon is occurring there. On the other hand, peat bogs occur because dead plant matter falls into a low oxygen environment and doesn't really decay, so there can be a build up of sequestered carbon there, but that is rather specific conditions.

Some of the biomass is also washed away and becomes marine sediments, which can store a significant amount of carbon. https://cbmjournal.biomedcentral.com/articles/10.1186/s13021...

> The matter is conserved: it becomes soil.

So for that mechanism to work, the ground level in old growth forests would slowly rise over the centuries, and more and more soil with some carbon content accumulates.

Is that what actually happens?

Some soil accumulates. Other is washed out to lower areas of elevation. My guess is it's another thing that reaches steady state.

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