To give a quick primer to people who would be tempted to only comment on the title, the issue with using plants for carbon sequestration is that they are a temporary sink. Plants use the atmosphere carbon to grow and that carbon is then released when the plants die and is decomposed.
So if you want to sequester more carbon than currently you need to either: have more plants or slow down plants decomposition.
Multiple ways exist to do that. Reforestation is one. A promising one is to farm algae which grow quite fast and either bury them in arid places, deeply submerge them or use them to recreate peat bogs. Another one is to augment the amount of decaying organic matter in the soil. This can be achieved either through change to how we farm or the article presents a project to use GMO crops with roots engineered for mass and slow decay.
The field is both promising and fascinating. It’s probably never going to be a panacea but everything that helps should be investigated.
According to wikipedia, during the azolla event, the azolla would grow in a fresh water layer at the top of the arctic ocean and then sink when it died. There was a layer of oxygen-free water at the bottom, preventing it from decomposing.
Ideally if we had a large fresh-water lake with a layer of anoxic water at the bottom, we could use it as a carbon-sequestering azolla farm with hardly any human input.
All the examples given in wikipedia are salt water, though. Apparently decomposition itself removes oxygen from the water, so I suppose you could use any sufficiently deep body of water as long as it has very little in the way of mixing currents.
I imagine that trying to have, say, lake Superior designated as an official azolla farm and deliberately rendered anoxic probably wouldn't be popular, even if it's possible.
I wonder if it's possible to make a salt-water tolerant variant of azolla or duckweed?
(I actually have some practical experience with azolla and duckweed, thanks to a pond I made during the pandemic. One or the other of those usually takes over in the spring and early summer, and then dies off in late summer, presumably because the water lettuce and water hyacinth out-compete them for nutrients. Azolla can survive the winter but duckweed does not. They both make good chicken food, but azolla seems to have more bulk to it.)
One thing missing from this fascinating Azolla story: if there's a huge green hat on the world, wouldn't something try to eat it? You'd think some sort of Azolla-eating creature would have a great time munching it and keeping the carbon up on the surface.
Rainforests, and the Sargasso Sea, continue to have very impressive amounts of plant biomass, despite the best efforts of all animals combined to eat them.
It's easy to imagine a current which sunk a Sargasso's worth of algae every year removing carbon. There's not enough oxygen on the bottom to support metabolizing that much sugar.
It's not poisonous to chickens, plenty of people grow it for chicken feed.
I think at this point it's unclear whether it's safe for human consumption. Azolla has a symbiotic relationship with a kind of cyanobacteria that might or might not be safe.
Not quite a temporary sink, not in every case. Both carbon and nitrogen are captured by plants, then what? Plants get eaten by livestock or people.
So these atoms get captured and enter a large and complex cycle forward.
But if you are talking about natural areas (non production), then the plant won't die anytime soon, essentially trapping carbon and nitrogen for a very long time.
And once they die, not all of it is released back to the air, I do not have an exact number now, but a lot of it is incorporated in soil.
We have several fields in animal science and agricultural science that are based on these exact mechanisms, for greener production and also for traditional production but with increased yields.
I'm always shocked at these kinds of comments. While true decomposition releases carbon, if you walk in any forest, you'll note very deep layers of carbon-rich top soil many feet deep from all the trees that fall. Fallen trees take a long time to decompose. Decomposing lignin takes a long time.
But it does reach a steady state. All the carbon we are releasing comes from a time when Lignin essential did not decompose at all. And we are releasing orders of magnitude more of it than current gets captured and stays captured. Even large intact rainforests do very little to actively capture CO2, they cycle it and they keep a large chunk of existing Carbon bound but they do not capture new Carbon from the atmosphere since they already reached a steady state between growth and decomposition.
Decomposition might take a "long time" from a human perspective, but from a geological perspective it's nothing at all.
Fungi can break down wood chips in a very short time since the wood is easily accessible. A whole solid tree will take longer, but the timescale is only years, not decades or longer.
It absolutely takes decades for trees to decompose. This is just one study of many:
> In woods stretching from Minnesota to Maine in the north and from Louisiana to Georgia in the south, technicians catalogued every downed log they found. They ranked each log on a scale of one to five, from freshly fallen to badly decomposed, and then returned to the same forests five years later to revisit and reassess. Using these figures, the researchers were able to create a model of decay for 36 different species of trees. “Some people said it couldn’t be done,” said Woodall, “but we did it.”
> The computer model calculates that the “residence times” (how long a tree will take to completely decompose) for conifer species range from 57 to 124 years, while hardwood species are typically around on the forest floor for 46 to 71 years.
Note that these estimates are for trees that have already fallen to the forest floor. It's common for trees to stand for decades after death, especially in dense forests where they have other trees to lean on.
You're just wrong about this. It's still possible to find large fallen American Chestnut trunks from the blight.
Trees will increase their biomass, hence sequestered carbon, for decades, and if left where they fall, it very much depends on the tree. A poplar will be gone in a couple years, a sycamore/plane will substantially still be there in a cenury.
Any processing which requires specific processing and handling is ... likely to scale poorly. The quantities involved are simply enormous.
Human CO2 emissions are about 43 billion tonnes annually. Let's say we're hoping to just take a bite out of that rather than compensate the whole thing, so we'll aim for a bit over 10% with 5 billion tonnes of stored CO2, to make the maths easier.
Let's say we're making and storing hay bales as a way of sequestering carbon.
A hay bale 16" * 18" * 36" weighs about 50 lb. (40 * 45 * 90 cm and 22 kg respectively). That's what you'll see as a traditional human-handled hay bale.
I'm not positive of the precise chemical composition, but I'll assume a 20% moisture content (by mass), and the remainder consisting of cellulose (C6 H10 O5). That leaves about a 35% net carbon content.
Note that carbon itself is only 27% of the mass of CO2, with most of the molecular weight being oxygen, so to sequester 1 billion tonnes of CO2, we need only bury 260 million tonnes of pure carbon.
We want to sequester five billion tonnes CO2 here, so 1.3 billion tonnes of carbon, or in the form of hay, 3.7 billion tonnes of hay bales. Let's round again, for convenience, and call it 4 billion tonnes of hay.
One tonne is roughly 40 bales.
One billion tonnes is 40 billion bales.
Four billion tonnes is 160 billion bales.
If we stack these to a height of 10m (33 feet), roughly 20 bales high, each tower weighs a half tonne. We need 8 billion of those stacks. Simply taking squares, that's a stack roughly 90,000 bales on a side, and 20 bales tall. (89,442.719 bales on a side if you want to be precise, I'm ... not.)
90k * 40 cm is 36 km (22 mi).
90k * 90 cm is 81 km (50 mi).
You'd need to bale, stack, and rack that much hay in a year. And repeat it every year.
And you're only accounting for 10% of annual human carbon emissions.
A process which captures, converts, and buries the carbon on its own would ... likely be preferable.
Note as well: large stacks of hay have a strong tendency to self-ignite through metabolic action. Or by other mechanisms in places such as Gävle.
Some of the plant decomposes to the atmosphere, but some stays in the soil too. It is pretty easy to demonstrate it to yourself, with fast-growing plants, after a few years you can have quite a bit of extra carbon-based matter in the soil.
There does appear to be a limit to carbon content in soil, though. It isn’t an endless sink.
Some interesting research which I can’t find great sources for indicates that microbial content of the soil can increase the carbon capacity, but I’d need to see better sources to verify that. An example of research around that would be BEAM/Johnson-Su bio reactors and the compost they use to improve agricultural yield. Part of their work indicates that the more they feed the soil with microbe and fungus-rich compost, the better the carbon-retention becomes. There are others like it, but there’s not a large enough body to work to show how well this scales and how well it works across different media.
If I had another life to live I think I’d love to do this kind of research.
A point made in the article, more than halfway down, is that modern intensive agriculture is less of a carbon sink than previous approaches.
There are people seeking to alter modern wheat | canola | et al to leave more carbon behind in larger root systems that remain within the soil and build deeper "peatier" soil bases.
This has the potential to knock a significant percentage from the global annual atmosperic carbon increase .. and every bit helps.
The article mentions a key to make it persistent - let them decompose under water, where oxygen supply is insufficient. Flooded open-pit mines are a good canidate. I imagine that turning them into dead zones would create bogs and sequester much carbon. Mines for brown coal are likely in a suitable geographical location since they used to be bogs before (this is how brown coal came to be).
>Plants use the atmosphere carbon to grow and that carbon is then released when the plants die and is decomposed.
Plants do not melt into thin air when they decompose. They compost, become part of the soil, feed bacteria, fungi and other plants. Some of these processes release various gases, but it's nowhere near 100% conversion. If it was, there would be no fossil fuels to begin with.
Yes, but most plants we currently have are in a pretty much 100% cycle, that is, why the biosphere is so stable. Take the Amazon region, the top soil layer is like 50cm. There is no deposit of carbon into the ground going on and its one of the most lush growing regions. To have a net carbon capture, the decay of dead plant material needs to be cut down by at least an order of magnitude. The article names options for that.
Note: from what I read, our fossil fuels come not from a constant process but from certain times in the past where the natural cycling of e.g. wood was way less than 100% which created concentrated deposits of carbon. Which makes sense if you consider how concentrated they are, if it was a truly steady process, they would be much more spread out.
Fossil fuels largely come from the Carboniferous period. The reason for this is that during this period the trees which covered the planet's surface did not decompose, and became buried in thick layers, which became coal seams. Whatever the reason, we know that the carbon cycle was not so much of a cycle at this point--the fixed carbon was retained and not recycled back into the atmosphere, resulting in the accumulation of vast quantities of carbon.
In the current environment, breakdown is rapid and carbon is rapidly recycled. Accumulation is minimal and while some longer-term storage can occur in places such as peat bogs, these are not stable long-term stores--they can easily burn if they dry out.
Plot twist: What we know as oil was buried by a former civilisation that fought climate change then went extinct. The trees we bury will be used as fossil fuels by the next civilisation.
If I understand right, most of our current fossil fuels are from an era when there weren't bacteria capable of breaking down lignin. So, plants would die and dry out, but wouldn't fully decompose. You'd have huge mounds of dead plants that would eventually get buried and turn into oil.
Those conditions don't exist anymore, so they're actually a non-renewable resource.
No, I mean that the conditions can't happen anymore the way they did then, because in the intervening time microbes have evolved that can eat lignin. The dead plants would be almost entirely decomposed before they have a chance to get buried.
Yeah, when I was in the solar business more than a decade ago funding was tough and so I used to refer to it as "wireless fusion power" in the hope that those two buzzwords would make people more interested.
Instead solar did fine with a not-so-slow-but steady path.
Yes I hope so! I am co-creating an open source solar powered farming robot specifically intended to be used in a regenerative organic system. Check it out!
Why are we talking about ferns and duckweed? It is the same reason wildlife conservation efforts tend to involve talking about the "charismatic megafauna"?
We would do well to remember that a full 70% of plant-based carbon sequestration takes place in the ocean (you know, 70% of Earth's surface), not inland, and is done by phytoplankton.
This doesn’t necessarily ignore or exclude utilizing the ocean, though. We should be aiming to do both.
The ocean is harder to control - at least with inland controlled experiments we can get a sense of what’s possible and how it can scale. With the ocean it seems our goal would be to simply let it do what it does best, and stop getting in its way. They seem like different endeavours.
I think all these approaches should be looked at. It's not obvious to me that farming algae to sequester carbon would be cheaper/easier per ton of CO2 removed than farming duckweed or azolla.
One way to do it is to produce more bioavailable nitrogen to encourage more plant growth, then have animals and humans eat the plants. Now the carbon is stuck in the animals. As long as those animal populations remain constant or grow you have a good carbon sink. You can even use the animals for food, as long as the animals never decompose it's fine.
Or another one, as we move to renewable fuels, we might just have a cycle that traps a significant portion of the carbon dioxide in the fuel between use and creation, so the added carbon would be part of it's own separate cycle, and we could keep using it for energy storage ad infinitum.
Why bury it? We can use it. As long as it's utility is high, we would only wind up with a slightly higher co2 concentration than baseline. If we bury it, then we have to take co2 from the natural carbon cycle that the biosphere needs to store energy, which means reduced biomass. But if we create a secondary carbon cycle for energy and food that only interacts with the natural one in the air we can store energy and food without impacting the biosphere as much.
You could also look into "regenerative agriculture" which also builds topsoil by capturing huge amounts of carbon, increases water retention, and nearly eliminates the use of industrial inputs like pesticides.
The fundamentals are, NEVER till the land, always have a cover crop, manage it carefully, and graze animals on it in a well managed manner.
I maintain a large garden and bury probably tons of organic matter underneath and in the beds. It is a pittance of carbon capture and no solution. Here is a fun video about carbon capture. https://youtu.be/MSZgoFyuHC8
If you can bury tonnes personally on the order of years then you're operating on a scale where it can contribute to undoing the harm providing we solve the cause of the problem first.
If regenerating all the devestated agricultural land can sequester a hundred billion tonnes, then that makes for 10 or 20% less total to remove.
A long time ago I read an article about burying plant materials.
Haven’t heard about it since, but it makes sense to me. Especially because many types of farming burns leftover bits of crops that can’t be used. Just bury it and you have sequestered carbon.
So if you want to sequester more carbon than currently you need to either: have more plants or slow down plants decomposition.
Multiple ways exist to do that. Reforestation is one. A promising one is to farm algae which grow quite fast and either bury them in arid places, deeply submerge them or use them to recreate peat bogs. Another one is to augment the amount of decaying organic matter in the soil. This can be achieved either through change to how we farm or the article presents a project to use GMO crops with roots engineered for mass and slow decay.
The field is both promising and fascinating. It’s probably never going to be a panacea but everything that helps should be investigated.