Phosphorus Depletion and Natural Farming Series
Chapter 6 / Essay
Chapter 6 № 06 · 2026

Pour the carbon of the air
into the soil.

Pouring atmospheric carbon into the soil

2026.05.11 🌐 日本語

In Chapter 5 we saw that soil microbes — mycorrhizal fungi in particular — can supply nutrients to plants in place of chemical fertilizer.

Here we add one more important fact.

The rise in atmospheric CO2 acts as a "tailwind" for this microbial form of agriculture.

Climate change is usually framed in negative terms. But seen from the angle of agriculture — and especially microbial agriculture — rising CO2 carries an unexpected benefit.

This chapter examines the mechanism, drawing primarily on the work of Christine Jones.

6.1 CO2 is "food" for plants

Restating the obvious.

For plants, CO2 is food.

Plants use photosynthesis to make glucose (carbohydrate) from CO2, water, and sunlight. That carbohydrate becomes the material from which the plant builds its body, ripens its seeds, and extends its roots.

Plant growth speed depends heavily on the CO2 concentration in the atmosphere. More CO2, more active photosynthesis. This is already established as the practice of intentionally raising CO2 in greenhouses (CO2 enrichment).

When CO2 is increased in greenhouses, tomato yield rises by 20-40% [unverified]. This is common knowledge in farming.

The change before and after the industrial revolution

The historical trajectory of atmospheric CO2 looks like this [unverified: exact figures].

Era CO2 concentration
Pre-industrial (~1800) ~280 ppm
1960 ~315 ppm
2000 ~370 ppm
2020 ~415 ppm
2026 ~425 ppm

Compared with pre-industrial levels, that is roughly a 50% increase.

This rise is causing serious effects on the climate — global warming, ocean acidification, and so on. That is a fact, and there is no intent to downplay it.

But at the same time, for plants it means an increase in the resources for photosynthesis. That, too, is an unavoidable fact.

The earth is "processing" this CO2 increase, in part, by accelerating plant growth. If humanity moves in a direction that puts plants to use, rising CO2 also functions as pressure to fix carbon into the soil.

6.2 Liquid carbon — plants paying microbes a "salary"

This is where the heart of Christine Jones's work comes in.

Plants do not use all of the carbon they capture through photosynthesis to build their own bodies. They release 20-60% of it from their roots into the soil [unverified].

This is not a "leak" — it is deliberate release. For what purpose?

To pay a "salary" to soil microbes.

The substances released are soluble organic compounds: sugars, organic acids, amino acids, enzymes. Collectively these are called root exudates, or liquid carbon.

Plant -> liquid carbon -> microbes Microbes -> minerals, water -> plant

This exchange is the economic transaction of the mycorrhizal symbiosis we saw in Chapter 5. The plant hands over carbon, and in return the microbes gather what the plant needs.

The size of the 20-60% share

What is striking is the magnitude of the share. Of the carbon a plant fixes in photosynthesis, up to 60% flows into the soil.

This is not a "cost" to the plant. It is a strategic investment. What is invested in microbes returns as more nutrients. Over the long run the return is large, which is why evolution has preserved the behavior.

Inside the soil, this liquid carbon is used in the following ways.

About half of the CO2 a plant draws from the atmosphere flows out from its roots into the soil and is fixed into the soil through microbes.

This means the plant itself functions as a natural carbon-sequestration device.

6.3 Two routes through which rising CO2 takes effect

Under this mechanism, rising CO2 affects agriculture along two routes.

Route 1: more photosynthesis

When CO2 increases, photosynthesis becomes more active. Plants simply fix more carbon.

This may show up as higher yield, or as more developed roots, more vigorous foliage, or thicker stems. Across the plant as a whole, the floor of productive capacity rises.

Route 2: more carbon supplied to microbes

More CO2 -> more photosynthesis -> more liquid carbon released from the roots.

That is, the amount of carbon the plant can hand to microbes goes up. Microbial activity rises. Mycorrhizal networks expand. Capacity to supply nitrogen, phosphorus, and minerals improves.

The result: the rate at which soil becomes fertile is accelerated by rising CO2.

This is the "tailwind" for microbial agriculture.

In particular, the network of arbuscular mycorrhizal fungi (AMF) — which form symbiosis with plants and supply nutrients such as phosphorus — is strongly affected by rising CO2. Interestingly, adaptive AMF communities that have survived in conventional agriculture (environments with tillage and chemical fertilizer applications) have been reported to deliver a powerful growth-promoting effect under elevated CO2 (eCO2) when supplied with abundant photosynthate from plants — boosting crop biomass by up to 63%.

Stabilization of soil structure by glomalin

As the hyphal network expands, production of a glycoprotein called glomalin also increases.

Glomalin

The aggressive mechanical tillage and biocide applications used in conventional agriculture physically and chemically destroy this hyphal network and glomalin, so conventional agriculture cannot capture the benefit of rising CO2 inside its system.

The nitrogen limitation paradox and "hidden hunger"

That said, the biomass increase under eCO2 triggers a complex biochemical limitation — the paradox of nitrogen (N) depletion.

Even when atmospheric CO2 rises (with ΔCO2 above 200 ppm), the amount of nitrogen supplied to the soil is fixed, so the soil's C:N ratio (carbon-to-nitrogen ratio) expands sharply. Microbes proliferate rapidly and consume (immobilize) nitrogen, and the rapidly growing plants also demand it, so the system as a whole falls into severe nitrogen shortage.

This nitrogen limitation has fatal effects on the nutritional value of crops. Crops grown under eCO2 (especially monocrops such as wheat and rice) have been widely reported to show markedly lower concentrations of essential nutrients — protein, minerals, vitamins — increasing the global malnutrition risk known as hidden hunger.

Here, again, the superiority of microbial agriculture (regenerative agriculture) is demonstrated. To fill this nitrogen shortage, conventional agriculture has no option but to pour in still more synthetic nitrogen fertilizer (urea and the like) at ever-rising prices, which collapses economically almost immediately (covered in detail in Chapter 3).

Regenerative agriculture, by contrast, assumes intercropping with legumes (broad bean, clover, and others) and the introduction of diverse cover crops. By making use of the biological nitrogen fixation of rhizobia symbiotic with legumes, the system can keep supplying the nitrogen it needs without buying expensive nitrogen fertilizer from outside.

So the claim "rising CO2 is a tailwind for microbial agriculture" is a sharp piece of scientific and economic analysis — true only within a system that does not depend on chemical fertilizer and that secures plant diversity (the middle road).

6.4 The Gabe Brown case

This is not theory only. There is a real example.

Gabe Brown, a farmer in North Dakota, is known for raising the soil organic matter (SOM) content of his fields from 1.7% to over 11% through more than 20 years of regenerative practice [unverified].

A rise in soil organic matter means carbon has been fixed into the soil. Concretely:

That alone increased SOM more than sixfold over 20 years.

The implications:

By NRCS (USDA) estimates, raising SOM by 1% adds about 20,000 gallons (75 tonnes) of water-holding capacity per acre [unverified].

This bears directly on irrigation cost and drought risk. SOM lowers not only fertilizer cost but also water cost.

In other words, the present — when CO2 is overflowing in the atmosphere — is an era in which the tailwind blows for the Gabe Brown style.

6.5 Why "now" is a good moment

To sum up:

On top of that:

This economic pressure and this biological tailwind exist at the same time.

Historically, this is a rare moment when the wind is at agriculture's back during a transition.

Chemical fertilizer was cheap, so industrial agriculture was viable. Chemical fertilizer became expensive and CO2 increased, so microbial agriculture becomes viable. In both cases, external conditions decide the farming method.

6.6 A caveat — the negative side of climate change

To be safe and avoid misreading, write this down explicitly.

That rising CO2 has positive effects for agriculture does not mean climate change as a whole is good.

The negative side of climate change is severe.

These are clear threats to agriculture as well. In Japan too — sudden torrential rains, more days of extreme heat, larger typhoons — adverse effects are already showing up.

Even so:

That is, the farming method that survives under climate change converges, in the end, on microbial agriculture.

Industrial agriculture is fragile under climate change because it depends on chemical fertilizer and follows the convention of tilling bare earth. Microbial agriculture turns the biological health of the soil itself into shock-absorbing material.

6.7 Chapter 6 summary

We have now seen why microbial agriculture is being chosen, why it is feasible, and why now is a good time.

The next chapter, Chapter 7, looks at how to implement it — concrete implementation plans for realistic scales: one-tan smallholder farming, abandoned farmland, balcony gardens, and so on.

References

CO2 concentration and photosynthesis

Liquid carbon pathway and glomalin

Gabe Brown and regenerative agriculture

Nitrogen limitation paradox and hidden hunger

Negative side of climate change