Carbon comes from below

Ask most people in agriculture where soil carbon comes from and they will point at the stubble. Leave the residue on the surface, let it rot, and over the years it turns into organic matter. It is a tidy story, and it has shaped a lot of practice and policy. The evidence of the last two decades says it is mostly wrong.

Most of the carbon that stays in soil for a long time comes from below: from roots, from what living roots leak into the soil, and from the microbes that feed on it.

I first heard soil described as a living system at Las Marías, when the owner handed me Ana Primavesi's books to help with the old yerba mate plantations. It took me years, and a lot of papers by other people, to understand how literally she meant it.

What the evidence says

The clearest field test I know comes from Uruguay. Sebastián Mazzilli, working with Armen Kemanian, Oswaldo Ernst, Rob Jackson and Gervasio Piñeiro, ran no-till maize and soybean plots near Paysandú and swapped the stubble between them. Maize and soybean carry different carbon isotope signatures, so the swap let them tell apart carbon from roots and carbon from the residue lying on top.

Roots were only about 10% of the plant carbon going into the soil, yet they supplied 60% to almost 80% of the new carbon in the particulate organic matter of the top 10 cm. Root carbon turned into soil organic matter at 10% to 24%; residue carbon at 0.5% to 1% (Mazzilli et al., 2015). Most of what we leave on the surface goes back to the atmosphere.

That result fits a larger pattern. In a review of soil carbon ecology, Jackson and colleagues estimated that root inputs are about five times more likely than an equal mass of aboveground litter to end up as stable soil organic matter (Jackson et al., 2017). Noah Sokol and Mark Bradford at Yale went a step further and separated living roots from dead ones. Working with an annual grass in a Connecticut forest, they found that carbon released by living roots was 2 to 13 times more efficient than root or shoot litter at forming soil carbon, including the slow-cycling, mineral-associated kind. They also warned that living roots and litter together could speed up the loss of older carbon (Sokol et al., 2019). It was not a crop field, but the mechanism matches what Mazzilli measured in maize and soybean.

Microbes do the building

The old picture treated soil organic matter as plant material that had rotted slowly into something dark and stable. The newer picture puts microbes in the middle.

At Colorado State University, Francesca Cotrufo, Jocelyn Lavallee and Jennifer Soong proposed splitting soil organic matter into two pools that behave very differently. Particulate organic matter is made of fragments of plants and fungi; it builds up fast and disappears fast. Mineral-associated organic matter is mostly microbial products bound to clay and silt, and it can last for centuries. Cotrufo compares the first to a checking account and the second to a savings account (ScienceDaily, 2019).

The savings account is built mostly by microbes. At the University of New Hampshire, Cynthia Kallenbach, Serita Frey and Stuart Grandy fed sterile model soils nothing but simple compounds and showed that microbes alone could build chemically diverse, stable organic matter, with no plant structures involved. The microbial community mattered more than the clay type (Kallenbach et al., 2016).

This is why roots matter so much. Living roots release sugars and other simple compounds right next to mineral surfaces, where microbes eat them, grow, die and leave their remains stuck to clay. A stalk lying on the surface feeds a different set of decomposers, far from the minerals, and most of its carbon goes back to the air as CO₂.

The story is not that simple everywhere. In a 20-year forest experiment in Oregon, removing live roots actually increased mineral-associated carbon, apparently because roots also stimulate microbes that break old carbon down (Pierson et al., 2021). Roots both build and spend soil carbon. Which effect wins depends on the soil and the system.

What changes in the field

Residue still matters. It protects the surface from rain and sun, slows erosion, keeps water in and feeds the particulate pool. I would not tell anyone to burn or bale their stubble. But if the goal is carbon that lasts, residue is the weaker lever, and counting tonnes of stubble left on the field tells you little about carbon gained.

The stronger lever is time with living roots in the ground. A field that sits bare between harvest and the next sowing is a field where nothing is feeding the microbes near the minerals. Cover crops, double cropping and pastures in rotation all add months of root activity. Seen this way, the question for a rotation is how many days a year something is alive and growing, more than how much biomass it leaves on top. That is also something satellites can measure.

It also changes what breeding should look for. Crops have been selected for a century to put more of their carbon into grain and less into roots, and in the last essays I argued that this trade-off is often worth making. For soil carbon, it has a cost. Jackson and colleagues point out that breeding for aboveground yield may come at the expense of root production (Jackson et al., 2017). If we want cover crops that build soil, roots and root exudation are traits worth selecting for directly.

And it matters for carbon markets, which often reward practices instead of measured outcomes. A protocol that credits residue retention and ignores how long living roots were in the ground is paying for the weaker mechanism.

Below the surface

In The plant is not the unit I argued that yield belongs to the population. In The crop after I argued that the unit is the sequence of seasons. This is a third version of the same mistake. We judge a crop by what we can see and weigh, the grain and the stubble, and the carbon that stays is built by what we can't see: roots, their exudates and the microbes living on them.

Primavesi would not have been surprised. She spent her career saying that the soil is alive and that you manage it by feeding that life. The papers caught up with her, and they added a detail she would have liked: most of the feeding happens underground, while the plant is still growing.