As any student of soil science will tell you, soil is not just dirt under your feet. Soil is so multifunctional that there are soil health advocates who sound like they must belong to some soil fan club. Healthy soil nourishes plants, nourishes humans, fights disease, fights drought, fights flooding, and recently soil has taken on another role — fighting climate change. In the wake of the 2015 Paris Climate Agreement, André Leu, president of the International Federation of Organic Agriculture Movements, spoke excitedly about the prospects for reversing climate change. Not slowing ‒ reversing. In response to the gloomy prediction of continued global temperature rise from carbon dioxide already emitted into the atmosphere, Leu advocates building up the organic matter in the soil through more conscientious agricultural practices. He calculates that if adopted on a global scale, such practices could draw down enough carbon from the atmosphere to bring the carbon dioxide concentration to pre-industrial levels in just 57 years, possibly sooner. Soil organic matter, the stuff that darkens soil brown to black, the stuff that is the basis for plant nutrition, disease resistance, drought resistance, etc., etc., provides a service that was not even conceivable a few decades ago.
The question then becomes, how does one build soil organic matter? Organic farmers have been working on this issue from the beginning of the organic movement and have made incorporation of manure and plant residues one of their central practices for achieving this goal. However, what became apparent from the work of researchers like Dr. Hollis Waldon was that microbes would then break down the added organic matter and exhale it as carbon dioxide back into the atmosphere, leaving little to no accumulation of organic matter in the soil. This season’s organic amendments have to be replaced come next season.
One method that has succeeded in increasing organic matter is no-till farming. In no-till, the tillage is replaced by herbicides or organic practices to manage weeds, and herbicides or crimping to terminate cover crops, and thus quantities of plant residue are left on the field. Most importantly, when the soil is not opened up, the microbes are deprived of oxygen and are slow to make use of the fuel in the organic matter. How does the organic matter get into the soil? How much organic matter might be contributed by the roots versus the aboveground portions of the plant, its “shoot”? How long does this season’s organic matter last in the soil? Is there a way to figure out where the carbon from the plant ends up after the plant dies?
There is. Researchers can essentially mark the carbon in carbon dioxide, feed it into the plant’s leaves, and detect it in plant residue and the environment afterward. This is done using rare variants of carbon. A well known carbon variant is carbon fourteen, abbreviated 14C, the same type of carbon at the heart of radiocarbon dating. The number 14 signifies that it has fourteen neutrons in the nucleus rather than the usual twelve, and this makes it radioactive. In carefully controlled laboratory experiments a researcher can use chemical compounds made with radioactive 14C in biological systems to discern with high sensitivity how the compounds move and where they end up, using techniques for detecting radioactivity. For open field experiments, though, researchers take advantage of a lesser known carbon variant, 13C. Its thirteen neutrons make its nucleus stable, so it can be handled without so much as gloves.
The non-radioactive 13C atoms are detected using a machine known as the mass spectrometer. The researcher takes a sample and burns it to turn the carbon back into carbon dioxide. The mass spectrometer gives the carbon dioxide molecules a charge and then shoots them through a magnetic field, which makes them curve toward a detector as they pass. A sample’s carbon dioxide will always contain a mix of regular and 13C. The extra neutron makes 13C heavier than the regular carbon, and that causes a carbon dioxide molecule made from it to travel slightly farther along the detector than an ordinary carbon dioxide molecule. The researcher records the proportion of heavy versus regular carbon in the various samples, and then with the appropriate bookkeeping determines exactly how much of the carbon dioxide that was originally applied has ended up in the samples of interest.
Dr. Emily Austin fed carbon dioxide containing extra 13C into a rye cover crop and followed it into the soil organic matter. She addressed the question of whether it is roots or shoots that contribute to organic matter accumulation by giving the 13C-enriched carbon dioxide to half of her cover crop patches and leaving the others open to the atmosphere. At termination, when she cut down the shoots, she switched the 13C shoots with the untreated ones. At that point, any 13C that was in the soil from which the 13C shoots were removed could only have come from the roots, and any in the plain-air patches that received 13C shoots would have to be from those shoots.
And the answer is, soil organic matter comes mainly from the roots. After five months ‒ the end of the summer corn-growing season ‒ there was four times as much organic matter from the roots and from the carbon that they had given off into the soil around them than there was from the shoots, and after a year there was seven times. By comparison, at the time that the shoots were first cut the roots had 1/6 less carbon than the shoots.
This result is consistent with the idea that lack of oxygen slows the breakdown of organic matter. Chopped leaves and stems sitting on top of the soil in this no-till system are exposed to oxygen and get broken down quickly by microbes, while buried roots are relatively protected. There is more to the story, though. Dr. Kris Nichols and other researchers who specialize in the beneficial fungi that live in roots claim that these fungi fill the soil with a sticky protein known as glomalin and that this glomalin is the source of recalcitrant organic matter, the organic matter that is hard to break down. With 13C flowing into the rye roots and their associated fungal partners, Dr. Austin was in a position to look for evidence of this claim.
Dr. Austin further processed soil samples from the experiment and extracted organic matter that had differing degrees of protectedness ‒ the unprotected free bits, the stickier stuff tucked away inside tiny clumps of mineral particles, and the small metabolites that form an intimate coating on the clay particles. She also chloroform-fumigated soil samples to extract total microbial biomass, that is, all the living fungi and bacteria. She then shot all these organic matter extracts through the mass spectrometer. The story that emerged from this finer analysis is that only about 2/3 of the carbon atoms that move into the roots are retained to build root tissue. The other third are sent directly to the soil. Growing root tips release carbon-rich mucilage that feeds microbes and coats soil particles. Slender short-lived root hairs and threads of fungi work their way into tiny aggregates of soil particles where they are relatively protected and deposit their carbon. Soil microbes incorporate the carbon into their cells within hours, and these cells turn over rapidly and release the transformed carbon into the soil, further contributing to the clumping and the coating of the particles. The organic matter that coats the clay particles is protected from degradation by the way it is bound to the mineral surface, suggesting that while the protein glomalin may play a role in binding aggregates together, it is not the only source of persistent organic matter. The shoots can enter the same pathway of transformation, but only at a later stage of breakdown.
What does this mean in practical terms? Dr. Austin’s focus is the loss of organic matter from farmland when the leftover stalks and leaves of a corn crop are hauled off for bioenergy. If the shoots of the corn account for a small fraction of the organic matter that builds up in soil, then the removal of a major portion of them can be fully compensated using the cover crop. Dr. Amélie Gaudin investigates the feasibility of increasing soil organic matter in fields where the shoots are grazed by livestock. Glomalin researcher Dr. Nichols insists that every field should have something growing on it at all times. In fact, at the end of Dr. Austin’s project, after a second corn crop, the amount of 13C detectable in the soil from the original cover crop had fallen to a very low level, and the difference in the relative contributions of root and shoot were negligible. Cover crops have to be used every year in order to build up soil organic matter and sequester carbon out of the atmosphere over the long term.
If soil organic matter buildup can play such a key role in pulling carbon dioxide out of the atmosphere, climate activists sometimes ask how to convince farmers to take on such a burden. One incentive is direct government payments to farmers who switch to methods that capture carbon dioxide, such as California’s Healthy Soils Program. However, increasing soil organic matter is in the farmer’s self-interest. One of the functions of soil organic matter is to increase the soil’s water holding capacity, and another is to decrease its erodibility. These are exactly the advantages a farmer needs to face a future of harsher droughts and bigger storms. Soil organic matter not only fights climate change, it helps farmers weather its effects. Soil organic matter lives up to the hype. Time to join the club.