That's the area between the soil surface and the water table.

Tiny sediment grains are covered with a very fine-grained, complex mixture of minerals in an open fabric that results in a large surface area in contact with water between the grains. Scientists at the U.S. Geological Survey (USGS) are studying this microscopic layer and finding that the mineral composition of these coatings on sediment grains in the unsaturated zone (i.e., between land surface and the water table) can have a substantial effect on the retention of nitrate and sulfate. . .

"The unsaturated zone is potentially a large reservoir for anions (negatively charged ions) like nitrate and sulfate. . .

Nutrient storage in the very fine-grained mineral coatings on the sediment grains was attributed to a complex combination of chemical and physical storage mechanisms.

Sounds like biochar doesn't it? It's good to recall that soil scientists and ag practitioners have been working these issues from a variety of angles for a very long time. Those who have just begun to think about soil, and are only doing so because of an interest in climate, might benefit from seeing the bigger picture.

Worrying abut the loss of anions due to the chemical and physical characteristics of soil is a significant part of land management, though it is more commonly discussed as soil PH and tilth. Carbon is a major issue in this, and it isn't only carbon in mineral form such as char, it is also carbon in organic form - combined in complex molecules. Organic carbon isn't durable - it's food for soil organisms and will be cycled from one form to another as each organism feeds on the wastes of another until in the end it is emitted back to the atmosphere, mainly as CO2 and CH4 but also as a variety of VOCs. Land management is an unending race to produce more organic matter than is lost so that the chemical and physical characteristics are favorable to life.

One of my eccentrics - I collect them - is Greg Retallack, a soil scientist at the University of Oregon in Eugene. He expands on the great race between producers and consumers.

After the dinosaurs disappeared 65 million years ago, wiped out by an asteroid impact or other calamity, plants seized their chance. The emergence of the first grasses was the breakthrough. Grass doesn't hold much CO2 itself, but it can create mollisols, soils that are very rich in organic matter and hence carbon. "Typically they are 10 per cent organic matter down to a depth of a metre, whereas forest soils are only that rich down to about 10 centimetres," says Retallack. So a grassland ecosystem can, despite appearances, contain more carbon than a forest ecosystem.

Over the past 40 million years or so, tall grasslands spread across the globe, taking over many formerly forested zones. These ecosystems, Retallack argues, took control of the planetary thermostat, securing lower CO2 levels for their own advantage. New grazing animals evolved to live on and coexist with the grasses. "The co-evolution of grasses and grazer created a carbon-hungry ecosystem of a kind never before seen," says Retallack. "I think mollisols are saving our skins right now. Without them the world would be a lot hotter."

Mollisol refresher.

Mollisols are the soils of grassland ecosystems. They are characterized by a thick, dark surface horizon. This fertile surface horizon, known as a mollic epipedon, results from the long-term addition of organic materials derived from plant roots.

Mollisols primarily occur in the middle latitudes and are extensive in prairie regions such as the Great Plains of the US. Globally, they occupy ~6.9% of the ice-free land area. In the US, they are the most extensive soil order, accounting for ~21.5% of the land area.

Notice the red area in S. America in the image at right. That's what I was lamenting in the post Jerked Beef, which talked about the conversion of grasslands to row crops in Argentina. It's the best soil in S. America and will now begin to spew carbon rather than storing it.

The carbon in mollisols isn't only from plant roots.

The general public normally only hears about microorganisms in terms of viruses or harmful bacteria, to be battled with soap, disinfectants, and other weapons in our modern arsenal. However, in general the functions of most microorganisms are benign or very much to our benefit, and to destroy them would be to destroy ourselves. The human body, for example, is teeming with microlife--about 10% of the average human’s body weight is made up of microorganisms! Each square centimeter human skin hosts an average of 100,000 bacteria, maintaining the health of the skin, and countless millions occupy our intestines. Our vital abilities to breathe and digest food are all intricately linked to the microorganisms that reside within us and make our life possible. As the biologist Lynn Margulis said, “Beneath our superficial differences we are all of us walking communities of bacteria.” The web of life depends on this vast network of microorganisms.

For agriculturalists, the greatest interest in microlife is in the complex communities of microorganisms that are part of the soil. One gram (the same weight as a small paperclip) of healthy soil can contain between one and ten billion microorganisms. If the microbes underground were spread over the land surface of Earth, they would make a layer 5 feet thick. Next time you see cows or sheep raised in a pasture, remember that the invisible soil microorganisms on that same piece of land can outweigh the livestock per hectare (2.4 acres) by factors of 10 or 100 times!

For the past century, the trend in university research and in modern farmer’s practice has been to focus on the physical and mechanical properties of soil, and only recently has the living component gained recognition for its central role in land productivity and plant health. Communities of bacteria, fungi, algae, protozoa, and other microorganisms aerate the soil, make nutrients available to plants, create water and air channels, maintain soil structure, and recycle nutrients and organic matter that allow vegetation to grow. Every chemical transformation that happens in soil involves microorganisms. Some microorganisms excrete enzymes and other growth substances that stimulate plant feeding. Microorganisms provide a living reserve of nutrients like nitrogen and sulfur that would otherwise be easily leached. A healthy population of soil microorganisms can also maintain ecological balance, preventing the onset of major problems from the viruses or other pathogens that live in the soil.

Many soil microorganisms are consumers of plant materials - not only roots but any dead plant material - and so don't increase soil carbon. But some are producers. Cyanobacteria photosynthesize, and some strains are also able to fix nitrogen from the air - just as legumes seem to do. It's really rhizobia bacteria that do that in symbiosis with legumes. A fungi, mycorrhizae, associates with plant roots, especially trees, and so in a sense extend plant roots widely so that they can reach more nutrients. Both rhizobia and mycorrhizae rely on plants for photosynthesis, but they do useful work that enables the plants to grow better in return for sugary plant secretions. Soils such as mollisols that are higher in organic matter, and to a greater depth than other soil, are also especially rich in soil microorganisms and macroorganisms.

Talk of using biochar as a sort of air capture technology to reduce CO2 concentrations in the air and durably sequester it is only part of the issue, and not the important part as I see it. It is a way to restore lost soil carbon that took eons to accumulate, and will improve soil chemical and physical characteristics to be more favorable to life, not least the retention of anions needed for plant growth.