Soil as Carbon Sink
The degradation of soils from unsustainable agriculture and other development has released billions of tons of carbon into the atmosphere. But new research shows how effective land restoration could play a major role in sequestering CO2 and slowing climate change.
Now, armed with rapidly expanding knowledge about carbon sequestration in soils, researchers are studying how land restoration programs in places like the former North American prairie, the North China Plain, and even the parched interior of Australia might help put carbon back into the soil.
Absent carbon and critical microbes, soil becomes mere dirt, a process of deterioration that’s been rampant around the globe. Many scientists say that regenerative agricultural practices can turn back the carbon clock, reducing atmospheric CO2 while also boosting soil productivity and increasing resilience to floods and drought. Such regenerative techniques include planting fields year-round in crops or other cover, and agroforestry that combines crops, trees, and animal husbandry.
Recognition of the vital role played by soil carbon could mark an important if subtle shift in the discussion about global warming, which has been heavily focused on curbing emissions of fossil fuels. But a look at soil brings a sharper focus on potential carbon sinks. Reducing emissions is crucial, but soil carbon sequestration needs to be part of the picture as well, says Lal. The top priorities, he says, are restoring degraded and eroded lands, as well as avoiding deforestation and the farming of peatlands, which are a major reservoir of carbon and are easily decomposed upon drainage and cultivation.
Rattan Lal: “Soils of the world must be part of any agenda to address climate change, as well as food and water security. I think there is now a general awareness of soil carbon, an awareness that soil isn’t just a medium for plant growth.”
He adds that bringing carbon back into soils has to be done not only to offset fossil fuels, but also to feed our growing global population. “We cannot feed people if soil is degraded,” he says.
Scientists say that more carbon resides in soil than in the atmosphere and all plant life combined; there are 2,500 billion tons of carbon in soil, compared with 800 billion tons in the atmosphere and 560 billion tons in plant and animal life. And compared to many proposed geoengineering fixes, storing carbon in soil is simple: It’s a matter of returning carbon where it belongs.
One promising strategy, says Goreau, is bolstering soil microbiology by adding beneficial microbes to stimulate the soil cycles where they have been interrupted by use of insecticides, herbicides, or fertilizers. As for agroforestry, programs with greater species diversity are better able to maximize the storage of carbon than monocultures. Many researchers are looking to biochar — produced when plant matter, manure, or other organic material is heated in a zero- or low-oxygen environment — for its ability to turn problem areas into productive sites while building soil carbon. Says Goreau, “Vast areas of deforested land that have been abandoned after soil degradation are excellent candidates for replanting and reforestation using biochar from the weeds now growing there.”
An important vehicle for moving carbon into soil is root, or mycorrhizal, fungi, which govern the give-and-take between plants and soil. According to Australian soil scientist Christine Jones, plants with mycorrhizal connections can transfer up to 15 percent more carbon to soil than their non-mycorrhizal counterparts. The most common mycorrhizal fungi are marked by threadlike filaments called hyphae that extend the reach of a plant, increasing access to nutrients and water. These hyphae are coated with a sticky substance called glomalin, discovered only in 1996, which is instrumental in soil structure and carbon storage. The U.S. Department of Agriculture (mentioned in: http://www.drjulieecoethics.com/442883016 ) advises land managers to protect glomalin by minimizing tillage and chemical inputs and using cover crops to keep living roots in the soil.
One implication of this research, says Goreau, is that “the effect of most landscape alterations is to convert them from systems that store carbon efficiently … toward ones that are inefficient in the use of nitrogen, and as a result are losing carbon storage.” By landscape alterations, he means from forest to cropland, or from small farms to industrial agriculture operations that use the chemicals that inhibit the mycorrhizal and microbial interactions that store carbon.
With practicing Holistic Planned Grazing, livestock are managed as a tool for large-scale land restoration, mimicking the herding and grazing patterns of wild ruminants that coevolved with grassland ecosystems. Animals are moved so that no plants are overgrazed, and grazing stimulates biological activity in the soil. Their waste adds fertility, and as they move in a herd their trampling aerates soil, presses in seeds, and pushes down dead plant matter so it can be acted upon by soil microorganisms. All of this generates soil carbon, plant carbon, and water retention. Savory says HPG doesn’t require more land — in fact it generally supports greater animal density — so it can be applied wherever livestock are raised.
Even at our current level of knowledge, many see great potential for storing carbon in soil. Lal of Ohio State says that restoring soils of degraded and desertified ecosystems has the potential to store in world soils an additional 1 billion to 3 billion tons of carbon annually, equivalent to roughly 3.5 billion to 11 billion tons of CO2 emissions. (Annual CO2 emissions from fossil fuel burning are roughly 32 billion tons.)
As basic as soil carbon is, there’s much scientists are just learning about it, including how to make the most of its CO2 sequestration capacity.
“If we treat soil carbon as a renewable resource, we can change the dynamics,” says an expert.