Wednesday, February 03, 2016

Healthy Ground, Healthy Atmosphere:  Recarbonizing the Earth’s Soils

Clover cover crop (Credit: Environ Health Perspectives) Click to Enlarge.
David Brandt hasn’t tilled his soil since 1972, when he rented his first 600 acres of farmland to grow wheat, corn, and soybeans.  And by keeping plants on his land in various stages of growth and decomposition, Brandt appears to have increased the amount of carbon in his soil over the years.  One study estimated that total organic carbon in the top foot of Brandt’s soil increased by 10% after six years of no-till, 35% after 20 years, and 61% after 35 years.1  (The data on which this estimate was based were not peer reviewed.)  Overall, Brandt’s soil stored, or sequestered, an estimated average 960 kg of carbon per hectare per year.1

With figures like those, soil scientists and climate researchers believe that returning carbon to the soil on a large scale could help mitigate climate change.  The world’s terrestrial carbon stores—the combined amount of carbon in soil and in plant matter—are much greater than the amount of carbon in the atmosphere:  3.12 trillion metric tons in the top meter of soil versus 780 billion metric tons in the atmosphere, by one estimate.2  Before the dawn of agriculture, there was even more carbon in the global soil pool—an estimated 55–78 billion additional metric tons,3 and the world’s plant biomass held still more, much of it lost to land use changes.2

That’s why Rattan Lal, one of world’s preeminent soil scientists and director of the Carbon Management and Sequestration Center (C-MASC) at The Ohio State University, has called for recarbonizing the world’s soils.2  Doing so, Lal says, “would be a truly win–win–win situation.”  In addition to carbon sequestration, increasing carbon in soil has many other co-benefits:  increased water storage in soil, increased length of the growing season, cooling of the ground via evapotranspiration, recharging groundwater aquifers, keeping springs and rivers flowing in the dry season.  Soil itself filters water, reduces flooding, and provides a water reserve for plants in times of drought.

Lal believes food security is an especially important co-benefit.  By one estimate, about 24% of total global land area shows evidence of impaired productivity,4 and each year some 1–2.9 million hectares is degraded so badly that it becomes unsuitable for farming.5  Increasing population levels, predicted to reach 9.2 billion by 2050, will place more pressure on the world’s farmlands to produce enough food.6

Lal has calculated that increasing organic carbon in the soil surrounding plant roots by 1 ton per hectare per year can increase grain production by 32 million tons per year.7,8  “This is especially important for small landholders of sub-Saharan Africa, South Asia, and the Caribbean,” he says.

Promising but Slow to Catch On
The same study that estimated the increased carbon levels in Brandt’s soils compared the productivity of two no-till fields on his farm—one with cover crops and one without—and found corn yields increased by 36–44% with the use of cover crops.1  In addition, Brandt estimates he uses 75% less fertilizer and herbicide on his land (he has eliminated pesticides altogether) and less fuel than if he were employing conventional methods that would require more passes over his fields.  “We operate this farm on 2.5 gallons of diesel fuel an acre, compared to 35 gallons an acre for conventional farms,” he says.

Brandt also has observed no soil erosion, and he says his crops are protected against weather extremes.  “Where we cover crop our soils,” he explains, “the soil never gets above 95–98°F in the summer, whereas in [his neighbor’s] conventionally tilled fields, the soil will get to 120–140°F.”

Despite those promising statistics, it has taken decades for Brandt to convince other local farmers to follow his lead, and many still resist.  Indeed, just across the road lies a vista of bare brown soil on his neighbor’s farm; it will remain like that all winter.  Nationally, no-till is used on about 13% of farms and cover crops on just 6%.9,10

Most conventional agricultural practices deplete rather than build up carbon.11  When farmers leave their fields bare between crops, for instance, only a small amount of organic matter is left to decompose and replenish the carbon stocks that are removed by harvesting the crops.  The situation is worse in developing countries, where farmers often remove every bit of plant material left after harvest to feed animals or to burn as cooking fuel.12  In addition, tilling the soil brings any leftover plant material into contact with soil microbes faster than if the plant were to slowly degrade on the surface of the ground, which speeds up the plant’s decomposition and the return of its carbon stores to the atmosphere.13
Biochar:  Another Opportunity to Increase Soil Carbon
Another option under exploration for increasing the carbon content of soil is biochar.  This highly stable substance is produced when plant matter is heated at high temperatures in a low-oxygen environment, a process known as pyrolysis.  Carbon is concentrated in the resulting biochar at levels twice that of ordinary plant materials.17

Biogeochemist Thomas J. Goreau, who is coordinator for the Soil Carbon Alliance information network, first encountered biochar years ago when someone brought him a sample of ash from rock layers marking the Cretaceous–Paleogene boundary, the geologic time period when scientists believe an asteroid hit the earth, causing massive fires and bringing 75% of plant and animal life to extinction.  “When we looked at the ash through a microscope,” Goreau says, “you could see every cell in the plant that had burned.  I mean, this sample was 65 million years old, and it was untouched.”

He tells that story to illustrate just how long biochar can persist in the ground.  Biochar deposits have helped produce extremely fertile soils, including the famous terra preta (“black earth”) soils in the Amazon.18  Yet, scientists say, it is only within the last decade—after the first international conference on the use of biochar to help mitigate climate change in 200719—that researchers began to seriously study it.

Read more at Healthy Ground, Healthy Atmosphere:  Recarbonizing the Earth’s Soils

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