Trapping carbon in soil key for protecting global food security, dealing with climate change
COLUMBUS, Ohio – Restoring soil carbon levels should be a top priority among the global community, according to a viewpoint article in this week's issue of the journal Science.
The amount of carbon that can be restored in the world's degraded agricultural soils will directly influence global food security and climate change within our lifetime, said Rattan Lal, author of the article and director of the carbon management and sequestration center at Ohio State University.
Scientists estimate that, since the mechanization of agriculture began a few hundred years ago, some 78 billion metric tons – more than 171 trillion pounds – of carbon once trapped in the soil have been lost to the atmosphere in the form of carbon dioxide (CO2).
"Converting natural ecosystems to fields for crop production and pastures depletes a soil's carbon content by as much as 75 percent," Lal said. "And the amount of carbon we emit into the atmosphere each year from industrial activity is on the rise."
With too little carbon in the soil, crop production is inefficient. Right now, the world's agricultural soils are alarmingly depleted of carbon, particularly in sub-Saharan Africa, south and central Asia and the Caribbean and Andean regions, Lal said.
He calls for adopting "recommended management practices" for increasing and keeping carbon in farmed soils. These practices include no-till farming – leaving residue from the previous year's crops on the field; agroforestry – planting trees or shrubs on or around cropland to enhance the quality of the soil; planting cover crops, which protect the soil from erosion during normal growing seasons; and using nutrients such as manure, compost or biosolids to fertilize crops.
Evidence shows that following such practices greatly increases and sustains crop yields.
Lal cited an 18-year experiment in Kenya: Farm fields managed by regular farming practices – tilling the land, using no fertilizer, leaving fields bare in the non-growing season – produced about 1 ton of maize and beans per hectare (a hectare is about the size of two football fields). But fields treated with manure, planted with cover crops and covered with mulch yielded six times that amount.
"This is the type of quantum jump in crop yield needed at the continental scale to ensure food security in Sub-Saharan Africa," said Lal, who is also a professor of natural resources. "Soil needs enough carbon in order to hold water and nutrients and to grow crops efficiently.
"But completely removing crop residue for animal fodder and fuel is the norm in many African and Asian countries," he continued. "This drastically reduces soil carbon levels, and we cannot achieve global food security without returning crop residues and putting carbon back in soil. Both are necessary for improving soil quality."
According to the report, soil carbon sequestration can offset global fossil-fuel emissions by 5 to 15 percent each year.
"The potential of carbon sequestration in world soils – about 1 billion tons per year, is equal to the renewable energy produced globally during 2001," Lal said. "Carbon sequestration is an important strategy to mitigate climate change, which can neither be ignored nor over-emphasized.
"Keeping carbon locked in soil reduces how much carbon dioxide enters the atmosphere, and it also improves and sustains soil productivity."
But soil has a finite capacity for holding carbon. And if the practices Lal outlines in his report are put to widespread use, the world's soils could reach their carbon-carrying limits in about 50 years.
That will hopefully buy researchers enough time to find alternatives to fossil fuels, which should reduce the amount of carbon released into the atmosphere as CO2.
He noted that the United States and some European Union countries already have CO2 emission-reduction policies in place. For example, the U.S. government has mandated an 18 percent decrease in CO2 and other greenhouse gas emissions by 2012.
But until we find replacements for fossil fuels, it may be difficult at best to meet those goals. In the meantime, some industrialized countries can take advantage of carbon trading markets – utility companies and other industries that rely on fossil fuels can essentially buy carbon credits from farmers who have sequestered carbon in their soils. This ensures that CO2 in the atmosphere is kept at a minimum while farmers are encouraged to use methods that increase carbon in their soils. It can also be an important source of income for struggling farmers.
"Carbon trading isn't a long-term solution, but it's one option that's being used to buy us enough time to hopefully find alternatives to fossil fuels," Lal said. "And in the short run if it helps to restore some of the degraded soil ecosystems in developing countries, why not use it? It gives resource-poor farmers income, increases productivity, restores degraded ecosystems and improves water quality. It is truly a win-win option.
"Soil carbon sequestration is a natural, cost-effective and environment-friendly process," he continued. "Once sequestered, carbon remains in the soil as long as restorative land use, no-till farming and other recommended management practices are followed."
Source: Eurekalert & othersLast reviewed: By John M. Grohol, Psy.D. on 21 Feb 2009
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