Agriculture: Problems and Solutions

Traditional tillage releases CO₂ into the atmosphere. But conservation tillage, like this cotton planted with no-till, can help store carbon in the soil.

Tillage Releases Carbon Dioxide

Decades of tillage have caused soils on American cropland to lose up to half their virgin organic matter. Much of it may literally be going up in a puff of gas--as CO₂.

"Carbon is the backbone of the organic matter that made our native prairie soils so black and fertile," says Donald C. Reicosky, an ARS soil scientist in Morris, Minnesota. "Soil carbon levels have been declining ever since the first plows tore up prairie land."

The worst of the short-term losses occurs within minutes after the moldboard plow fractures the soil, forcefully releasing CO₂ stored in soil pores and water. "It's just like opening a bottle of champagne. The gas in the air space above the liquid is released, and CO₂ bubbles out of solution to establish a new equilibrium in the air," he says.

"The CO₂ is a byproduct of microbial feeding on, and the biological oxidation of, soil organic matter," says Reicosky, who has measured CO₂ losses from soils in Alabama, Texas, and Minnesota. He gauges the amounts with a clear, plastic chamber equipped with an infrared CO₂ analyzer and carried by a tractor.

Studies by Reicosky and colleagues show that the soil releases as much as 260 pounds of CO₂ per acre per hour immediately after tillage. Over time, even more is lost because of the extra oxygen let in by tillage and the extra organic matter from crop residues plowed under. "That speeds up decomposition," notes Reicosky. "You're able to feed more soil microorganisms faster, and there goes your organic matter."

Technician Julie Roth prepares samples for soil carbon analysis.

Not coincidentally, as the amount of soil carbon has declined, atmospheric CO₂ has gone up. The intensive tillage seen in America's post-World War II farming boom increased the rate at which soil carbon was converted into CO₂, just as the Industrial Age's coal-burning smokestacks were turning coal carbon into CO₂ at a furious pace.

But if tillage is the cause, it's also the cure, Reicosky says. Crop residue management and conservation tillage reduced carbon losses by up to four-fifths in Reicosky's studies. These practices disturb the soil less and conserve organic matter by leaving dead roots undisturbed and crop residue on the surface after harvest.

"The trick is to use crop residue management and other soil management techniques to keep carbon where it belongs," he says. "Let the soil serve as a storage reservoir, or sink, for excess carbon created from human activity, ameliorating the potential environmental harm of rising levels of atmospheric carbon dioxide."

There are estimates that widespread adoption of improved crop residue management could return soil carbon levels to near those of our native prairies, storing or sequestering a portion of the carbon released through worldwide fossil fuel emissions, Reicosky says.

Of course, returning highly erodible cropland to perennial grasses would be even better, says program leader Mayeux.

"To date, that has been done on 36 million acres (15 million hectares) of land taken out of production and covered with grass or trees under the federal Conservation Reserve Program," he says.

"Each year, these CRP soils may be storing almost a third of the 38 million metric tons of carbon released annually into the atmosphere by all sources related to U.S. agriculture. Most of these lands are dryland farms in the Great Plains."

Story Contents

Inside the "sniffing corral" at Watkinsville, Georgia, soil chemist Ronald Sharpe (left) and soil scientist Lowry Harper check equipment designed to measure methane concentrations in air.

Animal Waste Gives Off Gases

Lowry A. Harper, an ARS agricultural microclimatologist in Watkinsville, Georgia, also measures CO₂--not from soil but from animal waste treatment ponds called lagoons.

He has devised a system for measuring CO₂ and other greenhouse gases with an array of outdoor "sniff," or sampling, tubes connected to a laser spectrometer or an infrared gas analyzer.

For the lagoons, Harper mounts the sniff tubes on a floating barge to detect CO₂, methane, nitrous oxide, and ammonia emissions. Harper's research will not only help computer modelers better evaluate the greenhouse gases emitted from animal waste lagoons, but also establish whether there's enough methane emitted to make it worthwhile for a farmer to use it as fuel for an electrical generator.

Harper uses a land-based version of the sniff tubes for measuring methane emissions from cattle breath. He uses similar techniques to detect nitrous oxide on land and has measured significant emissions where animal wastes have been spread.

Eventually, he and others plan to adapt the equipment to measure gas emissions from soil, landfills, rice paddies, animal manure, and termite mounds.

From his tests so far with cattle in Australia, Georgia, and Texas--the first such outdoor tests in the world--Harper has found that a cow grazing on pasture can emit more than 8 ounces (230 grams) of methane per day. "That is somewhat more than estimates from indoor tests of confined animals," he says. The studies also pointed to a solution: Higher quality diets reduced methane emissions. Cows fed grain rather than pasture grass emitted only 2.4 ounces (70 grams) per day, about half as much as previous tests indicated.

Story Contents