If You want to farm green, two decisions about how you grow your crops are likely to have the biggest impact on achieving your goal — how much tillage you use, and how well you manage nitrogen.

Although there probably are hundreds — or thousands — of other decisions, practices and technologies that can contribute to a green farming regime, these two factors alone can come close to eliminating your cropping system's global warming footprint. At the same time, you'll also reduce your operation's impact on soil and water erosion, as well as surface and groundwater degradation from fertilizers and other ag inputs. You also may be able to turn this environmental stewardship into extra cash by selling carbon credits associated with no-till. (See “Cash from carbon,” page 36, for details.)

This surprising finding — that intensive crop production practices in use today can be nearly carbon neutral — is emerging from research on both greenhouse gas production and greenhouse gas sequestration associated with production of annual crops.

The research shows that basic crop management decisions to reduce tillage and use nitrogen more efficiently can cut a farming operation's impact on global warming gases by 90% or more. In a best-case scenario, which would include no-till, these everyday practices come close to zeroing out a farming operation's greenhouse gas footprint.

How's that for farming green?

The greenhouse impact

The possibility of having only a minimal impact on greenhouse gases might seem unlikely in an industry that relies heavily on carbon-emitting diesel fuel, as well as pesticides and fertilizers, which generate carbon dioxide as they are manufactured.

But the soil's ability to sequester large amounts of carbon when it isn't tilled, along with the ability of most soils to impound small amounts of methane, goes a long way toward offsetting carbon dioxide emissions associated with burning fuel and manufacturing fertilizers and pesticides.

“Some people are surprised by this, but our research shows that no-till can almost completely mitigate all other sources of greenhouse warming in field crop systems,” says Philip Robertson, an environmental scientist at Michigan State University who has conducted extensive research on the impact of farming practices on global warming gases. Other reduced tillage systems, such as strip-till, store less carbon than no-till, but they still offer significant potential for mitigating greenhouse gases.

The second biggest opportunity for reducing greenhouse gases in crop farming is to boost nitrogen use efficiency, according to the research. This reduces off-gassing of the important greenhouse gas nitrous oxide.

Greenhouse gas basics

Although the list of greenhouse gases is long, Robertson's research focuses on the big three: carbon dioxide (CO₂), nitrous oxide (N₂O) and methane (CH₄). In terms of volume and total impact, these are the most important and pervasive greenhouse gases and the most prevalent in agricultural crop production.

Robertson and other scientists calculate the impact of these greenhouse gases in universal CO₂ units, which they call Global Warming Potential (GWP). This helps them assess various mitigation strategies, since some greenhouse gases are, in effect, more powerful than others.

For example, a molecule of nitrous oxide emitted to the atmosphere has an impact that is 300 times greater than that of a molecule of CO₂. This greater impact is due primarily to the facts that nitrous oxide has a greater ability to capture heat and it has a longer expected lifetime in the atmosphere (about 140 years as opposed to about 10 years for CO₂).

No-till vs. conventional vs. organic

Robertson's research compared the net GWP of corn-soybean-wheat rotations grown under conventional tillage, no-till, a low-input system with a legume cover and an organic system with a legume cover. He also examined the GWP of alfalfa and poplar trees, as well as the GWP of land in various stages of succession from grassland to forest.

To conduct the research, Robertson analyzed air samples taken multiple times each year from each plot over a period of 10 years. (For a description of this and other greenhouse gas research, visit www.kbs.msu.edu/faculty/robertson. Look for “Greenhouse Gases in Intensive Agriculture.”)

The research showed that no-till had the lowest net GWP of all annual cropping systems. It released about 125 lbs./acre of CO₂ equivalent into the atmosphere, whereas conventional tillage released the most — about 1,000 lbs./acre. Notably, no-till sequestered about 1,000 lbs./acre of GWP, which nearly offset the balance of GWP produced by fertilizer, lime, fuel and nitrous oxide released from the soil. No-till also sequestered about 45 lbs. of methane equivalent, as did the other cropping systems.

The organic system, which was in second place, produced a net GWP of about 365 lbs./acre. That's about three times the carbon footprint of the no-till system. But its footprint is far less than that of a conventional tillage system. That's primarily because no nitrogen fertilizer, pesticides and lime were applied and because a small amount of carbon was sequestered in the soil, versus none for conventional tillage. The low-input legume system produced slightly higher GWP, about 560 lbs./acre.

In comparison to the annual cropping systems, alfalfa sequestered, or removed, about 180 lbs./acre of GWP, and poplar sequestered about 1,000 lbs./acre. Plots just beginning the succession from grassland to forest removed the most GWP, about 1,900 lbs./acre. A late successional forest sequestered only about 35 lbs./acre.

Nitrous oxide

Nitrous oxide gas, which is given off by the soil as excess nitrates are eaten by bacteria, is drawing more attention from scientists who are examining greenhouse gases associated with farming systems. Until recently, nitrous oxide received little notice, except at the dentist office or the drag strip. (Nitrous oxide is the “laughing gas” sedative used by dentists — and the horsepower enhancer sometimes used by tractor pullers and drag racers.)

Scientists had thought that nitrous oxide was produced primarily in wetlands. But no one knew for sure, because it is difficult to measure, Robertson says.

That changed after new analytical instruments became available and scientists developed the labor-intensive procedures needed to test for nitrous oxide. They learned that virtually all soils produce this powerful greenhouse gas. How much is produced depends heavily on how much nitrogen is in the soil.

The greenhouse gas testing procedure involves plopping a box about a foot square on each plot, then taking gas samples about two hours later. This is done at multiple locations in each plot throughout the year, since nitrous oxide off-gases at varying rates, depending on the location and the time of year.

All the annual crop management systems in Robertson's study produced nitrous oxide in a narrow range centering on a GWP of about 500 lbs./acre. That included the organic system, whose nitrogen was supplied by manure, not supplemental fertilizer nitrogen.

This shows that the source of the nitrate does not matter. “Organic farming is good for the environment in one sense [because it reduces pesticide use], but there is little evidence that it keeps nitrogen out of the environment,” Robertson says.

Various strategies can be used to reduce nitrous oxide off-gassing without reducing yields, he says. These include side-dressing, use of time-release fertilizers and other approaches to supplying nitrogen when the crop can use it.

“There is relatively moderate nitrous oxide production until you saturate the crop with nitrogen,” he says. “If you spoon-feed nitrogen, you can reduce the amount of nitrous oxide a lot without affecting yield.”

Carbon credits to reduce N₂O?

Policy makers are expected to focus on ways of reducing nitrous oxide emissions. In Europe, for example, to earn farm subsidies, growers must apply nitrogen below capped levels.

Robertson is hoping that a carrot approach will be used in the U.S. through the emerging market for carbon credits. He is currently conducting research sponsored by the Electric Power Research Institute, a consortium of electric power companies, to examine the practicality of including nitrous oxide mitigation in the carbon market.

“If we can convince the prime exchanges to accept nitrous oxide as a carbon credit, that will expand the opportunities for farmers,” Robertson says.