Curbing greenhouse gas emissions from cultivated fields may require going beyond cutting back on nitrogen fertilizer and changing crop rotation cycles, according to work by USDA Agricultural Research (ARS) scientists.>

Jane Johnson, a soil scientist at the ARS North Central Soil Conservation Research Laboratory, near Morris, Minn., is looking for practical ways to keep carbon in the soil and reduce greenhouse gas emissions.

In a comprehensive study, Johnson grew corn, soybean, wheat and alfalfa in rotation so that each crop grew in the same year, on plots treated with and without fertilizer. She also used a less-aggressive tillage system known as strip tillage, in which only narrow bands of soil are tilled instead of an entire field.

For comparison, she replicated the cropping system adopted by many Minnesota farmers — growing corn and soybeans in a 2-year cycle on fertilized plots tilled with a chisel or moldboard plow.

Johnson used a hydraulic soil probe to measure the organic carbon sequestered in the soil, and closed-vented chambers to measure emissions of carbon dioxide, methane, and nitrous oxide.

She found that when measured over the course of a year, greenhouse gas releases were largely the same under 2-year and 4-year rotation systems, and that applying nitrogen fertilizer had less overall impact than anticipated on nitrous oxide emissions. Nitrous oxide emissions peaked during spring thaws when the sun warmed the soil, regardless of the tillage or rotation system.

Chisel and moldboard-plowing increased carbon dioxide emissions for a short time. But measured over the course of a year, carbon dioxide emissions were no different from plots with intensive tillage than plots without it. Johnson also found no consistent patterns to methane releases.

Johnson's work is part of a 5-year ARS project known as GRACEnet (Greenhouse gas Reduction through Agricultural Carbon Enhancement network). Researchers at more than 32 sites are examining strategies to help reduce agriculture's climate change footprint. The project supports the USDA's  priority of responding to climate change.

More information about this research appeared in the November/December 2009 issue of Agricultural Research magazine.

ARS is USDA's principal intramural scientific research agency.