News Release

MADISON, WI, FEBRUARY 18, 2008 – Agricultural practices contribute about 25% of total anthropogenic carbon dioxide emission, a greenhouse gas responsible for global warming. Soil can act both as sink or source of atmospheric carbon dioxide. Scientists at USDA-ARS, Sidney, MT have investigated the effects of irrigation, tillage, cropping system, and nitrogen fertilization on soil carbon dioxide emission and carbon storage under irrigated and dryland cropping systems in eastern Montana and western North Dakota.

The scientists measured carbon dioxide emission once a week from May to November 2005 and soil carbon levels under a combination of tillage and cropping system with or without irrigation and nitrogen fertilization and compared them under Conservation Reserve Program planting. Results from the study were published in the January–February 2008 issue of the Journal of Environmental Quality. Results were also presented in Workshop on Agricultural Air Quality: State of the Science, June 5-8, 2006, Potamac, MD and Fourth USDA Greenhouse Gas Symposium, February 5-8, 2007, Baltimore, MD.

They concluded that, although soil carbon storage was not altered, management practices influenced carbon dioxide emission within a short period due to changes in soil temperature, water content, and nutrient levels. Regardless of irrigation, carbon dioxide emission can be reduced from croplands to a level similar to that in Conservation Reserve Program planting by using no-tilled crops with or without N fertilization compared with other management practices.

Research revealed that irrigation increased carbon dioxide emission by 13% compared with non-irrigation by increasing soil water content during dry periods in western North Dakota. Tillage increased the emission by 62 to 118% compared with no-tillage in western North Dakota and eastern Montana. The emission was 1.5- to 2.5-fold greater with tilled than with non-tilled treatments following heavy rain or irrigation in western North Dakota and 1.5- to 2.0-fold greater with crops than with fallow following substantial rain in eastern Montana. Nitrogen fertilization increased the emission by 14% compared with no nitrogen fertilization in western North Dakota and cropping increased the emission by 79% compared with fallow without tillage and nitrogen fertilization in eastern Montana. The emission in undisturbed Conservation Reserve Program planting was similar to that in no-tilled crops. Carbon dioxide emission was linearly related with soil temperature and daily average air temperature at the time of measurement. Soil carbon storage was not influenced by treatments at both locations.

Carbon dioxide fixed in plant biomass through photosynthesis can be stored in soil as organic carbon by converting plant residue into soil organic matter after the residue is returned to the soil. While management practices, such as tillage, can increase carbon dioxide emission from soil by disrupting soil aggregates, incorporating plant residue, and oxidizing soil organic carbon, no-tillage practices and increased cropping intensity can increase soil carbon storage. Respiration by plant roots also contribute about half of carbon dioxide emitted from the soil. Carbon dioxide emission from soil to the atmosphere is the primary mechanism of soil carbon loss and provides an early indication of soil carbon level when changes in carbon storage due to management practices are not detectable within a short period.