A potting soil mixture depleted carbon monoxide in a test atmosphere from a concentration of 120 parts per million to near zero within 3 hours. Maximum activity occurred at 30 degrees C. Steam sterilization of the soil, the addition of antibiotics or 10 percent (by weight) saline solution, and anaerobic conditions all prevented carbon monoxide uptake. Sterilized soil inoculated with nonsterile soil acquired activity with time. Samples of various natural soils differed in their ability to remove carbon monoxide from the air. Acidic soils with a high content of organic matter were generally the most active. The soil's ability to remove carbon monoxide from the atmosphere is ascribed to the activity of soil micro-organisms.

The CO uptake capacity of soils was studied in the field in order t o refine previous estimates of the potential of soil as a CO sink. Soils representative of most of the major vegetative regions of North America as well as roadside soils and soils under cultivation were exposed in situ t o atmospheres containing initial concentrations of ca. 100 ppm CO in the air. The CO uptake capacities of the soils ranged from 7.6 to 115 mg CO/h/m2, with the tropical soils showing the greatest activity and the desert soils the least. Soils under cuItivation were consistently less active than the same soils nearby under natural vegetation. Laboratory investigations indicate that the differences were probably not due to agricultural chemicals but rather t o a lack of organic matter in the tilled soils. Roadside soils were consistently higher in CO uptake capacity than similar soils in remote regions. Laboratory studies support the idea that this increased rate was due to the constant exposure of roadside soils t o high levels of CO. Data for the field studies was corrected for the influence of environmental variables based on laboratory studies and the potential capacity of the soils of the conterminous United States was estimated t o be an uptake of 505 million tons of CO per year. This capacity, more than twice the estimated anthropogenic CO in the U.S. annually, indicates that potentially soil is a major sink for atmospheric CO.

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