"My assumption is that we'll go ahead with the second mission the way it is," he said. "We're searching for ideas that might be alternatives to taking the soil out."

The next experiment in human habitation is scheduled to begin Feb. 26 and to last one year. The shorter period of occupation -- one year versus two -- in itself will lessen the severity of the problem since the decline in oxygen becomes more severe and dangerous over time. The crew of men and women for the new mission has yet to be selected, although a group of candidates is now training at the Arizona site.

During the two-year trial run, which ended Sept. 26 amid celebratory fanfare, four men and four women gasped increasingly thin air inside the 3.15 acres of glass domes. Beginning in January, then again in August and September, project officials injected 23 tons of pure oxygen into the sprawling glass bubble to avoid a medical emergency, raising levels from a low of 14 percent oxygen to the normal 21 percent.

Work on Biosphere 2 (project officials refer to Earth as Biosphere 1) began in earnest around 1984, financed by Edward P. Bass, the Texas billionaire and oil heir. The plan was that human inhabitants would thrive in a miniature world whose environmental cycles were powered by an ocean, rain forest, marshland, desert and farm. Everything would be recycled. Most importantly, the project sought to show that humans could for the first time create a life-support system that was totally self-sufficient and esthetically pleasing -- sort of a new Eden.

Despite its New Age overtones, the project was promoted as highly scientific. Its officials were quick to note the prestigious talent hired to get things under way, including the Smithsonian Institution's Marine Systems Laboratory, the New York Botanical Garden's Institute of Economic Botany and the University of Arizona's Environmental Research Laboratory. Delicate Atmospheric Balance

The university laboratory used powerful computers to predict how various combinations of plants, animals and soils would affect the structure's atmosphere. Dr. Robert J. Frye, a senior researcher there, said in an interview that all his mathematical models assumed that the agricultural soils would contain 4 or 5 percent organic matter -- what he called "representative figures" for standard enrichment.

Dr. Frye said that some project officials "wanted to use much higher amounts" and that an important issue in discussions was whether bacterial respiration would upset the atmospheric balance. Those questions, he said, "came up frequently in meetings." Limited to giving advice, the university laboratory apparently had little idea that the enrichment of some soils would turn out to be quite high.