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Carbon-13 labeled formate, alanine, and glucose decompose when added in aqueous solution to soils collected from the "Mars-like’’ Yungay region ( S 24 degrees 4’ 9.6 ’’, W 69 degrees 51’) of the Atacama Desert. During the first 5 d of incubation, alanine ( 5 mM) and glucose (5 mM) solutions decomposed at rates of 0.1 to 0.2 mu mol/d, and formate solution ( 50 mM) decomposed at rates of 0.4 to 1.6 mu mol/ d. The observation of approximately equal (13)CO(2) initial production rates by soils treated with D-glucose and L-alanine, compared to soils treated with L-glucose and D-alanine, indicates the presence of one or more nonbiological chemical decomposition mechanisms. An increase in the decomposition rates of D-glucose and L-alanine, compared to L-glucose and D-alanine similar to 5 d after the addition of these organics, demonstrates that the soils are also biologically active. When treated with sodium formate solution, tested soils released (13)CO(2) gas in a manner that reproduces the initial gas release observed in the Mars Viking Labeled Release (LR) experiment. Our results indicate that the (13)CO(2) produced in Yungay soils is consistent with an initial phase of nonbiological decomposition followed by biological decomposition of added organics. Heat treatment of Yungay soils eliminated all CO(2) production, while in the Viking LR experiment, the initial rapid CO(2) release was eliminated by heat treatment, but a slower secondary CO(2) production was not. Our results indicate that the mechanism for the decomposition of organics in Yungay soils is different from the processes observed in the Viking LR experiment.