Mean growing season soil PCO, data were obtained for 19 regions of the woi.1 in nine countries. Bivariate anL multiple linear regression analysis with soil log(PC0,) as the dependent variable and TEMP, PRECIP, log (AET), and log(PET) as the four climatic independent variables demonstrated that AET was the best independent predictor of soil PCO, .An improved soil PC0,-AET model was developed by assuming (1) that as AET approaches zero, soil PCO, approaches the atmospheric value and (2) that there is an upper limit to soil PCO, at very high AET. This model has the form log(PC0,) = -3.47 + 2.09 (1 -e-0'00172 AET) where AET is in mm. It explains 67 per cent of the initial variation in the soil PCO, data, predicts a soil log(PC0,) of -3.47 at AET = 0, and an upper limit of 3.5 per cent (log(PC0,) = -1.45) for mean growing season soil PCO, at AET values of 2000mm and above. The results of this study suggest that soil PCO, levels in tropical areas are, on average, higher than those in temperate, alpine, and arctic regions.

Concentrations of secondary clay minerals (mica, vermiculite, montmorillonite, chlorite, and kaolinite) in the A horizon of 99 U.S. soils were used in a stepwise regression with climate, parent material, and soil property variables to develop models of the occurrence and abundance of each clay mineral species. Climate-especially water balance estimates of soil water surplus, water deficit, and indicators of seasonality-were especially useful variables. These models correctly predicted the dominant (most abundant) clay minerals for 80 of the 99 sites even though the RZ values for individual clays were modest. Maps of predicted and actual clay dominance are in good agreement. In addition, the models may be used to devise a weathering sequence of clay minerals from least to most stable (i.e., mica < montmorillonite < vermiculite < kaolinite). Weathering sequences based on existing models place vermiculite before montmorillonite. The models in this study suggest that vermiculite with aluminum as the interlaying cation is common in U.S. soils and is formed in a more intense weathering environment than is montmorillonite; vermiculite should therefore follow montmorillonite in the weathering sequence. This result agrees with many field studies and suggests that the generally accepted sequences need additions and revision. Finally these models support geochemical theories that imply that seasonal wetting and drying are important controls of the formation of secondary clay minerals.

The concentrations (per cent) of secondary clay minerals (mica, vermiculite, montmorillonite, chlorite and kaolinite) in the A-horizon of 99 soil sample sites were entered into a cluster program to determine the common clay mineral assemblages found in U S . soils. Five major assemblages were identified with the most common assemblage consisting of mica, montmorillonite and kaolinite. A suite of climatic variables was determined for each of the 99 sites, including the water balance variables and Arkley's leaching index. A discriminant analysis was then used to predict the occurrence of a clay assemblage from the climate data. Sixty per cent of the sites could be correctly predicted, although not all assemblages could be predicted equally well; two assemblages could be predicted with 88 and 82 per cent accuracy. The precipitation of the wettest month and Arkley's leaching index accounted for most of the predictive power. Thus this study supports theoretical models which suggest that seasonal moisture supply is an important control of clay genesis and persistence. A residuals analysis indicated that many of the palaeosols in the data set and many of the soils from glacial parent materials possessed clay mineral assemblages not expected on the basis of modem climate.

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