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A rapid and sensitive method of soil carbonate analysis, utilizing gas chromatography, is presented. The analysis of soil samples by gas chromatography and the Chittick methods produced comparable results (r= 0.96, slope = 1.11). However, the gas chromatograph method was much more rapid. There was also a linear relationship between the results of soils analyzed by both the gas chromatograph and Hg manometer methods (r= 0.99). The gas chromatograph underestimated CaCO3contents at low concentrations, however, and overestimated it at high concentrations relative to the manometer method. The gas chromatograph method was the most sensitive and had the lowest possible detection limit (0.008 mg CaCO3) of the methods compared. The results of the study indicate that the gas chromatograph method is much more rapid than previously reported chromatography methods and has the potential to be used for the rapid analysis of samples low in CaCO3.

We measured selected properties of a naturally saline and sodic soil in pedons that had been reclaimed a minimum of 0, 5, 8, 15, or 25 yr. More than 7.2 x 104kg of soluble salt were leached from the upper 100 cm of the soil after 5 yr, with little or no additional removal with time. A steady-state soil solution composition was reached after approximately 15 yr based on electrical conductivity values of the saturation extracts. The sodium adsorption ratios (SAR) of the steady-state solutions were either greater or less than the SAR of the irrigation water and appeared to be related to mineral weathering. Smectite minerals were unstable in the reclaimed soils and were transformed to kaolinite. This may have been the cause for the reduction in water-dispersible clay in soils that had been reclaimed for 15 yr or more.

This paper reports the changes in soil properties that have occurred since vegetation was established on unstabilized dunes approximately a century ago in Golden Gate Park, San Francisco, CA. The static and dynamic soil properties under artificially established eucalyptus (Eucalyptus globulusLabill.), Monterey pine (Pinus radiataD. Don), cypress (Cupressus macrocarpaHartw. ex. Gord.), as well as native live oak (Quercus agrifoliaNée), were compared with those of an unstabilized dune. The O and A horizon development occurred under all the vegetated sites with differences in the horizon thickness occurring in response to vegetation type. The organic C in the vegetated sites (40 000 to 80 000 kg C ha−1) was much greater than in the dune (5000 kg C ha−1). The vegetated soils all exhibited severe water repellency in the surface horizons while the dune exhibited none. The oxalate‐extractable Fe in the A horizon of the vegetated soils (1.3–2.3 g kg−1) was greater than that in the dune (≈0.9 g kg−1) suggesting the formation of organically bound Fe and/or poorly crystalline Fe minerals. Dithionite‐citrate‐bicarbonate extracts suggest the formation of more crystalline forms of Fe oxides in the vegetated sites. Soil temperatures were lower and less variable in the vegetated sites (isomesic regime) than the dune site (mesic or thermic regime), apparently slowing biological activity and reducing the soil CO2contents relative to the dune. Total soil respiration in the vegetated soils (350–400 mg CO2m−2h−1) was slightly greater than in the dune (300 mg CO2m−2h−1), which lacked any appreciable surface litter undergoing decomposition.

The C and O isotopic composition and radiometric ages of laminar pedogenic CaCO3coatings were examined in limestone‐derived alluvium along an elevational and climatic transect of the Mojave Desert of Nevada. The δ13C of the soil CO2decreased with increasing elevation and was related to plant density and available soil moisture. Laminar pedogenic carbonate coats which formed on the bottoms of clasts were separated into inner and outer laminae. The δ13C of carbonates in the outer laminae decreased with increasing elevation as a result of a decrease in the percentage of detrital carbonate in the sample and a decrease in the δ13C of the soil CO2. The δ18O of the outer laminae also decreased with elevation and, after correcting for contamination by detrital carbonate, appeared to correspond to changes in the isotopic composition of the precipitation. The δ13C of the carbonate in the inner laminae also decreased with increasing elevation; however, carbonate in the inner laminae was much purer than the outer laminae and appeared to be predominantly pedogenic in origin. The δ13C of these layers reflects the δ13C of soil CO2and, for most samples, corresponded to expected values for modern carbonate. Based on uncertainties in dating techniques and the estimated range in isotopic composition of pedogenic carbonate, however, it was not possible to determine if some of the inner laminae formed in the latest pluvial period and reflect past pedogenic conditions.

The stable C and O isotope chemistry of native and irrigated Typic Natrargids was examined in order to determine the relationship between the climate and vegetation and the δ13C and δ18O of the native pedogenic carbonates (CaCO3) and the effect of irrigation on the CaCO3in the cultivated soils. The native soil had a sparse vegetative cover consisting of grasses and salt tolerant shrubs, especially Atriplex polycarpa(Torr.), S. Wats. The δ13C of the CaCO3reflected the mixture of C3 and C4 vegetation and the δ18O of the CaCO3suggested that it had formed from soil H2O that had undergone extensive amounts of evaporation. At the time of sampling, alfalfa (Medicago sativaL.) was being grown on the irrigated soils. These soils had CO2partial pressures of up to 7 KPa, nearly 20 times that of the native soil. The δ13C of the CaCO3in the irrigated soils was not significantly different than that of the native soils which was due, in part, to the relatively small isotopic difference between the newly formed CaCO3in the irrigated soil and much of that which occurred naturally. However, the δ18O of CaCO3in the irrigated soils appeared to be shifted to more negative values than the δ18O of the native CaCO3. Based on the differences in the δ18O of the CaCO3in the irrigated and native soils, it was semiquantitatively estimated that between 10 to 40% of the carbonate in the irrigated soils was newly formed or had dissolved and reprecipitated as a result of irrigation, increased biological activity, and the application of chemical amendments.

The relationship between the stable C-isotope composition of the soil environment and modern climate and vegetation was determined empirically along a present-day climatic transect in the eastern Mojave Desert. The δ 13C of the soil CO 2and carbonates decreased with increasing elevation and plant density, even though plant assemblages at all elevations were isotopically similar. Several factors, including differences in the ratios of pedogenic of limestone calcite and differences in past vegetation, were considered as explanations of this trend, However, it appears that in the sparsely vegetated Mojave Desert, the δ 13C of pedogenic carbonate is controlled by differences in plant density and biological activity. This relationship may provide a tool for assessing past vegetational densities, as long as the vegetation is isotopically homogeneous.