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Stable Carbon Isotopic Composition of Carbonate in Holocene Grassland Soils

Dep. of Agronomy, Colorado State Univ., Ft. Collins, CO 80523

Ronald G. Amundson

Dep. of Soil Science, Univ. of California, Berkeley, CA 94720

Bruno D. Marino

Dep. of Earth and Planetary Sciences, Harvard Univ., Cambridge, MA 02138

Michael J. DeNiro

Dept. of Geological Sciences, Univ. of California, Santa Barbara, CA 93106

*Corresponding author.

ABSTRACT

Stable C isotope ratios (¹³C/¹²C) and amounts of soil carbonate were measured at six sites of differing temperature, moisture, and vegetational make-up in the northern Great Plains of the USA in order to determine climatic and biotic influences on the stable C isotope chemistry of these soils. With approximately constant precipitation, total carbonate decreased with increasing temperature. With decreasing precipitation, the total carbonate in the soils increased. For all but one soil, the ¹³C/¹²C ratios of segregated carbonate conformed to values expected if the carbonate formed from CO₂ derived from soil organic matter. In the nonconforming site, the ¹³C/¹²C ratios of the carbonate suggest that the vegetation and soil organic matter have recently become dominated by C₄ plants, which use the Hatch-Slack photosynthetic pathway, and organic matter derived from them. The ¹³C/¹²C ratios of disseminated carbonate reflect the proportion of pedogenic and parent-material carbonate in the sample. The ¹³C/¹²C ratios of pure, pedogenic carbonate at each site could be constrained by: (i) measured ¹³C/¹²C ratios of soil organic matter, (ii) measured ¹³C/¹²C ratios of carbonate nodules, and (iii) calculated ¹³C/¹²C ratios using a diffusion model designed by others. Estimates of the percentages of disseminated pedogenic carbonate in the soils suggested that, as the amount of weathering at sites increases and the total carbonate decreases, the percentage of carbonate that remains becomes more dominated by carbonate of pedogenic origin.

Contribution of the Dep. of Soil Science, Univ. of California, and Dep. of Agronomy, Colorado State Univ., Fort Collins.

Received for publication October 10, 1989.

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