A model for soil 14CO 2 and its implications for using 14C to date pedogenic carbonate

Wang Yang,Ronald Amundson,Susan Trumbore

doi:10.1016/0016-7037(94)90472-3

Wang Yang, Ronald Amundson + Show 1 more

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https://doi.org/10.1016/0016-7037(94)90472-3

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Abstract

A diffusion-reaction model for soil 14CO 2 is described that analyzes the 14CO 2 distribution in soils. It shows that the 14C content of soil CO 2 is not the same as that of atmospheric CO 2 and varies with depth depending on various factors. The most important factors affecting the 14C content of soil CO 2 include the 14C content of soil organic matter, the relative contribution of root respired CO 2 to total CO 2 production, soil respiration rate, atmospheric CO 2 concentration and 14CO 2 content, and soil properties such as temperature and moisture content etc. The 14C content of soil CO 2 not only can be a sensitive indicator of the residence time of decomposing organic matter in the soil, but also determines the 14C content of pedogenic carbonate. Our model suggests that soil CO 2 could be enriched or depleted in 14C relative to atmospheric CO 2, depending on the relative contribution of root respiration to total soil respiration and on the turnover rate of the soil organic matter contributing to the soil CO 2. Therefore, the initial 14C dates of soil carbonate could differ from the true ages of pedogenesis. The processes and factors considered by the model are a first step in determining whether the 14C content of soil carbonate could lead to reliable dates of pedogenesis.

Highlights

CARBON DIOXIDEISPRODUCEDby decomposition of organic matter and by root respiration in soils
It has been previously shown that the distribution of CO2 and its stable isotopic species (e.g., ‘*COZ, 13C02) in soils can be described by a diffusion-reaction model (CERLING, 1984, 199 1; SOLOMONand CERLING, 1987; CERLINGet al., 1989; QUADE et al, 1989)
Since the 6°C value of CO2 produced by root respiration is essentially the same as that of CO2 produced by decomposition of soil organic matter, the 6 13Cvalue of soil CO2 is determined by soil respiration rate and the d 13Cvalue of soil organic matter, which is more or less constant with depth and time in a stable soil ecosystem (CERLING, 1984, 1991; CERLING et al, 1989; QUADE et al, 1989)

Summary

Geochimico PI Cosmcchimiu Acla Vol 58, pp. 393-399 Copyright Q 1994 Elwier Science Ltd. The purpose of our modeling is ( 1) to help us better understand the 14C02 distribution in soils; (2) to evaluate the different factors affecting soil 14C02 such as the relative contribution of root respired CO* to total CO2 production, soil respiration rate, 14Ccontent of soil organic matter, atmospheric CO* concentration and 14C02 content, soil properties, and temperature etc.; and (3) to explore the potential appli~tion of using soil pedogenic carbonate to 14Cdate soils or landforms. When all soil CO2 is derived from root respiration and/ or decomposition of short-lived organic matter which has the same 14Ccontent as the atmospheric CO2, the 614Cvalues of soil CO2 are relatively enriched (up to 8.5% at 15°C) compared to 6 r4C values of atmospheric CO2 (Fig. 3 ) due to diffusion effects. In our test of the model against empirical data given below, various “‘C,, values are used, based on the reported data

OBSERVATIONS IN SOILS

Findings

CONCLUSIONS