Variations in Soil CO 2 Concentrations and Isotopic Values in a Semi-Arid Region Due to Biotic and Abiotic Processes in the Unsaturated Zone

• Published in: Radiocarbon
• Main Authors: Carmi, I, Yakir, D, Yechieli, Y, Kronfeld, J, Stiller, M
• Format: Article in Journal/Newspaper
• Language: English
• Published: Cambridge University Press (CUP) 2013
• Subjects: General Earth and Planetary Sciences; Archeology; Carbonic acid
• Online Access: http://dx.doi.org/10.1017/s0033822200058082; https://www.cambridge.org/core/services/aop-cambridge-core/content/view/S0033822200058082

A study of CO 2 in soil gas was conducted in a bare plot in the unsaturated zone (USZ) of Yatir Forest, northern Negev, Israel. In 2006, 6 tubes for sampling of soil gas were inserted into the USZ to depths of 30, 60, 90, 120, 200, and 240 cm. Profiles of soil gas in the USZ were collected from the tubes 5 times between October 2007 and September 2008. Measurements of the collected profiles of soil gas were of CO 2 (ppm), δ 13 C (′), and Δ 14 C (′). At all times, the concentration of CO 2 in the soil gas was higher than in the air at the surface (CO 2 ≃ 400 ppm; δ 13 C ≃ −9′). The main source of the CO 2 in soil gas is from biotic activity released through roots of trees and of seasonal plants close to the surface. In the winter, the CO 2 concentrations were lowest (6000 ppm) and the δ 13 C was −20′. In the spring and through the summer, the CO 2 concentration increased. It was estimated that the major source of CO 2 is at ≃240 cm depth (δ 13 C ≃ −22′; CO 2 ≃ 9000 ppm) or below. Above this level, the concentrations decrease and the δ 13 C (′) become more positive. The 14 C values in the measured profile are all less than atmospheric and biotic 14 C. It was deduced that biotic CO 2 dissolves in porewater to form carbonic acid, which then dissolves secondary carbonate (δ 13 C ≃ −8′; 14 C ≃ −900′) from the sediments of the USZ. With the 14 C data, the subsequent release of CO 2 into the soil gas was then estimated. The 14 C data, supported by the 13 C and CO 2 data, also indicate a biotic source at the root zone, at about 90 cm depth.