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A study of CO2 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 CO2 (ppm), delta C-13 (parts per thousand), and Delta C-14 (parts per thousand). At all times, the concentration of CO2 in the soil gas was higher than in the air at the surface (CO2 similar to 400 ppm; delta C-13 similar to -9 parts per thousand). The main source of the CO2 in soil gas is from biotic activity released through roots of trees and of seasonal plants close to the surface. In the winter, the CO2 concentrations were lowest (6000 ppm) and the delta C-13 was -20 parts per thousand. In the spring and through the summer, the CO2 concentration increased. It was estimated that the major source of CO2 is at similar to 240 cm depth (delta C-13 similar to -22 parts per thousand; CO2 similar to 9000 ppm) or below. Above this level, the concentrations decrease and the delta C-13 (parts per thousand) become more positive. The C-14 values in the measured profile are all less than atmospheric and biotic C-14. It was deduced that biotic CO2 dissolves in porewater to form carbonic acid, which then dissolves secondary carbonate (delta C-13 similar to -8 parts per thousand; C-14 similar to -900 parts per thousand) from the sediments of the USZ. With the C-14 data, the subsequent release of CO2 into the soil gas was then estimated. The C-14 data, supported by the C-13 and CO2 data, also indicate a biotic source at the root zone, at about 90 cm depth.