THE STABLE ISOTOPIC COMPOSITION AND MEASUREMENT OF CARBON IN SOIL CO2

The theoretical minimum difference between the delta(13)C of soil CO2 (a concentration) and soil-respired CO2 (a flux) was previously thought to be 4.4 parts per thousand. It is shown here that the difference is a function of the delta(13)C of soil-respired CO2, with possible minimum values less than 4.4%0. A simple equation is given which allows the delta(13)C of soil CO2 or soil-respired CO2 to be calculated from four measurable variables. Sampling methodology is investigated demonstrating the need for correcting measured soil-respired CO2 samples for CO2 contamination in base reagents, for contributions from atmospheric CO2, and for isotopic fraction at ion at the base-solution/air interface.