δC values of CO from soil respiration on sites with crops of C3 and C4 type of photosynthesis.

Based on the differential discrimination between CO and CO during photosynthetic CO fixation by C3 and C4 plants, the relative contribution of organic soil material of varying age can be checked by measuring the δC value of CO evolved during soil respiration.The CO from a soil cropped only with C3 plants for at least 7 years showed a δC value very close to that of C3 plant tissue. Correspondingly, a soil bearing the C4 plant maize for at least 6 years had a δC value for the respiratory CO quite near to that of a typical C4 plant. A soil cropped with maize in the year of study only showed intermediate δC values, with a tendency towards C4. With soil standardised for cultivation and cropping, a quantification of the part that fresh organic matter and especially the last year plant residues play in soil respiration, can be achieved.