Annual Variations of the 14C Content of Soil CO2

Abstract

A 6-year and a 2-year record of 14C measurements of soil CO2 in two soils are presented and discussed. The annual 14C variation of soil CO2 is controlled by the seasonally varying contribution of root respiration and of microbial decomposition of organic matter producing soil CO2. The Delta-14C soil CO2 difference between summer and winter is ca 50 per mil in a soil where turnover of organic matter is fast (r = 2.5a) and ca 100 per mil in a soil of slow turnover (r = 60a). A simple model describing the movement and turnover of organic matter is derived, giving the depth distributions of organic carbon and of 14C. The model needs a subdivision of the carbon reservoir into at least two reservoirs with residence times of r1= 1a and r2 = 100a, respectively, and with a vertical transfer velocity in the order of 0.6mm/a.