Soil Radiocarbon Record and Its Indication to the Dynamics of Soil Carbon Budget.

Soil organic matter formation is the balance between the net primary production input and the decomposition output. Since all the net primary production bears the atmospheric radiocarbon signature at the time of input and follows the first-order radiocarbon decay rule thereafter, soil radiocarbon record should be indicative of the dynamics of soil organic carbon cycle. Interpretation of soil radiocarbon record, however, has been complicated by two facts: First, soil organic matter does not decompose uniformly and no simple kinetic model could be applied to describe the dynamics. Second, although the atmospheric radiocarbon level had been relatively constant for the pre-1950 period, its level has been varied significantly since 1950, due to the nuclear bomb testing. The variable atmospheric radiocarbon level since 1950 invalidates the basic radiocarbon dating principle. The objective of this report is to discuss the strategy that we may use to overcome the difficulties in the interpretation of soil radiocarbon record in terms of soil organic carbon dynamics and its current sink/source relationship with the atmosphere. The strategy includes the used of active and stable soil carbon pools concept, the post-1950 “bomb” radiocarbon signature and the total soil radiocarbon inventory to constrain the soil carbon dynamics and deduce the current soil carbon sink/source relationship with the atmosphere. Data from long-term agricultural experiments and laboratory incubation studies are used to illustrate and examine the strategy and the theory. The strength and shortcoming of the strategy and theory are also discussed.