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Abstract The objective of this study was to investigate the long‐term cycling of organic C in cultivated soils by radiocarbon dating. Samples from a clayey Haploboroll soil (Indian Head) taken in 1963 and 1978 showed a general reduction in radiocarbon age of the whole soil of 20%. This was attributed to the enrichment of 14 C in the atmosphere because of nuclear explosions in the 1960s, and its subsequent incorporation into organic matter. Relative decreases in age were least for nonhydrolyzable C and the aromatic HA‐A, the oldest and least active fractions. The apparent age of the humin and clay‐associated HA‐B decreased much more, indicating their greater involvement in short‐term cycling. A second clayey Haploboroll soil (Melfort) was studied by chemical fractionation and separation into size fractions after ultrasonic dispersion. The HA‐A and coarse clay‐associated organic C were oldest, with a much younger date for fine‐clay‐associated organic C. Fulvic acids purified by dialysis to eliminate materials less than 2000 molecular weight had an old date of 1140 ± 35 yr. Size fractionation of two other Boroll soils indicated young radiocarbon ages (radiocarbon enrichment) of fine clay‐associated organic C. These findings are discussed in terms of the cycling of organic matter and nutrients. Particular attention is given to clay‐humus interactions and their importance in protecting otherwise labile humic materials to provide a substantial pool of moderately available nutrients.
Anderson et al. (Thu,) studied this question.