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SUMMARY Three methods of cultivation, conventional tillage (CT), superficial tillage (ST) and no‐tillage (NT), were applied for 17 years to continuous maize. Their effect on soil organic carbon content was investigated through measurements of carbon and 13 C/ 12 C ratios, using the natural difference in 13 C content between C 3 plants and maize, which is a C 4 plant. Because the soil had carried C 3 plants before the experiment started, the organic carbon remaining from that time (C 3 ,‐carbon), was distinguished from the carbon derived from maize. Comparison between continuous wheat and maize plots showed that organic matter from both maize and wheat decomposed without significant 13 C enrichment, whereas older C 3 ‐carbon was enriched by 1.5% compared to that of fresh wheat material. From the initial 3.6 kg C m −2 in the topsoil (0–30 cm), 0.95 were mineralized in the CT treatment, but only 0.45 in NT. The mineralization was the same in the tilled layer of ST as in CT. The CT treatment accumulated 1.1 kg C m −2 of maize‐derived carbon and the NT treatment 0.8. The mineralization of initial C 3 ‐carbon was the same at all depths between 0 and 30 cm in the NT treatment; 75% of the carbon derived from maize was found in the 0–5 cm layer.
Balesdent et al. (Sat,) studied this question.