| Summary: | The feasibility of implementing the "4 ppm" initiative, which assumes an annual increase in organic carbon stocks of agricultural soils in the layer 0-40 cm, was estimated with the dynamic carbon model RothC in two long-term DAOS experiments in the Moscow region, conducted in neighbouring fields for 74 and 76 years. Treatments included absolute control, application of organic, mineral, organic and mineral fertilizers at increasing rates. One of the experiments showed the growth of C stocks 12‰ in the layer 0-20 cm in the first 20 years in treatments with mineral fertilization, and 17‰ with the additional application of manure in an average annual rate of 10 Mg·ha-1. The accumulation of C allowed increasing its stock by 18-25%. Still, with the subsequent decline in crop rotation productivity, there was a loss of part of the previously accumulated C. In another experiment, at close values of annual C input, there was a loss of initial C stock due to the history of land use. The crop rotation adjustment provided a 3-8 ‰ increase of soil C in the 0-20 cm layer in the first 20 years after introduction but was insufficient to match the "4 ppm" initiative. In the long term, the organic fertilizer system had an advantage over the mineral one in ensuring the stability of organic C stocks in the arable layer. However, the management of C sequestration was complicated in the non-equilibrium state of the carbon system "plant residues-organic fertilizer-soil".
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