Humus State of Agrochernozems in the Chulym–Yenisei Forest-Steppe under Transition to Organic Farming

The study is concerned with changes in the humus state of long-term fallow agrochernozems during the transition to an organic farming system. The work was carried out in the conditions of the Chulym–Yenisei forest-steppe of Krasnoyarsk krai. The content, structure, and pool of humus substances were determined in two fields of spring wheat cultivated using a one-summer fallow as a predecessor against an intensive background and tilled 27-year-old sod-grass long-fallow. The content of C_humus varied from 5519 to 5681 mg/100 g (C_V = 8–18%) in the soil of the wheat-wheat crop rotation link against an intensive background on naked fallow. In the tilled long-fallow in the second year after plowing, a significant increase in C_humus up to 6880 mg/100 g (C_V = 11%) was observed compared with the intensive background. In crop rotation fields after long-fallow, the content of alkaline-hydrolyzable humus compounds in the soil increased 1.3–2-fold compared with the intensive background. After plowing down (return) of long-fallow plant residues in 2020, an increase was recorded in the amount of water-soluble humus by 20% and alkaline-hydrolyzable humus substances by 49% (p = 0.01) as early as in spring of 2021 compared with the intensive background. An appreciable increase (by 96%) in humic acids in the composition of alkaline-hydrolyzable compounds points toward a better quality of mobile humus in long-fallow soil. Involvement of long-fallow in agricultural turnover contributes to an increase in the C_humus pool in the 0–20 cm layer of agrochernozems by 13 tC/ha and replenishment of the pool of mobile humus substances due to the alkaline-hydrolyzable fraction by 9 tC/ha.