Effect of repeated drying-wetting-freezing-thawing cycles on the active soil organic carbon pool

Samples of soddy-podzolic soil (long-term overgrown fallow and continuous bare fallow), gray forest soil (forest, farming agrocenosis), and a typical chernozem (virgin steppe, forest area, farming agrocenosis, continuous bare fallow) have been incubated under stable conditions; other samples of these soils have been subjected to six drying-wetting-incubation-freezing-thawing-incubation cycles during 136 days. The wetting of dried soils and the thawing of frozen soils result in an abrupt but short increase in the emission rate of C-CO2 by 2.7–12.4 and 1.6–2.7 times, respectively, compared to the stable incubation conditions. As the soil is depleted in potentially mineralizable organic matter, the rate of the C-CO2 emission pulses initiated by disturbing impacts decreases. The cumulative extra production of C-CO2 by soils of natural lands for six cycles makes up 21–40% of that in the treatments with stable incubation conditions; the corresponding value for cultivated soils, including continuous clean fallow, is in the range of 45–82%. The content of potentially mineralizable organic matter in the soils subjected to recurrent drying-wetting-freezingthawing cycles decreased compared to the soils without disturbing impacts by 1.6–4.4 times, and the mineralization constants decreased by 1.9–3.6 times. It has been emphasized that the cumulative effect of drying-wetting-freezing-thawing cycles is manifested not only in the decrease in the total Corg from the soil but also in the reduction of the mineralization potential of the soil organic matter.