Effect of the Frost-Free Period Duration on the Fractions of Active Carbon Pool in the Freezing Agrosoils of the Steppe and Forest-Steppe Zones

The data of a long-term laboratory and microfield experiment assessing the effect of the duration of the freezing period on the state of organic matter and biological activity of agrochernozems (Voronezh oblast, Russia) and arable chestnut soils (Rostov oblast, Russia) are summarized. Water-soluble organic and inorganic carbon contents, the amount of soil organic matter (SOM) mineralizable fractions, and the content of microbial biomass carbon (Cmic) were measured. Agrosoils, previously kept at negative temperature for 160 days, display the maximum C–CO2 emission intensity over the warm period (185 days at 22°C). The minimum C–CO2 emission is observed in the treatment with prior freezing–thawing for 84 days. The 5-year experiment demonstrates that characteristic of the nonfreezing soils and the soils with short-term freezing is the decrease in the content of potentially mineralizable organic matter and its other fractions, as well as in microbial biomass carbon. A similar pattern is observed for the share of organic compounds in total Corg content. An abnormal share of Cmic in the potentially mineralizable carbon (>95%) is recorded in the agrochernozem. The arable chestnut soil is more sensitive to winter temperature changes, displaying a longer turnover time and half-life period, as well as a lower index of SOM biological stability. Freezing for 160 days leads to an increase in the content of water-soluble carbon in arable chestnut soils.