Stable and radioactive carbon isotopes as indicators of restructuring the pedogenic carbonates in Chernozems during their transition from arable to abandoned state (Cis‐Azov steppe, Russia).

Background: Pedogenic carbonates are a key component in the steppe soil system that determined soil‐forming processes, classification position, and also responsible for a carbon cycle. Aim: Investigating the processes of carbonate status transformation in Chernozems transiting from arable to self‐restoration state. Methods: The consequences of land‐use change from arable to self‐restoration abandoned soils for carbonate status have been studied in the chronosequence of Chernozems within the Cis‐Azov steppe, Russia, based on complex examination with the focus on stable and radioactive C isotopes in carbonates. Results: Pedogenic carbonate transformation starts immediately after land‐use change with altering of hydrothermal regime of soils and establishing of a permanent vegetation cover on the surface of abandoned soils when plowing stopped. The main changes are: (1) accumulation of carbonates moves deeper in the profile, (2) decreasing of carbonate C stock, (3) the soft carbonate concretions in the carbonate‐accumulative horizons Bk of aged abandoned soils gain vague diffuse boundaries with the soil mass in contrast to clear boundaries of those concretions in the youngest abandoned and arable soils, (4) the soft concretions with hard core completely disappear from the lowermost part of profile of aged abandoned soils, (5) decrease in the age of carbonates (14C values), (6) and the δ13С values in carbonates increase (become less negative) with time after stop plowing. In approximately 30 years of abandonment, Chernozems transformed to a "new" quasi‐stable state of carbonate profile. The movement of soil solutions in abandoned soils with permanent vegetation occurs under transpiration influence that prevailed over physical evaporation, and that results in a change of the carbon isotopic composition in carbonates. After the soil has been abandoned, carbonates are gradually displaced from the soil horizons of the former arable soils, being simultaneously renewed due to active recrystallization under the influence of plant roots and mesofauna. Conclusions: In the Chernozems of aged abandoned soils, both radioactive and stable C isotopes in carbonates show renewal as compared with the arable soils, mostly in the nonsegregated carbonates. Thus, the main pathway of the carbonate transformation is through the intrapedal and intercracks soil mass where the nonsegregated carbonates predominate. [ABSTRACT FROM AUTHOR]