Distribution of stable carbon isotopes in an agrochernozem during the transition from C₃ vegetation to a corn monoculture.

The distribution of carbon in an agrochernozem’s profile was studied by the natural 13C abun dance method during the C3–C4 vegetation transition and the analysis of the soil phytolith complex under a continuous corn monoculture. A young pool of soil organic matter (SOM) formed during 43 years of monoc ulture growing was detected by the isotope analysis in the 0 to 60cm layer, while the analysis of the phytolith complex identified this pool deeper: corn phytoliths were detected in the 0 to 80cm layer. The maximum size of the young pool was found in the upper soil horizon; it reached 6.4% of the SOM in the 0 to 20cm layer. The apparent time of the SOM turnover was 635 and 2225 years in the 0 to 20 and 40 to 60cm layers, respectively. The high values of the mean residence time were related to the low input of plant residues to the soil at the growing of corn for silage and the high initial content of organic carbon in the chernozem. The changes in the isotope composition after the decalcification of the soil to remove carbonates and the variation of the δ13C in the corn biomass during the vegetation period significantly affected the calculated value of the mean residence time