Age and genesis of Fe-Mn concretions in gray forest soils of the Southern Taiga, according to the results of isotope and metagenomic studies

Abstract

The high sensitivity of plants, and, consequently, the 13C/12C isotope ratio in their tissues, to fluctuations in air humidity and temperature, as well as to the concentration of carbon dioxide in the atmosphere, makes it possible to use this parameter as an accurate paleoclimatic indicator and biomarker of nodule genesis. Concretions from the gray forest soil of the Moscow region were studied using computed microtomography, biochemical analysis, and metagenomic sequencing. The composition of stable and radioactive carbon isotopes was determined in ferruginous-manganese nodules. It has been shown for the first time that the age of the concretions of light gray forest soils formed on cover noncalcareous loess-like heavy loamy parent rocks exceeds 1600 years. At the same time, the carbon isotopic composition of different fractions of nodules reveals in them the remains of modern plant species (from –26,3 to –27,4 δ13C, ‰) and confirms the leading role of woody plant lignin in the formation of nodules. The species composition of microorganisms differs in different fractions of concretions, makes the isotope composition of fine fractions heavier, and changes as soil hydromorphism increases. Drainage causes irreversible degradation of concretions, and first of all, their large fractions.