The Age and Genesis of Fe–Mn Nodules in Gray Forest Soils of the Southern Taiga Based on the Results of Isotope and Metagenomic Studies

The high sensitivity of plants and, consequently, the 13 C/ 12 C isotope ratio in their tissues to fluctuations in air humidity and temperature, as well as to the carbon dioxide concentration in the atmosphere, makes it possible to use this parameter as an accurate paleoclimatic indicator and biomarker of nodule genesis. Nodules of a gray forest soil of Moscow oblast were studied with the computed microtomography, biochemical analysis, and metagenomic sequencing. The composition of stable and radioactive carbon isotopes was determined in ferromanganese nodules. It has been shown for the first time that the age of the nodules of light gray forest soils formed on mantle noncalcareous loess-like clay loamy parent rocks exceeds 1600 years. At the same time, the carbon isotopic composition ( δ 13 C) of different fractions of nodules shows in them the remains of modern plant species (from –26.3 to –27.4 ‰) and confirms that woody plant lignin plays a leading role in the formation of nodules. The species composition of microorganisms varies in different fractions of nodules, makes the isotope composition of fine fractions heavier, and changes as soil hydromorphism increases. Drainage causes irreversible degradation of nodules, and first of all, their large fractions.