Pedogenic carbonate concretions in the Russian Chernozem

Pedogenic carbonate concretions are commonly found in grassland soils, but their origin is not fully understood. This study was conducted to determine the radiocarbon age, the stable isotope geochemistry, and chemical composition of carbonate concretions in the Russian Cher-nozem, one of the typical soils in grasslands. Three sites were sampled: a native grassland field (not cultivated for at least 300 years), an adjacent 50-year continuous fallow field in the V. V. Alekhin Central-Chernozem Biosphere State Reserve in the Kursk region of Russia, and a continuously cropped field in the Experimental Station of the Kursk Institute of Agronomy and Soil Erosion Control. All sampled soils were classified as fine-silty, mixed, frigid Pachic Hapludolls. The mineralogical composition of concretions varies from low-magnesium calcite to pure calcite. The concretion contains 0.05% N, 6.4% C, and has delta C-13 and 8180 values of -10.9 parts per thousand (the per mill symbol, parts per thousand) and -7.8 parts per thousand, respectively. The outside part of the carbonate concretion is 1909 40 C-14 age Before Present (B.P.) compared with 1693 +/- 40 C-14 age B.P. for the inside part of the same concretion, even though the concretion is found in the C horizon of much older age (10,902 +/- 63 C-14 age B.P.). Remnants of soil organic matter in concretions are closely associated with the cropped and fallow/plowed soils by pyrolysis molecular beam mass spectrometry.