Carbon isotope geochemistry and nanomorphology of soil black carbon: Black chernozemic soils in central Europe originate from ancient biomass burning

Abstract

A common paradigm is that chernozem soils developed in the Holocene under grassland steppes, with their formation largely determined by three factors, parent material, climate and faunal mixing. For European chernozems, however, pollen records show that steppes were rare. Here, using high‐resolution transmission electron microscopy, electron energy loss spectroscopy, micro Raman spectroscopy and radiocarbon dating, we characterized the nanomorphology and chemical structure of soil organic carbon (SOC) from central European chernozems. We identified submicron remnants of burned biomass (15–45 percent of SOC), coexisting as amorphous char‐black carbon (BC) derived from pyrolized cellulose or soot‐BC. The BC was several millenia in age (1160–5040 carbon‐14 years) and up to 3990 radiocarbon years older than bulk SOC, indicating significant residence times for BC in soils. These results challenge common paradigms on chernozem formation and add fire as an important novel factor. It is also clear that the role of fire in soil formation has been underestimated outside classical fire prone biomes. Furthermore, our results demonstrate the importance of quantifying BC in soils because of its large contribution, longevity and potential role in the global biogeochemical carbon cycle.