Pyrogenic carbon quantity and quality unchanged after 55 years of organic matter depletion in a Chernozem

Chernozems typically have large stocks of organic carbon and of fire-derived, pyrogenic carbon (PyC). PyC had been considered to be slowly released but new results challenged this assumption, indicating that PyC can be lost within decades. We analyzed total soil organic carbon and PyC content (detectable as benzene polycarboxylic acids) in bulk samples, light and heavy fractions from a 55 year old bare fallow and a nearby steppe soil. Loss of PyC stock due to the long-term fallow management was much smaller (6%) than for soil organic carbon (33%), and we detected no changes in the degree of aromatic condensation of PyC. Most (70%) of the PyC was associated with the heavy fraction, and less with the light fraction (30%) pointing to organo-mineral interactions as important stabilizing processes.