Rapid degradation of pyrogenic carbon

Pyrogenic carbon (PC- charcoal, biochar or black carbon) represents a poorly understood component of the global carbon (C) cycle, but one that has considerable potential to mitigate climate change through provision of long-term soil C sequestration. Mass balance calculations suggest global PC production and stocks are not in balance, indicating a major gap in our understanding of the processes by which PC is re-mineralized. We collected PC samples derived from the same wood material and exposed to natural environmental conditions for 1 and 11 similar to years. We subjected these materials to repeated laboratory incubation studies at temperatures of up to 60°C, as ground surface temperatures above 30°C and up to 60°C occur regularly over a significant area of the tropics and sub-tropics. Mineralization rates were not different for the two samples and followed an exponential Arrhenius function that suggest an average turnover time of 67 similar to years for conditions typical of a tropical savannah environment. Microbial biomass as measured by chloroform fumigation and DNA extractions was the same for the two samples, but abiotic CO₂ production was lower for the fresh PC sample than that for the aged sample. Nuclear magnetic resonance spectroscopy, hydrogen pyrolysis and scanning electron microscopy demonstrate that the measured CO₂ production originates dominantly from polycyclic aromatic compounds rather than any minor labile components. Therefore, rapid, sub-centennial rates of re-mineralization of PC on the soil surface in tropical and sub-tropical environments may represent a major and hitherto unidentified mechanism for balancing the PC production at the global scale