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Biochar: Is it inert or reactive?

Kurt Spokas
U.S. Department of Agriculture, Agricultural Research Service

Laboratory incubation studies and the discovery of historic charcoal fragments in soil and sediment profiles have supported the hypothesis that biochar is an inert material, which as one characteristic possesses a very slow microbial degradation rate (>10 yr half-life).  While these observations are valid, biochar also is a fascinating reactive material.  The reactions with oxygen, carbon dioxide, nitrogen, and water are particularly of importance for its utilization in a soil system.  Water uptake contributes to the physical breakdown of the structure due to expansion forces on the heterogeneous structural layering.  Simultaneously, carbon dioxide and water vapor steadily disappear from the headspace of sealed incubations of biochar with a low O:C ratio (<0.2).  The alteration in microclimate conditions (particularly relative humidity) require additional attention, since the rate of sorption of water vapor from the gas phase to biochar is a very rapid process, when compared to water evaporation rates or even liquid water entry into pores.  This presentation will highlight results comparing the sorption of water vapor, liquid water uptake, heat of immersion, and sorption/reaction with carbon dioxide of various biochars.  These observations have been supported with detailed nano-scale inspection by SEM/TEM and companion carbon isotopic labeling to confirm these pathways.  This data questions the concept of ”static equilibrium” for biochar and whether this reaction rate balance is actually ever achieved.  Furthermore, biochar aging commences immediately following formation, and some of these reactions are expedited by temperature; so the cooling process is a very critical stage of biochar chemical alteration.  These results stress the importance of time and perceived equilibrium in the interpretation of biochar interactions, in particular when assigning mechanisms for the observed impacts following soil incorporation. 

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