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Physical disintegration of biochar: An overlooked process
Spokas, K., J. Novak, C. Masiello, M. Johnson, E. Colosky, J. Ippolito, AND C. Trigo. Physical disintegration of biochar: An overlooked process. Environmental Science & Technology Letters. American Chemical Society, Washington, DC, 1:326-332, (2014).
The name biochar is given to biomass that through chemical/thermal processes is transformed into a form that has been shown to be more resistant to microbial degradation than the original biomass, thereby an agent for carbon sequestration. However, biochar undergoes chemical and physical alterations even under ambient storage conditions. Subsequently, once biochar is placed in the soil system it is further subjected to physical, chemical, and microbial degradation pathways. The main focus of current biochar mineralization/degradation research has been on microbial mineralization rates. Recent data collected from both artificially and naturally weathered biochars suggest that a potential significant pathway of biochar disappearance is through physical breakdown of the biochar structure. Through scanning electron microscopy (SEM) we characterized this physical weathering which increased the spacing between the graphic sheets due to the expansion accompanying water sorption, freeze-thaw, as well as desiccation and rewetting. As these sheets expand (exfoliate) this further accelerates physical break-down of the biochar. Soil texture plays an important role in this process, since clay particles were observed to seal pores and the spaces between the graphic sheets limiting the physical disintegration processes. In addition, these soil particles are virtually impossible to remove quantitatively, thereby skewing the resulting chemical analyses of the weathered biochar making it appear like chemical alteration rather than physical alteration was occurring. Lastly, the micro and nano-scale biochar particles resulting from this physical disintegration are still carbon-rich particles with no detectable alteration in the O:C ratio of the carbon structure, but are now easily suspended and moved by infiltration. There is a need to understand how to produce a biochar that is resistant to physical degradation in order to maximize the long-term C-sequestration potential of biochar in the soil system. These results are significant to farmers and policy makers and will assist scientists and engineers in developing improved biochars based on properties to improve soil carbon management.
Biochar is a form of black carbon (BC) that has been documented to be resistant to both microbial and chemical degradation pathways as well as simultaneously sequestering atmospheric carbon (C) and postulated to act as a soil fertility agent by providing critical inorganic plant nutrients. Much attention has been focused on the stability of BC in soil, especially mineralization by microbial activity and modifications by chemical (abiotic) reactions. On the other hand, less is known about the impact of comminution with subsequent solubilization and transport of dissolved black carbon (DBC) fragments. This physical deterioration is hypothesized to be important for directly impacting the longevity of BC in soils as well as its mobilization into fluvial systems. However, no data exists to support this claim for C loss from biochar. Here we present data on the physical disintegration of biochar. Despite its documented recalcitrant nature to microbial reactions, biochar is extremely susceptible to physical deterioration, abrasion, and subsequent dissolution by water. By examining a set of biochars that were exposed to a 24-hr water extraction (1:20; biochar:di. water), the amount of biochar lost into solution ranged from 4 to 17% (w/w) of the initial mass. To put this loss in perspective, less than 0.09% of biochar C was mineralized by soil microbes over a 100 d incubation (1% w/w amendment in an agricultural soil). It is important to note that not all biochars were equally susceptible to disintegration by water. We offer that physical comminution is a previously overlooked loss mechanism of BC and should be accounted for in predictions of biochar C sequestration potential and the interpretations of BC presence in the archeological/geologic record.