Aromatic structural components but not their degree of condensation are responsible for the persistence of biochars produced above 370 ˚C

Abstract:

Charred plant residues are more resistant to decomposition in soil than fresh biomass, rendering biochar a promising technology for increasing soil C content and mitigating climate change. Due to its persistence in soils and its surface properties, biochar is also considered as a tool for managing soil fertility in the long term. Biochar properties change with degree of carbonization and the feedstock material it is produced from. With large differences existing among biochar products, characterizing these materials is the first step in devising a biochar technology plan. We therefore measured various chemical and structural properties of a series of biochars prepared from corncob and miscanthus grass at 250 to 800 ˚C pyrolysis temperatures. We also measured the persistence of biochar in soil and devised an incubation method that eliminates likely errors in measuring natural abundance ¹³C isotopes. In our search for reliable estimators of biochar properties, we focused on the identification of structural properties responsible for the unique inherent stability of biochar, including the presence of molecular markers of benzene polycarboxylic acids (BPCAs) and aromaticity from nuclear magnetic resonance (NMR).

We found that the increased residence time of biochars, which were on average 60 times that of fresh plant residues, is mostly developed at pyrolysis temperatures up to 370 ˚C and does not appear to significantly increase with pyrolysis beyond this temperature threshold. Aromatic structural components were formed to a much greater extent above 370 ˚C, which can explain resistance to decomposition in soil. The fraction of C that is BPCA C correlated well with estimated biochar half-lives. However, aromatic condensation degree developed above 500 ˚C, and biochars produced at the highest production temperatures having the highest aromatic condensation degree were not found to be the most stable forms of biochar. This is an indication that the formation of aromatic structures and not necessarily their condensation into larger sheets determines the persistence of biochar in soil. This work helps our understanding of the origin of biochar stability and provides an aid for the development of a useful indicator for biochar stability.