Criteria to Select Biochars for Field Studies based on Biochar Chemical Properties

One factor limiting the understanding and evaluation of biochar for soil amendment and carbon sequestration applications is the scarcity of long-term, large-scale field studies. Limited land, time, and material resources require that biochars for field trials be carefully selected. In this study, 17 biochars from the fast pyrolysis, slow pyrolysis, and gasification of corn stover, switchgrass, and wood were thoroughly characterized and subjected to an 8-week soil incubation as a way to select the most promising biochars for a field trial. The methods used to characterize the biochars included proximate analysis, CHNS elemental analysis, Brunauer-Emmett-Teller surface (BET) area, photo-acoustic Fourier transform infrared spectroscopy, and quantitative (13) C solid-state nuclear magnetic resonance (NMR) spectroscopy. The soil incubation study was used to relate biochar properties to three soil responses: pH, cation exchange capacity (CEC), and water leachate electrical conductivity (EC). Characterization results suggest that biochars made in a kiln process where some oxygen was present in the reaction atmosphere have properties intermediate between slow pyrolysis and gasification and therefore, should be grouped separately. A close correlation was observed between aromaticity determined by NMR and fixed carbon fraction determined by proximate analysis, suggesting that the simpler, less expensive proximate analysis method can be used to gain aromaticity information. Of the 17 biochars originally assessed, four biochars were ultimately selected for their potential to improve soil properties and to provide soil data to refine the selection scheme: corn stover low-temperature fast pyrolysis (highest amended soil CEC, information on high volatile matter/O-C ratio biochar), switchgrass O-2/steam gasification (relatively high BET surface area, and amended soil pH, EC, and CEC), switchgrass slow pyrolysis (higher-amended soil pH and EC), and hardwood kiln carbonization (information on slow pyrolysis, gasification and kiln-produced differences).