Characterization of biochars to evaluate recalcitrance and agronomic performance

Biochars (n = 94) were found to have ash contents from 0.4% to 88.2%, volatile matter from 13.2% to 70.0%, and fixed carbon from 0% to 77.4% (w/w). Greater pyrolysis temperature for low-ash biochars increased fixed carbon, but decreased it for biochars with more than 20% ash. Nitrogen recovery varied depending on feedstock used to a greater extent (12–68%) than organic (25–45%) or total C (41–76%) at a pyrolysis temperature of 600 °C. Fixed carbon production ranged from no enrichment in poultry biochar to a 10-fold increase in corn biochar (at 600 °C). Prediction of biochar stability was improved by a combination of volatile matter and H:C ratios corrected for inorganic C. In contrast to stability, agronomic utility of biochars is not an absolute value, as it needs to meet local soil constraints. Woody feedstock demonstrated the greatest versatility with pH values ranging from 4 to 9.