Cultivar variation has a stronger influence than crop type on the properties of maize and sorghum residues and derived biochars

Conversion of crop residues to biochar can enhance soil carbon storage and reduce carbon dioxide (CO 2 ) emissions from agricultural soils, with these effects being influenced by crop type and cultivar, and pyrolysis conditions. This study characterized residues from two maize cultivars (SC701 and R201) and two sorghum cultivars (AS8 and PAN8816), and the biochars produced from them at 350 and 650 °C in a muffle furnace and at about 400 °C in a kiln. Feedstocks and biochars were analysed for proximate composition, elemental composition, chemical properties, surface morphology and functional groups. Significant differences were observed among cultivars within each crop, whereas crop type had comparatively smaller effects. Increasing pyrolysis temperature decreased moisture content from 12.79% in feedstocks to as low as 2.08% in biochars and increased ash content from 6.50 to 11.48% to 41.82-46.13% and pH from 4.26 to 5.09 to 10.58-11.40. Fixed carbon increased from 12.23 to 14.10% in feedstocks to a maximum of 47.13% in R201-B650, while kiln-produced PAN8816 and SC701 biochars retained the highest nitrogen contents (4.55% and 3.77%, respectively). Increasing pyrolysis temperature also reduced volatile matter and the H: C and O: C ratios. Cultivar-specific differences in feedstock physicochemical characteristics, including lignin content, elemental composition and other chemical properties, together with pyrolysis conditions, were more important than crop type in determining the properties of the resulting biochars. These findings highlight the importance of selecting cultivars with desirable feedstock characteristics and appropriate pyrolysis conditions when producing biochars for carbon sequestration and soil fertility improvement.