Phytotoxic condensed organic compounds are common in fast but not slow pyrolysis biochars

Abstract

Biochar chemistry varies with feedstock and pyrolysis parameters: fast pyrolysis may not provide sufficient time for volatile compounds – some phytotoxic – to escape from biochar particles. Lettuce and radish seeds were treated with biochar leachate derived from 100 slow pyrolysis biochars (20 wood species, each pyrolyzed at 300–700 °C for 60 min), and 20 fast pyrolysis biochars (two species pyrolyzed at 500 and 550 °C for 1–5 min). Slow pyrolysis biochar leachates enhanced radicle extension growth by (60% for lettuce; 16% for radish), while fast pyrolysis biochar leachates inhibited radicle growth by (63% for lettuce; 8% for radish). Slow pyrolysis biochars showed higher concentrations of Ca, Mg, K, Fe, Mn, and Zn than fast pyrolysis biochars, whereas fast pyrolysis biochars generally showed higher concentrations of phytotoxic volatile fatty acids, such as acetic and pentanoic acid. These results suggest the importance of extended pyrolysis residence times in producing high-nutrient biochars free from toxicity effects.