Pyrolysis methods impact biosolids-derived biochar composition, maize growth and nutrition

Land-applied biosolids (sludge) can improve food production sustainability through nutrient recycling. Biosolids-derived biochar may enhance soil fertility and overall soil health. However, there is little information on the conversion of biosolids to biochar using traditional kilns, or effects on biochar characteristics and plant growth. Biochar was produced from biosolids using two pyrolysis methods: 1) a traditional retort kiln (Top-lid Updraft-TLUD) intended for use by small farmers and gardeners, and 2) a laboratory muffle furnace, with the aim of evaluating biochar characteristics and its effects on Zea mays L. (corn) seed germination, growth and nutrition. Biochar produced in a muffle furnace contained 70% more ash, 78% more fixed carbon, and 63% less volatile matter than biochar produced by TLUD, which raised concern regarding TLUD-derived biochar toxicity The TLUD-derived biochar inhibited corn seed germination in a petri dish bioassay at biochar application rates from 2.5 to 100 Mg ha−1. However, germination increased from 29% (control) to approximately 60%, at 60 Mg ha−1 or greater rates, with muffle furnace biochar. A greenhouse experiment was conducted to evaluate the growth and nutrition of corn grown in soil treated with 0, 5, 10, 20 and 60 Mg ha−1 biochar pre-incubated for two weeks in moistened soil. The muffle furnace biochar had no negative effect on plant growth and N nutrition, whereas the TLUD biochar at a 60 Mg ha−1rate, reduced plant growth and increased plant N concentrations four-fold, compared to the control. Both biochars increased plant P concentrations with increasing application rates. Biosolids biochar produced via TLUD at rates below 20 Mg ha−1 may benefit crop production, although an incubation or weathering period may be necessary to limit potential short-term, phytotoxic effects. Future research needs include optimizing TLUD operational parameters and identifying weathering processes that improve biochar product quality for agronomic use.