Biochar is commonly defined as “the fine-grained or granular charcoal made from heating vegetative biomass, bones, manure solids, or other plant-derived organic residues in an oxygen-free or oxygen-limited environment and used as a soil amendment for agricultural and environmental purposes” (Guo et al. 2016b). Theoretically, all solid biomass materials can be used to manufacture biochar. Researchers have tested a wide variety of bio-derived organic residues as biochar feedstocks, including hard wood, soft wood, saw dust, scrap wood and shavings, tree barks, tree leaves, pine needles, greenwaste, switchgrass, Miscanthus, fescue straw, corn stover, corn stalk, corn cobs, cotton stalk, wheat straw, rice straw, rice husk, soybean straw, rapeseed straw, pepper straw, peanut hull, peacan shell, palm shell, coconut shell, orange peel, olive pomace, sugarcane bagasse, cottonseed meal, poultry litter, cow manure, swine manure solids, sewage sludge, cattle carcass, bone meal, and solid organic municipal waste (Guo, Song et al. 2020). The feedstock materials were generally dried; processed into small pieces, grains, or pellets; and then converted to biochar through pyrolysis (heating at 300–700°C without air) or gasification (heating at 500–900°C with controlled air supply) (Guo, Xiao et al. 2020). During the thermochemical carbonization processes, 10–65% of the feedstock OC was recovered in biochar, phosphorus (P) was nearly preserved, while the loss of nitrogen (N) was tremendous (Song and Guo 2012; Guo, Song et al. 2020). Considering the accessibility, collection infrastructure, and economic viability, crop residues (e.g., corn stover, wheat straw, and rice husk) and forest debris (e.g., floor litter, tree trimmings, and wood waste) are promising feedstocks for field-scale production of agricultural-use biochar. Specialty food-processing wastes like nutshells, olive pomace, sugarcane bagasse, and cottonseed meal are limited in supply and, therefore, may serve a localized biochar feedstock or be simply added to the main feedstock stream. Organic refuses in municipal solid waste may contain toxic elements (e.g., heavy metals) that contaminate the resulting biochar products. Conversion to biochar is an effective method to sterilize and stabilize animal wastes such as carcass, bones, manures, and biosolids, yet the significant feedstock N losses incurred by high-temperature thermochemical treatment (e.g., at more than 400°C) are a major concern. Biochar is also a byproduct of wood pyrolysis to harvest bio-oil (a liquid biofuel) and wood gasification to produce syngas (a gaseous biofuel) (Guo, Xiao et al. 2020). Utilization of such biochar by-products in agricultural production is a value-added approach to promote the bioenergy indu...