Objectives Biowastes from agricultural production exist in various forms. These wastes are produced in large quantities, having consequent environmental risks due to antibiotics, plant pathogens, and pesticide residues in them. Producing biochar by pyrolysis is an environmentally friendly way of dealing with biowastes. This study assessed the efficacy of different biowastes for biochar production and their effects on crop yield and quality.

Methods A pot experiment was conducted using pakchoi as test crop materials. A total of 26 types of agro-biowastes were collected from primary biowastes (e.g. straws), secondary biowastes (e.g. livestock dung) and by-products from food processing. All the feedstocks were pyrolyzed under the same condition to make biochar. We analyzed the biochar yield of different feedstocks and their physicochemical properties. The feasibility of pyrolysis and the effect of biochar on crop production were assessed.

Results The yield of biochar from the tested biowastes ranged in 22%–71%. The biochar recovery of carbon and nitrogen ranged in 22%–81% and in 21%–67%, respectively. The coefficient of variation (CV) of the pH and cation exchange capacity of the biochars was 10%; that for total organic carbon, total nitrogen, moisture, and ash contents were 60%–70%; and that for electrical conductivity, dissolved organic carbon and total P contents were > 90%. In the pot experiment, biochar was amended to 1%. The pakchoi biomass increase ranged from –34% to 314% compared to the no biochar control, with a CV of 100%. However, the quality index increase of pakchoi ranged in –14%–228%, with a CV of 59%. Meanwhile, soil fertility improvement ranged in 20%–360%, significantly increased by 0–24 folds (359% on average). Moreover, changes in biochar yield and carbon and nitrogen recovery were found inconsistent with the improvements in biomass and quality of pakchoi across the biochars from different feedstocks. Some biochars showed tradeoffs between biomass production and pakchoi quality, while a few showed synergism, in the pot experiment. Significant positive correlations were found between biochar yield and ash content and between biochar recovery of organic carbon and nitrogen and their content in the feedstock.

Conclusions The biochar yield, recovery of organic carbon and nitrogen, and the synergistic improvement of the soil-plant system were synthesized. Of the 26 biowastes tested, silkworm residue, rice husk dung, bone powder, agaricus bisporus residue, rabbit manure, sheep manure and corn residue had higher yield for biochar recovery and had a high synergistic improvement of the soil-plant system (> 50%). These biowastes should be prioritized for pyrolysis and biochar application in agriculture. On the other hand, xlose residue, rice husk powder, coconut residue, brewery sorghum residue, walnut residue, wormcast, and black tea residue were not recommended for pyrolysis and biochar use in agriculture due to either low biochar yield and element recovery or inconsistent improvement of the soil-plant system. The others could be considered conditionally potential for biochar production and application in agriculture due to acceptable resource recovery and soil-plant improvement of their pyrolyzed biochar.