Mixed biochar produced from agroforestry by-product and livestock manure feedstocks as a solution of heavy metals and salts problems

Soil application of biochar from livestock manure (LM) is an environmentally friendly way to treat excess LM and for enhanced soil carbon sequestration to remove atmosphere CO 2 .However, high copper, zinc, and salt contents in LM bring concerns on the wide use of LM biochar for soil application.In this study, the copper (Cu), zinc (Zn), and salt contents of LM biochar were estimated using national monitoring database of the heavy metals and salt in the LM composts and the experimental measurement on biochar yield from cow manure including bedding materials.The average of the estimated Zn and salt contents, except of Cu, of LM biochar exceeded the threshold of the national standard.A feasible solution of this problem is increasing the mixing ratio of agroforestry by-products (AB) with low heavy metals and salt content when producing biochar using LM to meet the national standard.Mixed biochar, proposed a new biochar type in the present study, is defined as a biochar made from mixed feedstocks of LM and AB at varying mixing ratios above 30% AB feedstock as the current national standard for LM biochar allows mixing AB below 30% to LM.The mixed biochar may be highly useful as a soil amendment because of the co-advantages from LM (e.g., high nutrient contents) and AB (e.g., high recalcitrance) feedstocks.Therefore, it is expected that the inclusion of the mixed biochar as a new type of biochar in the legal standards will be of help for increased and efficiency production and utilization of biochar in agricultural, forestry, and livestock industry sectors.