A novel strategy to reduce trace element supplementation in the semi-solid anaerobic digestion with gradient ammonia concentration: the role of biochar

•Semi-solid anaerobic digestion of chicken manure was focused.•Biochar could reduce 50% of optimal TEs in semi-solid anaerobic digestion.•Biochar increased the proportion of TE’ exchangeable fraction.•Biochar reduced the formation of precipitation between TEs and H2S.