Nutrient and heavy metal contents in edible fungus residues in Chongqing

Objectives The nutrient and heavy metal concentrations in the residual substrate of edible fungus production were investigated for their safety and efficient recycling as organic material resources.

Methods The residual substrate samples were collected from 84 producers across nine districts of Chongqing City for the determination of the concentrations of organic carbon (C), total N, P, K, Ca, Mg, and heavy metals (As, Cd, Cr, Cu, Hg, Pb, and Zn).

Results 1) The concentrations of C, N, P, K, Ca, and Mg in the residual substrates were 354, 13.9, 3.48, 8.93, 35.0, and 7.07 g/kg, respectively. The C/N ratios of the residue substrates were generally high (22.9–42.2), and the K/Mg and Ca/Mg ratios were 1.36 and 5.70, respectively. The K/Mg ratio was markedly greater than 0.6 (except for black fungus and mushroom residues), whereas the Ca/Mg ratio was less than 7.0. 2) The average concentrations of As, Cd, Cr, Cu, Hg, Pb, and Zn in the residual substrates were 0.69, 0.30, 1.54, 12.5, 0.07, 4.36, and 55.8 mg/kg, respectively, with high variations (CV 46.9%–179%). According to the NY 525—2021 standard for heavy metals limits in organic fertilizers in China, all the heavy metal concentrations were within the permissible limits.

Conclusions In the residual substrate of edible fungus produced in Chongqing, the average content of heavy metals As, Cd, Cr, Hg and Pb were only 1.03%–10.0% of the threshold value in the national standard for organic fertilizer, which indicated a high ecological security. Although not rich in N, P₂O₅, K₂O, edible fungus residue has high organic matter content (61.0%) which makes it suitable as an auxiliary material for livestock manure co-composting.