The Role of γ-Polyglutamic Acid, Superphosphate, and Smectite-Silica Clay Additives in Stabilizing Organic Matter, Reducing the Bioavailability of Heavy Metals, and Limiting the Ecotoxicity of Composts Made from Poultry Litter and Biochar

A mixture of poultry litter (PL) and biochar (BC) was composted over 120 days in a bioreactor. To assess the impact on the stability of organic matter, the bioavailability of heavy metals, and ecotoxicity, the PL+BC biomass was supplemented with 0.5% (w/w) γ-polyglutamic acid (PGA), superphosphate (SPP) and smectite-silica clay (SSC) relative to the dry matter. Incorporating PGA, SPP, and SSC additives into PL+BC increased total carbon content by an average of 6%, compared to PL+BC without additives. The SSC additive proved to be more effective in increasing the humic acid carbon content, raising Cha by an average of 23% relative to PGA and SPP treatments. The incorporation of biochar into PL led to a substantial increase in nonhydrolizing carbon content, while the enrichment of composts with PGA, SPP, or SSC resulted in an escalation in this form of carbon by an average of over 7% compared to PL+BC. The lowest amounts of metals extracted with water and the lowest RAC values were obtained for PL+BC+SPP compost. The additives used stabilized the composts more quickly and reduced their toxicity. The classification of PL compost was designated as class III, whereas composts that incorporated additives were classified as class II toxicity. The study findings substantiated the necessity to incorporate additives during the biological processing of poultry litter.