Carbon-mediated modulation pathways of phytotoxicity in chicken manure composting

Compost phytotoxicity affects the safety of organic fertilizers returned to the field, thus hindering the breeding cycle, so it is essential to reduce the compost phytotoxicity. The phytotoxicity of compost was estimated utilizing the germination index (GI) and the aqueous substances (organics and ions) present in compost correlated closely with GI. This study assessed the effect of carbon additives from different plant sources (mushroom substrates (MS), cornstalks (CS) and garden substrates (GS)) on maturity parameters (temperature, pH, EC, C/N), content of aqueous carbon and nitrogen matters, salt ions, heavy metal ions, and microbiome of piles when composting with chicken manure and especially focused on their effect on GI. Results showed that all additives significantly improved GI (85.25%-106.28%). The primary factors influencing seed germination were Mg²⁺ and SO₄^2- in CM compost, acetic acid and NH₄⁺ in CM + MS compost, humic acid in CM + CS compost, and dissolved total nitrogen in CM + GS compost. During composting, the growth of heavy metal passivating bacteria (Bacillus) and organic matter degrading bacteria (Desemzia and Turicibacter) can be promoted by decreasing aqueous carbon and nitrogen substances (volatile fatty acids, NH₄⁺, dissolved total nitrogen, amino acids) and increasing the content of humic acid, which improved the composting environment and provided favorable conditions for the germination of seeds, thereby increasing GI. Therefore, GS showed the best potential for accelerating degradation of organic matter and improving GI during composting with chicken manure.