Abstract:A pot experiment was conducted to evaluate the effects of different proportions of biogas residue (BR) on growth of ryegrass (Lolium perenne L.) and physico-chemical properties of lead-zinc smelting slag. BR was incorporated into slag at rates of 0, 1%, 3%, 5%, 7% and 9%, respectively. The comprehensive effects of biogas residue on nutrient content, microbiological characteristics, forms of heavy metals of lead-zinc smelting slag and biological characteristics of ryegrass were studied. The results showed that addition of BR improved slags properties and thus facilitated growth of ryegrass. Compared with the control, BR application significantly increased nutrients (nitrogen, phosphorus and potassium), organic matter, enzyme activity (alkaline phosphatase, invertase, urease and catalase), the number of microbes (bacteria, fungi and actinomycetes) and microbial activity (respiratory intensity) (p<0.05). Different proportions of BR also significantly enhanced the total fresh weight, root length and plant height of ryegrass and significantly reduced the heavy metal content of plant tissues (the ground and underground part) (p<0.05). BR addition and ryegrass growth promoted the transformation of acid-extractable state of heavy metals (Cu, Cd, Zn and Pb) into the residual state, and then decreased the bioavailability of heavy metals in slag. Canonical correspondence analysis (CCA) revealed that the application amount of BR at 5%, 7% and 9% effectively improved the micro-environmental conditions of slag, among which, the lower addition amount of BR (5%) could also significantly improve the micro-environmental conditions of slag and promote the healthy growth of plants for restoration. Therefore, considering the relationship between the cost and effects caused by BR, the 5% addition amount of biogas residue could be used as the optimal application ratio for the improvement and phytoremediation of lead-zinc smelting slag.