Biochar Mitigates the Negative Effects of Microplastics on Sugarcane Growth by Changing Soil Nutrients and Microbial Community Structure and Function

Microplastics pollution in sugarcane areas of China is severe, and reducing their ecological risks is critical. This study aims to explore the effects and mechanisms of microplastics combined with or without biochar on sugarcane growth, soil biochemical properties in red soil by a potted experiment. The results showed that compared with control (CK), microplastic alone treatments reduced the dry biomass of sugarcane, soil pH and the nitrogen (N) and phosphorus (P) contents, obviously decreased the observed_OTUs, Chao1 and Shannon indices of soil total bacteria (16S rRNA gene-based bacteria) while increasing them in phoD-harbouring bacteria. Interestingly, microplastics combined with biochar could alleviate the negative effects of microplastic accumulation on sugarcane growth and soil quality. There were significant differences in the bacterial community compositions among different treatments. Compared with CK, all other treatments significantly decreased the relative abundance of Gemmatimonadota while only microplastic combined with biochar treatments significantly increased the abundance of Subgroup_10 for the 16S rRNA gene, and only microplastic alone treatments significantly increased the relative abundance of Streptomyces for the phoD gene. Moreover, the treatments with microplastics combined with biochar increased the relative abundance of Subgroup_10 and Bradyrhizobium while decreased the IMCC26256 and Streptomyces compared with that in the treatment with microplastics alone. Correlation analysis showed that Subgroup_10 and Bradyrhizobium were significantly positively correlated with sugarcane biomass and several soil properties, while IMCC26256 and Streptomyces were just the opposite. Additionally, different treatments also changed the abundance of potential microbial functional genes. Compared to CK, other treatments increased the abundance of aerobic_ammonia_oxidation, denitrification while decreased the abundance of nitrate_respiration, nitrogen_respiration; meanwhile, these four functional genes involved in N cycling processes were obviously higher in treatments with microplastics combined with biochar than in treatments with microplastic alone. In conclusion, microplastics combined with biochar could alleviate the negative effects of microplastic accumulation on sugarcane growth by improve soil nutrients and microbial community structure and function.