Biochar alters chemical and microbial properties of microplastic-contaminated soil

The occurrence of microplastics (MPs) in soils can negatively affect soil biodiversity and function. Soil amendments applied to MP-contaminated soil can alter the overall soil properties and enhance its functions and processes. However, little is known about how soil amendments improve the quality of MP-contaminated soils. Thus, the present study used a microcosm experiment to explore the potential effects of four types of biochar on the chemical and microbial properties of low-density polyethylene (LDPE) MP-contaminated soil under both drought and well-watered conditions. The results show that the biochars altered soil pH, electrical conductivity (EC), available phosphorous, and total exchangeable cations (TEC) with some variability depending on the biochar type. Oilseed rape straw (OSR)-derived biochars increased soil pH, EC, and TEC under both water conditions with the highest values of 7.94, 0.54 dS m^-1 and 22.0 cmol₍₊₎ kg^-1, respectively. Soil enzyme activities varied under all treatments; in particular, under drought conditions, the fluorescein diacetate activity increased in soils with high temperature (700 °C) biochar. The application of soft wood pellet biochar (700 °C) to MP-contaminated soil increased urease activity by 146% under well-watered conditions. OSR-derived biochars significantly reduced soil acid phosphatase activity under both water conditions. With biochar supplementation, the diversity indices of the bacterial community increased in well-watered soil but not in soil under drought conditions. The abundance of bacterial phyla, such as Firmicutes, Proteobacteria, Actinobacteria, Dictyoglomi, and Gemmatimonadetes, was relatively high in all treatments. Biochar application resulted in negligible variations in bacterial communities under drought conditions but significant variations under well-watered conditions. The findings of this study imply that biochar can be used as a soil amendment to improve the overall soil quality of MP-contaminated soil, but its impact varies depending on the pyrolysis feedstock and temperature. Thus, selecting a suitable biochar is important for improving the soil quality in MP-contaminated soils.