Biodegradable plastics (BPs) have been increasingly used as alternatives to conventional plastics to alleviate soil pollution of conventional (micro)plastics. However, BPs in soils can break down into microplastics (MPs), nanoplastics (NPs), oligomers and monomers, which may pose risks to ecosystems. In comparison with the extensive study of conventional MPs and NPs, much less knowledge was available about behavior and risks of biodegradable MPs (BMPs), especially biodegradable NPs (BNPs) and oligomers in soils due to unavailability of reliable methodologies. Despite already published reviews in this field, the gap from BMPs to BNPs and oligomers to be bridged was more or less overlooked. This review systematically and critically summarized the latest advances and future research priorities. In contrast with the well studied aging of BPs, fragmentation process and intermediates releases of BPs, and their interactions between themselves and other pollutants in soils are poorly known. Despite relatively well explored effects on soil physicochemical properties, eco-toxicological impacts on microbes, plants and animals of BPs and BMPs, behavior and ecological risks of BNPs and oligomers are nearly fully neglected. Significant knowledge gaps and future research priorities in identification & quantification methodology, behavior and risks of BNPs and oliogomers in complex soil environments are demonstrated, including the unique challenges of BMPs and BNPs experiments from dynamic change of particle size and components due to hydrolysis in short term. This review underscores the need for further research on the formation, environmental behavior, and toxic effects of BNPs, oligomers, and monomers in soil. It improves understanding of the potential risks posed by BNPs and oligomers, and provides reference for future BP management strategies.