Biodegradable microplastics enhance soil microbial network complexity and ecological stochasticity.

Biodegradable plastics have emerged as an ecological alternative to conventional petroleum-based plastics. Despite the recent advances in the effects of conventional microplastic on soil ecosystems, the ecological impact of biodegradable microplastics in soil environments remains poorly understood. Here, we performed soil microcosms with conventional (polyethylene and polystyrene) and biodegradable (polybutylene succinate and polylactic acid) microplastics to estimate their effects on the success patterns, co-occurrence networks, and the assembly mechanisms of soil bacterial communities. Biodegradable microplastics significantly altered the soil bacterial community composition with steeper temporal turnovers (rate: 0.317 - 0.514) compared to the conventional microplastic treatments (rate: 0.211 - 0.220). Network under biodegradable microplastics showed greater network complexity, including network size, connectivity, average clustering coefficient, and the number of keystone species, as compared with the conventional microplastic treatments. Additionally, the biodegradable microplastic network had higher robustness, which may be potentially due to the enhanced dissolved organic carbon contents in the soil treated with biodegradable microplastics. The bacterial community assembly was initially governed by deterministic homogeneous selection (93 - 100 %) under the stress of microplastics, but was progressively structured by increasing stochastic homogeneous dispersal (17.8 - 73.3 %) over time. The normalized stochasticity ratio also revealed that the application of microplastics increased the importance of stochastic processes following incubation. These findings greatly enhanced our understanding of the ecological mechanisms and interactions of soil bacterial communities in response to microplastic stress.