Depth-dependent response of soil microbial community and greenhouse gas efflux to polylactic acid microplastics and tidal cycles in a mangrove ecosystem

Abstract

The impacts of microplastics (MPs) on greenhouse gas emissions from mangrove soil remain poorly understood. Previous studies mostly focused on the topsoil in stable inundation state, ignoring the effects of natural tidal cycle and deep soil under different soil oxygen conditions. In this study, we analyzed soil microbial communities and greenhouse gas emissions from mangrove soils across various depths and tidal conditions (by adding seawater to create different inundation durations) in response to polylactic acid (PLA) MP exposure. Results indicated that PLA MPs addition enhances CO₂ and CH₄ release from the continuously anaerobic subsoil (100-120 cm). With increasing submersion duration, PLA MPs facilitate the emission of CH₄ from the topsoil (0-5 cm). An elevated C:N ratio may promote microbial nitrogen mining and organic carbon mineralization, indicating the threat of PLA MPs to soil carbon and nitrogen pools. PLA MPs addition significantly altered the bacterial community structure and reduced bacterial diversity in the subsoil. Increases in the abundance and functioning of communities associated with methanogenesis and sulfate reduction contributed to the release of CO₂ and CH₄. The duration of inundation had no significant impact on the microbial community structure in the topsoil. These findings demonstrate the accelerating effect of PLA MPs on organic carbon mineralization and carbon release, which was critically regulated by the soil depth and tidal inundation.

Environmental Implication

Microplastics (MPs), as emerging contaminants, are ubiquitous in mangrove ecosystems and may impact soil microbial communities and greenhouse gas emissions. However, the effects of MPs on microbial communities and carbon cycling in different soil layers remain inadequately elucidated, especially under the dynamic conditions of tidal cycles. This manuscript investigates the controlling effects of depth and tidal inundation on the impact of MPs on microbial communities and greenhouse gas emissions in mangrove soil. The findings are crucial for understanding the impact of MPs on the carbon pool in mangrove soils.