Biodegradable and Non-Biodegradable Microplastics Drive Divergent Microbial Carbon and Nitrogen Metabolism Regimes in Soil-Plant Systems.

The accumulation of microplastics (MPs) in soils worldwide has raised concerns about their potential impacts on microbially mediated carbon (C) and nitrogen (N) cycling. However, whether biodegradable and non-biodegradable MPs differently affect microbial C and N metabolism and coupling, and whether these responses are modulated by co-occurring plant identity remain uncertain. We combined a meta-analysis with a controlled pot experiment comparing soil responses under two congeneric plant species exposed to two biodegradable and two non-biodegradable MPs at doses ranging from 0% to 2% (w/w). In the meta-analysis, MPs generally increased CO2 emission and microbial biomass N (MBN), but had no significant overall effect on microbial biomass C (MBC). In the pot experiment, MPs likewise increased CO2 emission, whereas overall effect sizes of MBC and MBN were nonsignificant. Nevertheless, these overall responses did not capture the distinct patterns of microbial C-N metabolic coordination between MP types. Under biodegradable MPs, tighter MBC-MBN coupling with a dose-dependent shift toward greater respiratory C loss relative to MBC collectively indicated a maintenance-associated metabolic pattern. In contrast, MBC-MBN decoupling and stronger MBC-CO2 coupling under non-biodegradable MPs indicated a biomass-linked metabolic pattern. In addition, plant identity modulated these microbial metabolisms in an MP-type manner, particularly in N-related processes. These metabolic responses were accompanied with changes in soil property conditions and substrate availability, weakened DOC-DON coupling, and shifts in microbial community structure and life history strategy. RDA-based hierarchical and variation partitioning and SEM further indicated that modeled soil property pathways were more prominent under non-biodegradable MPs, while substrate-related pathways were prominent under biodegradable MPs. Together, the meta-analysis reveals the general microbial metabolic patterns under MP type, and the pot experiment further reveals how MP type and plant identity jointly influence microbial C-N metabolism through contrasting associations with soil substrates and property conditions.