Polylactic acid microplastics induced negative priming and improved carbon sequestration via microbial processes in different paddy soils

Biodegradable microplastics (MPs), which are starting to be used in large quantities in croplands, may affect the mineralization of soil organic carbon (SOC). These priming effects induced by biodegradable MPs are a very new issue, and their mechanisms as well as consequences for various soils are nearly unknown. Using stable carbon isotope signature (δ13C), we quantified the priming effects by adding corn (C4 plant) -based polylactic acid (PLA, δ13C = 11.9‰) MPs to three paddy soils with solely C3 signature: Ferralsol, Alfisol and Mollisol at two rates (0.5 and 1.0 wt%, based on the mass of MPs). After the incubation (180 days), PLA-MPs reduced the SOC mineralization in all three soils, triggering a negative priming effect. This negative priming effect was strongest in Mollisol (210–220 mg CO2–C kg−1). The net C balance in Mollisol was positive and clearly larger than the C amounts initially added with PLA-MPs to soils, indicating C accrual. The two main mechanisms of the negative priming effects were: i) sorptive protection of SOC and especially of dissolved organic carbon (DOC) by PLA-MPs, and ii) reduction of microbial biomass and fungal diversity after PLA-MPs addition. Additionally, “switching of microbial decomposition from SOC to PLA-MPs” was pronounced in Mollisol, indicated by more PLA-MPs being mineralized. PLA-MPs thus changed the soil C stock and dynamics mediated in part by the changes of microbial biomass, diversity, and community composition, switch of utilization to new resources and decrease of SOC mineralization, all of them leading to C accumulation in soil.