Biodegradable microplastic increases CO2 emission and alters microbial biomass and bacterial community composition in different soil types

Plastic input to the terrestrial environment is of global concern and the still increasing production and release worldwide reinforces this problem. It has been shown that microplastics (MPs) can affect soil structure and soil organisms, possibly leading to an increase in soil carbon turnover, microbial activity and resulting CO2 emissions. Yet, the response of soil CO2 emissions to various types, quantities, and sizes of microplastic is not well understood. The aim of this work was to investigate the effect of conventional and biodegradable microplastics on soil microbial biomass, bacterial community composition and CO2 development. Two types of plastics, LDPE (low-density polyethylene) and PBAT (polybutylene adipate-co-terephthalate), at low (0.1 %) and high (1 %) concentrations and in three different size ranges (50–200 μm, 200–500 μm, and 0.63–1.2 mm) were amended to a sandy loam and a loamy soil and CO2 emissions were measured over four weeks. Afterwards, microbial biomass and growth were estimated, and prokaryotic community shifts were inferred by amplicon sequencing. No effect of LDPE on soil CO2 emissions could be detected, but higher CO2 emissions (13–57 %), microbial biomass (1–7 %), and a shift in community composition was induced by addition of the biodegradable PBAT when added at high concentration. Soil CO2 emissions were 10–13 % greater when small PBAT particles were added compared to large ones. PBAT addition at low concentration had no significant effect independent of its size. Overall, the effect of PBAT addition on soil CO2 emissions was larger in sandy loam than in loam. Several bacterial lineages known to degrade polyesters and other biodegradable MPs, such as members of the Caulobacteraceae and Comamonadaceae were found enriched after PBAT amendment, but effects were soil specific. We conclude that direct impacts of plastic on soil properties are not the main reason for increased soil CO2 emissions, but rather relate to the different recalcitrance of polymer types. Soils contaminated with biodegradable plastic may emit larger amounts of CO2, which needs to be considered in predictions of global impacts of plastic pollution and its mitigation.