Stimulated soil CO2 and CH4 emissions by microplastics: A hierarchical perspective

Microplastics have become ubiquitous in soils on a global scale, which may elicit profound deleterious impacts on soil health. However, contradictory case studies have prevented a clear understanding of how microplastics hierarchically impact soil carbon cycling. A meta-analysis was performed to systematically elucidate the effects of microplastics on carbon cycling, particularly in terms of CO2 and CH4 emissions. Upon exposure to microplastics, the overall soil CO2 emissions increased by 54.3%, as were the soil organic carbon (22.6%), dissolved organic carbon (13.3%), and microbial biomass carbon (5.5%). Sufficient carbon substrates supported the replication of functional genes that governed the decomposition of carbon (4.0%; e.g., abfA, sga, and manB, etc.), whereas the gene copies for carbon fixation (accA and pccA) were negligibly affected. Moreover, soil enzymatic (e.g., dehydrogenase, fluorescein diacetate hydrolase, and chitinase) activities were also enhanced. Meanwhile, the CO2 emissions were positively correlated with the gene copy numbers of carbon-decomposing microorganisms. Similarly, the soil CH4 emissions increased by 9.7%, where the increment of methanogen (41.1%; mcrA) was slightly higher than that of methanotrophs (37.9%; pmoA, mmoX and mxaF). Moreover, CH4 emissions were positively correlated with methanogen gene copies, and the increment of CH4 emissions was more pronounced in paddy soil (11.3%) than in upland soil. Microplastics with smaller dimensions, round shapes, higher concentrations, and biodegradability leaded to higher soil CO2 and CH4 emissions. Meanwhile, the effect sizes of CO2 and CH4 emissions decreased over time. These findings revealed that soil carbon cycling was expedited following microplastics exposure, which should be factored into future environmental risk assessments.