Effect of polyethylene microplastics on soil organic carbon pool dynamics in the soil-rhizosphere-alfalfa system

The effect of microplastics (MPs) on soil organic carbon (SOC) has become a growing concern. However, most studies have focused on the effect in a single soil system, and little is known about how MPs affect the soil carbon pool in the soil-plant system. In this study, a 90-day pot experiment was conducted with different concentrations (0 %, 0.5 %, 1 %, and 5 % w/w) and exposure times (30d, 60d, and 90d) of polyethylene microplastics to investigate their effects on SOC pools. The results showed that MPs increased particulate organic carbon (POC) and mineral-associated organic carbon (MAOC) pools, while decreasing microbial biomass carbon (MBC). Initially, MPs increased POC by disguise and their effects on enzyme activities and C degradation genes, and indirectly increased MAOC by affecting dissolved organic carbon (DOC) levels (via soil pH) and stimulating enzyme activities. However, as alfalfa rapidly grew from the branching to the flowering stage, plants inputted C into the POC pool via rhizodeposition, while activating and absorbing the MAOC pool for growth through roots, resulting in an increase in POC and a decrease in MAOC. Moreover, plants had priming effects on soil MBC, enzyme activities, and C degradation genes, further influencing SOC pools. Overall, MPs affected plant growth and soil carbon pools by altering soil properties, while plants in turn influenced soil organic carbon pools and the impact of MPs on soil properties. This study provides deeper insights into the effect of MPs on SOC dynamics in the soil-rhizosphere-plant system.