Objective  Microplastics interact with soil particles and aggregates and affect soil stability. It is of great significance to investigate the effect of microplastic concentration on the characteristics and stability of black soil aggregates,

  Method  Soybean pot experiment was conducted at the different microplastics concentrations l (0%, 0.1%, 0.5%, 1%, 2%, 5%) in soil under natural conditions. The indices of aggregates, such as macroaggregate content (R _{> 0.25}), mean weight diameter (MWD), geometric mean diameter (GMD) and fractal dimension (FD) of Black soil aggregates were observed and analyzed.

  Result  In different treatments of microplastic concentration, the content proportion of mechanical stable aggregates with particle < 0.25 mm was the lowest, and the content proportion of aggregates with particle > 2 mm and < 0.25 mm increased with the increase of microplastic concentration. The content proportion of water-stable aggregates with particle > 2 mm was the lowest, and the content proportion of water-stable aggregates with particle < 0.25 mm increased with the increase of microplastic concentration. However, the content proportion of mechanical and water stable aggregates was contrary to the trend of other concentrations in the treatment with 1% microplastic. Planting was similar to the change of aggregates without planting. The content proportion of soil macroaggregates (R _{> 0.25}) decreased significantly with the increase of microplastic concentration, except for the content proportion in treatment with 1% of microplastic addition. Regarding to the different sampling periods, the MWD and GMD of soil mechanical and water stable aggregates at soybean maturity stage were lower than that at flowering stage while FD at soybean maturity stage were greater than that at flowering stage, indicating that the stability of soil aggregates reduced with the plant and microplastic interaction. Correlation analysis showed that MWD and GMD were significantly positively correlated, and both of them were significantly negatively correlated with FD, indicating that the capacity of soil aggregating declined. In addition, our findings showed that FD was the lowest in the treatment with 1% of microplastic addition among other treatments, indicating that soil aggregate stability seems improved at this level.

  Conclusion  The higher the cumulative concentration of microplastics in soil is, the stronger the damage of soil aggregates will occur, and the aggregation ability of soil particles is weakened. Whether the microplastics concentration of 1% in soil could be the threshold of microplastic effect on black soil aggregate stability. Further work should be done to explore the interaction of microplastic on soil quality.