Effect and microbial mechanism of suspended sediments particle size on nitrous oxide emission in eutrophic lakes

Suspended sediment (SPS) is an important environmental factor in eutrophic lakes, where they may play a significant role in the microbial nitrogen cycle and thus affect the N₂O source and sink function. This study investigated the correlation and corresponding microbial mechanisms between N₂O emission fluxes and SPS particle sizes. N₂O emission characteristics were investigated in four parallel operated lab-scale microcosmic systems, in which different sizes of SPS particles were inoculated (i.e., <75, 75-150, 150-300, and >300 μm). The results show that, N₂O emission fluxes in the eutrophic lakes were exponentially correlated with the lake trophic level index (TLI) (R² = 0.94, p < 0.01) and the specific surface area of the SPS (R² = 0.38, p < 0.05). In the microcosmic systems, SPS with 75-150 μm particles had the highest N₂O emission rate of 5.94 ± 0.007 μg N/L/d, which was 2.6 times that of the <75 μm particle size system. The microcosmic system with particle size >300 μm had the highest N₂O reduction rate (Vmax) of 6.776 μmol/L/h, which was 16-50 times that of the other three groups. Larger particle size SPS have a smaller specific surface area, which could affect the microenvironment on SPS surface and thus affect the microbe functions. The microbial community structure results indicated that the dominant microorganisms on the SPS surface were denitrifying bacteria. The maximum (nirS + nirK)/nosZ ratio was 30.2 for the 75-150 μm system, which was nearly 2 times higher than the other systems. The >300 μm system had the highest nosZ abundance, indicating a strong ability to reduce N₂O. The co-occurrence networks analysis indicated that the cooperation and competition among nitrifiers and denitrifiers determined N₂O emissions. These results provide fundamental insights into the influence of SPS size on N₂O emissions in eutrophic lakes.