Humic substances suppress Fusarium oxysporum by regulating soil microbial community in the rhizosphere of cucumber (Cucumis sativus L.)

Fusarium wilt causes severe yield loss in greenhouse cucumbers. Humic substances refer to organic amendments typically applied in the soil to suppress pathogens and promote vegetable growth. The present study evaluated the effects of two humic substances (humic acid (HA)) from weathered coal and sodium humate from peat (HN) on the relative abundance of Fusarium oxysporum (F. oxysporum) and the greenhouse cucumber growth. HA and HN showed different chemical structures as per the Fourier transform infrared spectroscopy. HA and HN had different impacts on soil bacterial and fungal community compositions as well as on their network topological patterns. Specifically, both HA and HN increased the relative abundance of Firmicutes , Planctomycetes and Chloroflexi. HA increased the bacterial chao1 index and the observed_OTUs, while HN improved both the bac-terial and fungal alpha-diversities. Both HA and the HN also increased the network interactions, as well as the clustering coefficients (21.2%-21.5%), degrees (387%-554%) and densities (300%-467%) compared to the unamended control. The structural equation model indicated that the HA and HN demonstrate positive corre-lations with the bacterial and fungal community structure and density; besides, the fungal community structure was negatively correlated with network density. The latter showed a negative correlation with the disease index. Hence, HA and the HN increase the fungal network complexity of the rhizosphere soil and downgrade the network density and subsequently the disease index; as a result, the aboveground biomass of the greenhouse cucumbers increases. Overall, this study demonstrates the link between humic substances and microbial com-munity structures and highlights the significance of network topological patterns in suppressing Fusarium wilt disease and promoting cucumber growth.