Nitrate Stabilizes the Rhizospheric Fungal Community to Suppress Fusarium Wilt Disease in Cucumber

Nitrogen forms can regulate soil-borne Fusarium wilt suppression, but the related mechanisms are largely unknown, especially possible action via the rhizospheric microbial community. Soil analysis, MiSeq high-throughput sequencing analysis, community diversity, and network analysis were used to characterize the impact of different nitrogen forms (nitrate and ammonium) on rhizospheric fungal communities and the contribution of nitrate to the suppression to Fusarium oxysporum f. sp. cucumerinum compared with ammonium. Nitrate-grown cucumber showed a lower disease index and F. oxysporum f. sp. cucumerinum abundance in the rhizosphere. In comparisons with ammonium nutrients, nitrate-fed plants maintained a higher soil rhizosphere pH, microbial biomass carbon content, microbial biomass nitrogen content, as well as fungal community richness and diversity following F. oxysporumf. sp. cucumerinum incubation. All these factors were negatively related with disease index. Network analysis showed fewer nodes and edges in the ammonium treatments compared with nitrate treatments. The relative abundance of Pathotroph-Saprotroph, Pathotroph-Saprotroph-Symbiotroph, and Saprotroph fungi explained 82% of the variability of rhizosphere F. oxysporum f. sp. cucumerinum abundance. In conclusion, after pathogen inoculation under nitrate nutrition, the less-affected microbial composition, community diversity, and community internal relations, which resulted from the more diverse and robust microbial population, potentially contributed to greater Fusarium wilt suppression.