Long-Term Excess Nitrogen Fertilizer Reduces Sorghum Yield by Affecting Soil Bacterial Community

The application of nitrogen (N) fertilizer is one of the most important measures to affect crop yield and soil bacterial communities. In this study, the four rates of N (namely N0F 0 kg N ha^-1, N1F 90 kg N ha^-1, N2F 180 kg N ha^-1, and N3F 270 kg N ha^-1) along with a control (no fertilization, CK) were evaluated for their influence on sorghum yield, soil chemical properties, bacterial community, and diversity. The results showed that the yield-increasing effect was reduced by the higher dose of N input. Compared with N0F, sorghum yield increased by 58.8% in N1F and 68.2% in N2F but decreased by 8.1% in N3F relative to N2F. The soil pH decreased significantly with increasing N application. Compared with CK or N0F, N3F treatment increased the available P content by up to 18.6% or 32.2% but decreased the alkaline hydrolysis N, available K, organic matter, and total N contents by 8.4% or 23.8%, 5.5% or 10.6%, 8.4% or 28.8%, and 11.1% or 39.6%, respectively. In addition, different fertilization treatments altered the soil bacterial communities. Excess N fertilizer led to a decrease in bacterial abundance, and compared with N0F, the absolute abundance of bacteria increased by 18.7% in N1F, while it decreased by 31.8% in N3F. The predominant phyla, including Acidobacteria, Proteobacteria, and Chloroflexi, in the microbiome shift under different N application levels. The redundancy analysis (RDA) and Pearson's correlation analyses indicated that the soil properties, especially soil pH, available P, total P, total N, and organic matter, were the key environmental factors that defined the bacterial community in the ecosystem. Within the scope of the present experiment, N application at 90 kg N ha^-1 (N1F) optimized soil bacterial community abundance in sorghum-cultivated soil, while N2F (180 kg N ha^-1) achieved the highest sorghum yield, suggesting a trade-off between optimizing the soil microbiome and maximizing crop yield under long-term fertilization.