Nitrogen-Fixing Bacterial Inoculation Can Enhance Maize Yield and Alter Soil Microbial Community Structure Under Fertilizer Reduction

Excessive fertilizer application is a common practice in agricultural production in the North China Plain. To determine an optimal fertilization strategy for summer maize with nitrogen-fixing bacterial inoculation, we conducted a two-year field experiment (2022–2023) using the conventional fertilization rate (600 kg ha−1 NPK; N:P2O5:K2O = 28:8:10; 100F by default) as a control and examined the effects of fertilizer reduction (at 90%, 80%, 62.5%, and 50% of 100F) combined with Azotobacter chroococcum inoculation on maize plants and soil. Although fertilizer reduction increased free amino acid content, soluble sugars, proteins, and fatty acids contents were reduced. However, bacterial inoculation significantly enhanced all the above nutritional indices in maize leaves. Bacterial inoculation under fertilizer reduction conditions can enhance the activity of key nitrogen metabolism enzymes (i.e., GS and GOGAT), which further supports nitrogen, sugar, and lipid metabolism in maize plants. Additionally, bacterial inoculation promoted root development, biomass accumulation, and grain nutritional value while significantly increasing yield under reduced fertilizer conditions. The highest yield (11,454 kg ha−1) was achieved with bacterial inoculation at approximately 87F (≈522 kg ha−1 NPK), while the non-inoculated control reached a peak yield (11,032 kg ha−1) only at around 90.5F (≈543 kg ha−1). The complementary effects of bacterial inoculation with fertilizer reduction resulted in improved nutrient supply and modulation of soil microbial diversity. Inoculation of A. chroococcum increased soil ammonium and nitrate levels and decreased soil pH, though it was associated with a decline in overall bacterial richness, which may have persistent and adverse effects on the soil. Both fertilizer reduction and bacterial inoculation significantly altered microbial community structure, with notable interannual variation. Collectively, our findings suggest that moderate fertilizer reduction (9.5–13%) combined with nitrogen-fixing bacteria inoculation can support sustainable maize production by maintaining higher yield, enhancing nutrient use efficiency, and improving soil health. However, due to pH-lowering effects, long-term monitoring is necessary to assess the ecological impact of nitrogen-fixing bacteria inoculation on soil microbial balance.