Nitrogen Application Under Integrated Water–Fertilizer Management Regulates Rhizosphere Nitrogen Dynamics, Microbial Communities, and Maize Productivity

Nitrogen management strongly regulates rhizosphere nutrient availability, microbial ecological processes, and maize productivity in fertigation-based systems. Yet, the mechanisms by which nitrogen application gradients shape rhizosphere nitrogen transformations and microbial community stability remain insufficiently understood. In this study, a field experiment with four nitrogen levels (N0 0 kg·hm⁻², N1 100 kg·hm⁻², N2 140 kg·hm⁻², and N3 180 kg·hm⁻²) was conducted to clarify how nitrogen inputs influence rhizosphere nitrogen forms, microbial community assembly, and yield performance in maize under an integrated water-fertilizer regime. Moderate nitrogen application (N2) significantly enhanced rhizosphere nitrogen availability throughout the jointing, silking, and grain-filling stages. Nitrate-N and dissolved organic nitrogen (DON) reached 297.15 mg·kg⁻¹ and 152.14 mg·kg⁻¹ at jointing-substantially higher than N0-and remained elevated during subsequent stages, indicating sustained nitrogen supply. This improved nutrient status promoted plant growth and resulted in the highest grain yield (5281.85 kg·hm⁻²), representing a 25.7% increase compared with N0. In contrast, excessive nitrogen input (N3) failed to further elevate nitrogen fractions and may have reduced nitrogen-use efficiency. Nitrogen application influenced rhizosphere microbial communities. Moderate nitrogen (N2) increased bacterial richness and altered community composition, with several dominant bacterial and fungal taxa showing changes in relative abundance. Core microbial taxa remained largely stable, whereas transient taxa exhibited modest variations. Co-occurrence network analysis indicated increased cohesiveness in bacterial networks and reduced connectivity in fungal networks under nitrogen input. Functional predictions revealed that nitrogen application reduced bacterial chemoheterotrophy, aerobic-chemoheterotrophy, and nitrogen-cycling-related functions, while enhancing aromatic-compound-degradation potential. Predicted abundance of potential plant pathogenic fungi decreased following nitrogen application. Overall, moderate nitrogen input optimized rhizosphere nitrogen fractions, influenced microbial community composition and functional potential, and improved maize productivity, providing insight into microbiological responses to nitrogen management in semi-arid fertigation systems.