Given the shortage of water resources and excessive application of nitrogen fertilizers in irrigated areas, we explored the effect of water–nitrogen coupling on soil microbial diversity in maize fields irrigated using shallow buried droppers. A field experiment (split-plot design) was used with irrigation amounts set at 40%, 50%, and 60% of the conventional amount; furthermore, 13 water and nitrogen coupling treatments were designed. The secondary area was the nitrogen application level, corresponding to 50%, 70%, and the original conventional application amounts. The results showed that the effect of irrigation amount on bacterial community composition was greater than that of nitrogen, whereas the effect of nitrogen on fungi was greater than that on bacteria. No significant difference was detected in the α diversity index or species richness of bacteria and fungi. Available phosphorus and organic carbon contents significantly correlated with the community structure of soil bacteria (p < 0.05). The relative abundances of bacteria and fungi were stable with the decrease of nitrogen application rate at the irrigation rate of 2000 m³ ha⁻¹. With the decrease of irrigation amount, the relative abundance of bacteria and fungi was stable under the treatment of 210 kg ha⁻¹ nitrogen fertilizer. Moreover, the relative abundance of nitrogen-fixing bacteria related to the nitrogen cycle was increased by irrigation of 2000 m³ ha⁻¹ and nitrogen application of 210 kg ha⁻¹. Moderate reduction of subsequent N supply should be as a prior soil management option in a high N input agroecosystem.

Biochar application can alter soil properties and enhance soil quality. However, the effects of the regulatory mechanisms underlying nitrogen fertilizer application on soil physicochemical properties and microbial characteristics remain unclear. We conducted a three-year maize field experiment to investigate the effects of four biochar application rates (0, 8, 16, and 24 t/ha) and three nitrogen fertilizer rates (0, 150, and 300 kg/ha) on soil physicochemical properties, enzyme activities, microbial community structure, and maize yield. In the 0–20 cm soil layer, the combined application of biochar and nitrogen fertilizer significantly increased soil porosity (SP) and soil water content (SWC), reduced soil bulk density (SBD), and increased soil organic carbon (SOC) and total nitrogen (TN) levels. Additionally, β-D-cellobiohydrolase and leucine aminopeptidase activities were enhanced, along with nitrogen acquisition potential. The relative abundance of Proteobacteria was highest at a biochar application rate of 16 t/ha. The combined 16 t/ha biochar and 150 kg/ha nitrogen treatment exhibited the highest maize yield. Maize yield was positively correlated with SP, SWC, SOC, and TN. Consequently, the combined application of 16 t/ha biochar and 150 kg/ha nitrogen improved soil physicochemical properties, enhanced soil enzyme activities, regulated key soil microbial taxa, and increased maize yield.