Humic acid substitution improves soil quality and maize yield under drought and saline–alkaline stress by regulating bacterial community structure

Humic acid (HA) substitution provides a potential strategy for reducing chemical fertilizer inputs while maintaining crop productivity and soil functions in drought-prone saline–alkaline agroecosystems. However, the optimal HA application rate and its effects on soil quality and microbial communities under reduced water and fertilizer input remain unclear. A two-year field trial was conducted in Xinjiang, China, including two irrigation levels (100% and 80% crop evapotranspiration, ET c ) and five fertilization treatments, including conventional fertilization (CT: 300 kg N ha −1 , 120 kg P ha −1 , and 150 kg K ha −1 ), reduced fertilization (R, 20% less than CT), and partial substitution of chemical fertilizer with HA at 60 (HAL, 3% substitution rate), 150 (HAM, 7% substitution rate), and 300 (HAH, 14% substitution rate) L ha −1 , with total N, P, and K inputs kept equal among the R and HA treatments. Deficit irrigation (80% ET c ) reduced maize yield by approximately 5% compared with full irrigation (100% ET c ), whereas HA substitution significantly improved crop productivity. Compared with CT, HAM increased maize yield by approximately 12%, achieving the highest yield among fertilization treatments. HA substitution improved soil quality, with HAH showing the greatest improvement in soil quality but a lower yield response than HAM. Compared with chemical fertilizer alone, HA substitution applied in the two-year experiment reduced soil pH, electrical conductivity, total nitrogen, and available nitrogen contents, while increasing total organic carbon, available phosphorus and potassium contents, as well as enzyme activities. HAM enhanced bacterial α-diversity, whereas HAH reduced it, with HA effects on β-diversity varied with irrigation level. Moreover, moderate HA promoted more compact and structured bacterial networks, while high HA input weakened network connectivity. Partial least squares path modeling indicated that bacterial community characteristics exerted stronger associations with soil quality and maize yield than soil physicochemical properties, while enzyme activities positively related to soil quality improvement, which subsequently promoted maize yield. Overall, 150 L ha −1 HA fertilizer substitution combined with 80% ET c is a valuable strategy for sustainable maize production and soil quality improvement in arid saline agricultural regions.