Effects of subsurface drainage configuration on soil bacterial communities, water and nitrogen use, and yield in severe saline-alkali cotton fields

To investigate the effects of subsurface drainage configuration on soil bacterial communities, cotton yield, and water and nitrogen use in saline-alkaline soils of Xinjiang, a field experiment was conducted from April to October in 2024 with three drain burial depths, namely 0.8 m (M1), 1.1 m (M2), and 1.4 m (M3), and three drain spacings, namely 10 m (J1), 15 m (J2), and 20 m (J3). Aboveground dry matter, seed cotton yield, water and nitrogen use efficiency, and plant nitrogen accumulation were measured, while bacterial alpha diversity (Chao1 and Shannon indices), beta diversity (Bray-Curtis distance and principal coordinates analysis), community structure, and community assembly mechanisms were analyzed. The effects of different drainage configurations on soil bacterial communities, cotton yield, and water and nitrogen use were systematically evaluated. The results showed that: 1) in terms of yield and water and nitrogen use, treatment M2J1 achieved the highest seed cotton yield and water and nitrogen use efficiency, with a seed cotton yield of 7 418.98 kg/hm². Under M2J2, seed cotton yield increased by 29.66% compared with M3J2, and under M2J3, it increased by 21.67% compared with M3J3. At 125 days after emergence, nitrogen accumulation under M2J1 reached 320.26 kg/hm², which was significantly higher than that of the other treatments. 2) Regarding the soil bacterial community, alpha diversity did not differ significantly among drain burial depth treatments, whereas it showed a decreasing trend with increasing drain spacing. Principal coordinates analysis indicated that drain burial depth significantly altered soil bacterial community structure (P < 0.001), whereas drain spacing had no significant effect on community structure (P = 0.116). Bray-Curtis distance increased significantly with increasing burial depth. Null model analysis showed that the median betaNTI values in all treatments ranged from -2 to 2, indicating that soil bacterial community assembly was mainly governed by stochastic processes; however, the role of deterministic processes was markedly enhanced in the M2 treatment. Structural equation modeling further showed that drainage configuration could influence nitrogen accumulation in cotton organs by regulating the soil environment, water use efficiency, and bacterial community characteristics. These findings provide a theoretical basis and technical reference for optimizing subsurface drainage configuration, improving cotton yield, and enhancing efficient water and nitrogen use in saline-alkaline cotton fields in Xinjiang.