Biochar and deficit irrigation are associated with enhanced saline soil mechanical stability and shifts in bacterial assembly

Saline soil structural degradation in arid regions is exacerbated by water scarcity. Deficit irrigation (DI) combined with biochar can mitigate this, but the bacterial mechanisms remain poorly understood. Here we show, from a two‑year field experiment in the Hetao Irrigation District, that DI with high‑dose biochar (30 t ha⁻¹) restructures the soil bacterial community assembly: the normalized stochasticity ratio (NST) decreased below 0.5, indicating a shift toward deterministic processes, while the contribution of heterogeneous selection increased by 22.9%. This shift in assembly processes was associated with a more stable bacterial co-occurrence network, with network robustness increasing by 13.1% and vulnerability decreasing by 44.2%. Fungal communities showed no statistically significant response. Mechanical aggregate stability, measured as mean weight diameter (MWD), improved by 14.4% under DI+B 30 relative to DI alone. However, water-stable aggregates did not show corresponding improvement under the same treatment; geometric mean diameter decreased. This contrast suggests that the observed stabilization was more evident under dry-sieving conditions than under wet-sieving conditions. The effect thus appears to be limited to mechanical resistance and does not extend to hydraulic stability, within the scope of our measurements. Random Forest and structural equation modeling revealed that microbial assembly processes and network stability were associated with MWD in our statistical models. The statistical models were consistent with the interpretation that the association between water-biochar synergy and soil structure was indirectly statistically associated with changes in microbial community properties. Our findings indicate that water-biochar management covaries with a shift toward greater deterministic influence in bacterial assembly and with enhanced network stability, and that these microbial properties correlate with improved mechanical aggregation in saline soils. The decoupled response of water-stable aggregates highlights the need for further investigation into the mechanisms underlying aggregate stabilization under combined water and biochar management.