Mitigating soil salinity–alkalinity and reshaping bacterial community to improve soil organic carbon sequestration in the Hetao Irrigation District: a combined approach of organic ameliorant and microbial agents

Microbial agents promote the decomposition and the transformation of organic materials, enhancing the utilization of organic ameliorants in saline-alkali soil. However, the effects of the combination of organic ameliorants and microbial agents on the soil organic carbon (SOC) pool and the underlying regulatory mechanisms are poorly understood. Consequently, a 2-year field experiment was conducted in 2022-2023 to investigate changes in the SOC fractions and pools, as well as the regulatory pathways under different organic ameliorants: no organic ameliorants (CK), sole organic ameliorants (O), and organic ameliorants combined with microbial agents (O+M). O+M was superior to O treatment in improving the soil saline-alkali and nutrient conditions, alleviating the microbial carbon (C) and phosphorus (P) limitation and increasing the bacterial diversity and richness in saline-alkali soil. Community analysis revealed that O+M preferentially enriched the K-strategists (Actinobacteriota, +16%; Acidobacteriota, +6%), increasing the bacterial K/r ratio by 9% compared with O treatment. Partial least squares path modeling (PLS-PM) identified distinct SOC accrual pathways: the O and O+M treatments directly enhanced SOC, the O treatment increased the particulate organic carbon (POC) via bacterial diversity-mediated nutrient cycling, whereas the O+M treatment promoted POC through K/r ratio shifts and the mineral-associated organic carbon (MAOC) via soil content and pH modulation. Ultimately, O+M achieved the highest SOC (8.26 g kg^-1, +34% <i>vs</i>. CK), with MAOC contributing 32% of the total increment, 61% higher than that of O treatment. This mechanistic study elucidates how microbial-metabolic coordination and abiotic stabilization jointly regulate the persistence of SOC in degraded soils, providing a synergistic bioremediation strategy for saline-alkali ecosystems.