Response of Soil Organic Carbon and Bacterial Community to Amendments in Saline-Alkali Soils of the Yellow River Delta

The salt-alkali barrier and low fertility of coastal saline soil are bottlenecks that restrict the high-quality development of agriculture, and optimising measures to control salinity and increase carbon content has become an urgent task. The application of gypsum and the retention of straw are increasingly recognised practices aimed at mitigating soil salinization and enhancing soil fertility. However, the combined effects of these practices on soil organic carbon (SOC) sequestration and microbial community remain unclear. A field experiment was performed to analyse the influence of straw (0 and 10 t·ha⁻¹) and desulfurization gypsum (0 and 29 t·ha⁻¹) on soil chemical properties, aggregate-related carbon fractions and bacterial community features. Both straw and straw plus desulfurization gypsum treatments enhanced Shannon and Chao1 indices in all aggregates, though straw plus desulfurization gypsum reduced Chao1 in small macroaggregates compared to straw alone. The multidimensional scaling analysis suggested that the bacterial β-diversity was obviously impacted among different treatments. Both straw and straw plus desulfurization gypsum favoured the growth of Chloroflexi, Actinobacteria in clay-silt aggregates, Planctomycetes in microaggregates, Acidobacteria in microaggregates and clay-silt aggregates, Woeseia in macroaggregates, and inhibited the growth of Proteobacteria in clay-silt aggregates and Bacteroidetes in microaggregates and clay-silt aggregates. Straw plus desulfurization gypsum reduced the inhibitory effect of straw treatment on Gemmatimonadetes, Bacteroidetes, and Tumebacillus. Straw plus desulfurization gypsum decreased soil pH, sodium adsorption ratio and dissolved organic carbon (DOC), and increased sequestered SOC, enzyme activity and mean weight diameter more than only straw addition. Both enhanced microbial biomass carbon (MBC) and reduced exchangeable sodium percentage. MBC, SOC, DOC, and β-glucosidase were closely correlated with bacterial community composition. It is feasible to achieve greater carbon content while enhancing soil aggregate stability through the optimisation of straw incorporation or straw plus desulfurization gypsum in saline-alkali soil. These findings offer significant insights to improve saline-alkali soil.