Synergistic nano-bioorganic amendments enhance soil properties and microbial structure in coastal saline soils

Introduction Soil salinization threatens global food security and sustainable land use. Ameliorating coastal saline soils with exogenous amendments is crucial. Bio-organic fertilizer (OF) and nano-carbon (NC) are promising green amendments, but their comparative and combined effects on soil properties and microbial communities are not fully understood. Methods A field experiment was conducted in coastal saline soil (Ninghe District, Tianjin, China). Four treatments were established: control (CK, no amendment), OF application, NC application, and combined application of OF and NC (FC). Soil physicochemical properties and microbial community structure (via 16S and ITS rRNA gene sequencing) were analyzed. Results The FC treatment most effectively improved soil properties, significantly reducing bulk density, pH, salinity, and sodium adsorption ratio (SAR), while increasing porosity, water content, and nutrient (N, P) availability. Soil bacterial diversity (Ace, Chao1, Shannon indices) increased significantly in all amendment treatments compared to CK, with the highest values in NC and FC treatments. Amendment application altered microbial community composition, enriching specific bacterial taxa (e.g., Firmicutes, Desulfobacterota in FC) and fungal taxa. Redundancy analysis identified soil salinity and pH as key drivers of bacterial community structure, whereas fungal communities showed a distinct, less correlated response pattern. Discussion The synergistic application of nano-carbon and bio-organic fertilizer (FC) created a more favorable soil habitat, rapidly ameliorating physicochemical conditions which directionally shaped the bacterial community. Bacterial and fungal communities responded differently to amendments, suggesting divergent assembly mechanisms. The FC strategy demonstrates high potential for the initial restoration of saline-alkali soils by enhancing soil health primarily through rapid physicochemical improvement and modulation of the soil microbiome, particularly bacteria. Future work should focus on functional validation of predicted metabolic shifts and assessment of agronomic outcomes.