Higher C/N or C/P enhanced interrelationship and deterministic assembly of bacterial community in oil-contaminated saline–alkali soil

Supplementation with nitrogen (N) and phosphorus (P) is a straightforward, cost-effective strategy for enhancing the bioremediation of total petroleum hydrocarbons (TPH). However, the mechanisms underlying this process in saline–alkali soils with high osmotic pressure remain unclear. This study demonstrates that bioaugmentation mediated by higher C/N or C/P ratios enhances soil basal respiration (SBR) and the activities of alkane hydroxylase and catechol 1,2-dioxygenase, resulting in TPH biodegradation efficiencies of 80.5% at a C/N ratio of 180 and 78.7% at a C/P ratio of 1800. The regulation of soil C/N and C/P ratios significantly altered the microbial community structure and composition, notably increasing the relative abundance of Pseudomonas , Alcanivorax , and Bacillus . The timely addition of N or P enhances overall metabolic pathways and functional genes (e.g., fad , alkB , aldh , frmA , paa , dmp , and nah ) in microbial communities. Elevated C/N and C/P ratios increased the contribution of deterministic processes to microbial community assembly, promoting the directional enrichment of specific microbial groups and enhancing TPH degradation. These findings provide practical guidance for the targeted regulation of bioaugmentation in oil-contaminated saline–alkali soils. • The SBR, enzyme activities and TPH removal were enhanced by higher C/N and C/P. • Timely inputs of N or P strengthened overall metabolic pathways and functional genes. • Homogeneous selection mainly contributed to community assembly after N or P inputs. • Driving mechanism of nutrient inputs for excellent degradation of TPH were proposed.