Biostimulation holds promise for remediating deep-layer oil-polluted land but it often faces unexpected challenges. Despite initial efforts, outcomes frequently fall short, with underlying mechanisms remaining elusive. This study, based on four oil-contaminated sites that received amendments for three years in Saskatchewan, Canada, is to investigate the factors responsible for the declined efficacy of biostimulation over time. The amendments were designed to provide potential electron acceptors (iron, nitrate, sulfate) and nutrients (nitrogen and phosphate) to promote benzene degradation under anerobic conditions. Linear model displayed that soil water-soluble Ca 2+ and SO 4 2- were positively correlated with the ratio of declined remediation outcomes at site scale (P<0.05). The generalized linear mixed model identified soil pH, along with soluble PO 4 3-, Ca 2+, SO 4 2-, NO 3 - and NO 2 -as significant contributors to the effectiveness of biostimulation at sample scale. Random forest model showed that Ca 2+ and PO 4 3- have equal importance but opposite roles in determining whether the soils can be remediated. The study revealed that at sites with averagely high background SO 4 2-, decade-long natural attenuation left the benzene more recalcitrant. High soil-soluble Ca 2+ could sequester the phosphate introduced by amendments, forming precipitates that reduced phosphorus availability. An increase in pH or a decrease in electrical conductivity during the biostimulation may indicate that the clogging of infiltration pathway, preventing the amendments from reaching the plume area, as observed in the third year at Site 2 and Site 3. Moreover, the decline in functional genes linked to anaerobic benzene degradation suggests insufficient microbial capacity to utilize the amendments. To achieve successful in situ biostimulation, it emphasizes the importance of tailoring biostimulation strategies to ensure the effective delivery of amendments, particularly for long-term remediation practices, and to sustain the activity of microorganisms under field conditions.