Rice paddies are a significant agricultural land type and a notable source of greenhouse gas (GHG) emissions, making mitigation efforts critical for ecological sustainability. Both steel slag and biochar mitigate GHG emissions, and their co-application may enhance mitigation through complementary mechanisms; however, evaluations of their combined effects remain limited. This study explored the innovative use of a biochar-based silicate fertilizer (BSF), created by combining steel slag and biochar, in a field experiment across two rice-growing seasons. By measuring GHG fluxes, soil properties, plant biomass and microbial biomass carbon, we evaluated its efficacy and mechanistic pathways for GHG mitigation. Results showed that BSF application significantly increased early- and late-season rice yields. It significantly reduced CO2, CH4, and N2O emissions during the early rice season, decreasing GHG CO2-equivalent emissions by 7.4 %–29.1 %, but had no significant effect on late-season emissions. Mechanistically, combined results from variance analysis, correlation analysis, and structural equation modeling demonstrated that BSF indirectly reduced GHG emissions in early rice paddies by regulating soil, plant and microbial parameters. Specifically, BSF reduced CO2 flux primarily by increasing soil pH, which inhibited both plant autotrophic respiration and microbial heterotrophic respiration, with the latter playing a dominant role. It reduced CH4 production through enhanced Fe(III) oxidation and lower soil bulk density, limiting methanogenic activity. Furthermore, BSF mitigated N2O emissions by inhibiting denitrification. Differences in rice varieties, growth stages, and soil properties during the late season influenced GHG drivers, potentially masking BSF's effects. Results highlight the potential of seasonally targeted BSF application to reduce rice paddy GHG emissions while promoting the beneficial reuse of industrial and agricultural waste in climate mitigation efforts.