Nitrous oxide (N2O) is a potent greenhouse gas and ozone-depleting substance, with a high emission risk in intensive agricultural systems. As a typical high-input agroecosystem, tea plantations are characterized by long-term excessive nitrogen application and strong soil acidity, resulting in N2O emission factors significantly higher than those of conventional croplands, making them a global “hotspot” of agricultural N2O emissions. However, the regulatory mechanism by which different types of carbon sources reshape N2O emission pathways remains unclear, limiting the scientific design of mitigation strategies in tea plantations. In this study, a randomized block field experiment was conducted in a typical hilly tea plantation in Lujiang, Anhui, with four fertilization treatments: no fertilizer (CK), chemical fertilizer alone (FF), mixed application of chemical fertilizer and rapeseed cake (easily decomposable carbon source, FO), and mixed application of chemical fertilizer and biochar (recalcitrant carbon source, FB). The effects of carbon source type on N2O emission dynamics and microbial mechanisms were systematically evaluated. The results showed that FO treatment induced a significant N2O emission peak on day 30 after fertilization, which was highly synchronized with the peak of microbial biomass carbon (MBC), while the peak of microbial biomass nitrogen (MBN) was delayed to day 60. This pattern reflects a temporal allocation of carbon and nitrogen resources, in which active organic carbon input first promoted heterotrophic metabolism and denitrification, followed by nitrogen assimilation. In contrast, FB treatment significantly suppressed N2O emissions throughout the experiment, with an 88.79% reduction compared to FO, and no significant fluctuation in MBC or MBN, indicating a weaker microbial metabolic response. Functional gene and network analyses showed that FO treatment mainly activated intermediate denitrification genes such as narG and norB, promoting the reduction of NO??-N and NO??-N to N2O. In contrast, FB treatment upregulated genes such as hao, AOA, AOB, and nrfA, enhancing nitrification and dissimilatory nitrate reduction to ammonium (DNRA), while norB expression was suppressed, effectively blocking the formation of N2O. Partial least squares path modeling further revealed that FB treatment did not significantly enhance nosZ expression (involved in N2O reduction), but rather reduced N2O precursors by strengthening DNRA and microbial nitrogen assimilation, indicating a mitigation strategy based on “process avoidance” rather than “terminal reduction”. This study clearly demonstrates that the mitigation advantage of FB lies in the restructuring of nitrogen cycling pathways rather than compensatory N2O-to-N2 reduction, providing novel theoretical insights into carbon–nitrogen interactions in soil nitrogen regulation. The findings offer important guidance for optimizing carbon–nitrogen coupled fertilization strategies in tea plantations and contribute a reference framework for other high-nitrogen agroecosystems.