Glucose Elevates N2O Emissions by Promoting Fungal and Incomplete Denitrification in North China Vegetable Soils

Agricultural soils are hotspots of nitrous oxide (N2O) emissions, where carbon substrates act as a critical factor influencing microbial community composition. However, how carbon availability modulates microbial denitrifying pathways and further influences N2O emissions remains poorly understood. Here, we conducted anaerobic incubations to investigate North China vegetable soil N2O production and consumption in response to varied glucose concentrations (0, 0.5 (Glu_0.5), 1.0 (Glu_1.0), and 2.0 (Glu_2.0) g C kg−1 d.w. of soil). In this study, the δ15NSP/δ18O mapping approach (δ15NSP/δ18O MAP) and acetylene inhibition technique (AIT) were used to quantify the residual N2O ratio (rN2O) and the relative contributions of bacterial (fBD) and fungal (fFD) denitrification to N2O production. The results showed that increasing glucose concentrations significantly increased CO2 and N2O emissions, with peak fluxes observed at Glu_2.0 on day 1 (116.22 ± 2.80 mg CO2-C kg−1 and 1.08 ± 0.02 mg N2O-N kg−1). Concurrently, δ15NSP was also significantly elevated (p < 0.001), indicating enhanced fFD, which was further corroborated by positive correlations between fFD and glucose concentration (r = 0.48–0.56, p < 0.001). Nevertheless, bacterial denitrification (BD) still dominated N2O production throughout the incubation period, except on day 1 in Glu_1.0 and Glu_2.0 of Case 2. Bland–Altman analysis with 95% limits of agreement (LoA) demonstrated strong agreement between the MAP and AIT in rN2O estimation, particularly under Glu_2.0. All the above revealed glucose-induced denitrifying microbial shifts from BD to fungal denitrification (FD), which consequently modulated N2O emissions and promoted incomplete denitrification. These findings collectively demonstrate that in vegetable cropping systems, rational carbon management strategies can promote N2O reduction to N2, thereby achieving effective N2O mitigation.