Impact of New N Fertilizers on N Supply and Greenhouse Gas Emissions in Vegetable Soils

Soil mineral nitrogen (N) supply and the ammonium-to-nitrate ratio (NH4+-N:NO3−-N) are critical for vegetable yield and quality. Under current urea N-reduction practices, inadequate soil N often limits continuous vegetable growth. This study evaluated novel N fertilizer prepared by adsorbing different ratios of ammonium nitrogen and nitrate nitrogen onto rice straw extract (AX1–AX5), different N gradient conventional urea (U1, U2) and a control without N fertilizer (CK) in an incubation experiment. Results showed that ammonium-loaded novel N fertilizers (AX1, AX2) maintained a stable soil NH4+-N:NO3−-N (~1:1), while nitrate-loaded ones (AX4, AX5) exhibited slow-release effects, increasing cumulative mineral N by ~70% over U1. Novel N fertilizers also raised cumulative CO2 (3451–4513 μg kg−1) and N2O (9.6–12.0 μg kg−1) emissions versus U1, reflecting stimulated microbial activity supported by higher sucrase and nitrate reductase activities. The treatment (AX2) showed “fast-early, stable-late” N release, with mineral N consistently exceeding U1 and a maintained NH4+-N:NO3−-N near 1:1. Field validation is needed to assess agronomic and environmental performance under real farming conditions.