In situ 15N tracer quantification of fertilizer- and soil-derived NH3, NO, and N2O emissions in maize fields

Agricultural soils are major sources of reactive nitrogen (N) gases, yet distinguishing fertilizer-derived emissions from those originating in the soil remains a key challenge for accurate N management. We conducted an in situ 15 N tracing experiment in a maize field in Northeast China using 15 N-labeled urea (49.7 % 15 N, 200 kg N ha⁻¹), integrating passive adsorption and static chamber techniques to quantify source-specific emissions of ammonia (NH 3 ), nitric oxide (NO), and nitrous oxide (N 2 O). The results revealed distinct timing in the peak emissions of these N gases. NH 3 emission peaked first (6.4 kg N ha⁻¹ cumulative loss) and was mainly driven by soil ammonium levels whereas subsequent NO (3.8 kg N ha⁻¹) and N 2 O (1.4 kg N ha⁻¹) peaks were primarily regulated by temperature and soil nitrate availability. The synchronous bimodal 15 N dynamics of NO and N₂O indicated coupled nitrification-denitrification processes, with higher 15 N enrichment in NO (mean 20 %) than in N 2 O (11 %), suggesting stronger nitrification control on NO production. Fertilizer-derived N accounted for 67 %, 52 %, and 30 % of total NH 3 , NO, and N 2 O emissions, respectively. However, fertilizer-induced soil N transformations via priming and legacy effects led to underestimation of the total influence of fertilizer in 15 N tracing. These findings challenge conventional emission factor models, which may overlook indirect N emissions from agricultural inputs, and highlight the need to incorporate soil N priming and legacy dynamics into agricultural N footprint assessments.