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 15N tracing experiment in a maize field in Northeast China using 15N-labeled urea (49.7 % 15N, 200 kg N ha-1), integrating passive adsorption and static chamber techniques to quantify source-specific emissions of ammonia (NH3), nitric oxide (NO), and nitrous oxide (N2O). The results revealed distinct timing in the peak emissions of these N gases. NH3 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 N2O (1.4 kg N ha-1) peaks were primarily regulated by temperature and soil nitrate availability. The synchronous bimodal 15N dynamics of NO and N2O indicated coupled nitrification-denitrification processes, with higher 15N enrichment in NO (mean 20 %) than in N2O (11 %), suggesting stronger nitrification control on NO production. Fertilizer-derived N accounted for 67 %, 52 %, and 30 % of total NH3, NO, and N2O emissions, respectively. However, fertilizer-induced soil N transformations via priming and legacy effects led to underestimation of the total influence of fertilizer in 15N 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.