Isotopic Composition of In Situ Soil NOx Emissions in Manure‐Fertilized Cropland.

Soil emissions represent ~25% of the global annual atmospheric budget of nitrogen oxides (NOx). The nitrogen isotopic composition of soil NOx emissions (δ15N‐NOx) is potentially useful to track soil emission contributions to NOx budgets, yet its in situ variations with fertilizer management and meteorology are unknown. We quantify in situ δ15N‐NOx distributions from liquid dairy manure‐fertilized cropland in State College, Pennsylvania at hourly resolution during spring 2016 and 2017. δ15N‐NOx (n = 37) ranged from −44.2 to −14.0‰ and was distinct between injected (−32.2 ± 12.1‰) and broadcast manure without tillage (−23.4 ± 2.1‰). δ15N‐NOx was not correlated with order of magnitude emission variations due to large changes in soil moisture. δ15N‐NOx differences between treatments were associated with nitrification and NO consumption contributions. Our results suggest that δ15N‐NOx can be a valuable observational tracer of soil emissions and varies with fertilizer management practices. Plain Language Summary: Soil processes contribute about one quarter of all global emissions of nitrogen oxides, with important impacts on air quality and ecosystem health. The ratios of heavier to lighter nitrogen atoms (isotopes) are potentially useful to track soil emissions, but it is uncertain how they vary with agricultural management choices and weather conditions. We measure isotopes of soil nitrogen oxides emitted from manure‐fertilized cropland and find large differences based on whether manure was injected below or applied onto soil surfaces. Our results provide a new potential way to track agricultural emissions and their impacts on air pollution and ecosystem health. Key Points: Novel hourly, in situ N isotopic composition of soil NOx emission observations were performed across manure‐fertilized croplandsδ15N‐NOx in injected manure (−32.2 ± 12.1‰) was enriched over time, while that for broadcast manure (−23.4 ± 2.1‰) was less negativeδ15N‐NOx was insensitive to emission variations associated with soil moisture, and mainly linked with nitrification of soil NH4+ [ABSTRACT FROM AUTHOR]