Isotopic Composition of In Situ Soil NO_x Emissions in Manure-Fertilized Cropland

Soil emissions represent ~25% of the global annual atmospheric budget of nitrogen oxides (NO_x). The nitrogen isotopic composition of soil NO_x emissions (δ¹⁵N-NO_x) is potentially useful to track soil emission contributions to NO_x budgets, yet its in situ variations with fertilizer management and meteorology are unknown. We quantify in situ δ¹⁵N-NO_x distributions from liquid dairy manure-fertilized cropland in State College, Pennsylvania at hourly resolution during spring 2016 and 2017. δ¹⁵N-NO_x (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-NO_x was not correlated with order of magnitude emission variations due to large changes in soil moisture. δ¹⁵N-NO_x differences between treatments were associated with nitrification and NO consumption contributions. Our results suggest that δ¹⁵N-NO_x can be a valuable observational tracer of soil emissions and varies with fertilizer management practices.