Measuring the nitrogen isotope compositions (δ¹⁵N) of nitric oxide (NO) from different sources helps to quantify the relative contributions of atmospheric NO_x. Soil is one of the most important sources of atmospheric NO_x, but only limited measurements on the δ¹⁵N of soil emitted NO exist, hampering our ability to partition sources to air pollution. Here we conducted soil incubations to measure the δ¹⁵N-NO under defined aerobic or anaerobic conditions, favoring either nitrification or denitrification. Soils were collected from seven sites spanning three ecosystems in northern China (two agricultural, two forest, and three grassland sites). We found that the δ¹⁵N-NO and their associated N isotope fractionations were significant different between anaerobic and aerobic conditions in seven soils. Under aerobic condition, the δ¹⁵N-NO ranged from -62‰ to -50‰ (averaged -56 ± 4‰), being significantly more negative (by 23‰) than those under anaerobic condition (-45‰ to -23‰, averaged -33 ± 7‰). The apparent N isotope fractionation for NO production under aerobic condition (¹⁵ε_aerobic = 61 ± 3‰) was significantly higher (by 26‰) than under anaerobic condition (¹⁵ε_anaerobic = 35 ± 6‰), with a small variability among ecosystem types. Our study demonstrates that the δ¹⁵N-NO from different soils are very different from fuel combustions (mainly from 0 to +20‰), supporting that measuring ¹⁵N is a useful tool to partition the contributions of soil NO to atmospheric NO_x. Our results also imply δ¹⁵N-NO produced by nitrification and denitrification distinctly different, as these two processes are dominant processes producing NO under aerobic and anaerobic conditions, respectively.

We conducted laboratory incubation experiments with three types of soils collected from seven sites, including two temperate forest (QY-LF,QY-MF), two agricultural (SY, JL) and three grassland soils (EG, HB,DL) under anaerobic or aerobic conditions. We set 5 sampling times, on days 0, 1, 3, 5, and 7. For each sampling time, we determined the concentration and isotope compositions of cumulative nitric oxide (NO) produced and the remaining soil KCl-extractable NH₄⁺ and NO₃^-, we also calculated the isotope fractionations during NO producing.