δ15N of nitric oxide produced under aerobic or anaerobic conditions from seven soils and their associated N isotope fractionations

Measuring the nitrogen isotope compositions (δ15N) of nitric oxide (NO)
from different sources helps to quantify the relative contributions of
atmospheric NOx. Soil is one of the most important sources of atmospheric
NOx, 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
(15εaerobic = 61 ± 3‰) was significantly higher (by 26‰) than under
anaerobic condition (15εanaerobic = 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 NOx. 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.