2.3 Soil δ15N and
δ18O-NO3- stable
isotope analysis
The concentration of δ15N and δ18O
of natural nitrate NO3- was
measured using denitrifier method (Sigman
et al., 2001). Briefly, soil NO3- was
extracted from 10.0 g fresh soil sample by ultrapure water at a
soil/water ratio of 1:5 with shaking (180 rpm) for 1 h. Then
δ15N and δ18O isotope ratios of the
extracted NO3- was determined based on
N2O produced by cultured denitrifying Pseudomonas
aureofaciens that converts NO3- to
N2O and analyzed on a
Trace-Gas/Isotope
Ratio Mass Spectrometry (IRMS, Isoprime100, UK) calibrated with
ultra-high purity N2 gas against air N in Chinese
Academy of Agricultural Sciences. The stable isotope ratios are
expressed in delta (δ) units and a per mil (‰) notation relative to an
international standard.