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.