3.1 Soil physiochemical properties and δ15N and δ18O-NO3-composition along the soil profile
Soil physiochemical properties varied significantly between maize field and woodland in two seasons along the depth (Figures 2 and S1). Generally, all the soil properties except NH4+-N, NO3--N and C/N significantly decreased along the depth, especially between the top 0-20 cm layer and other layers. While soil C/N ratio showed the opposite trend along the depth for both land uses. Soil NH4+-N and NO3--N contents showed inconsistent trend along the depth in two land uses and seasons.
Among different land uses, soil DON, DOC, C/N, NH4+-N and NO3--N contents were all significantly higher in maize field than in woodland irrespective of season (Table 1). Among different seasons, soil C/N was significantly higher in spring than in autumn irrespective of land use. Soil NH4+-N content was significantly higher in spring than in autumn only in maize field, and no significant difference of soil NO3--N content was found in maize field irrespective of season.
The dual isotope ratios of soil δ15N and δ18O-NO3- are shown in Figure 3. It clearly showed that isotopes abundance from different land uses were separated from each other along the Y axis. Soil δ15N-NO3-composition from maize field fall between +5‰ and 25‰, while those from woodland fall between -5‰ and 15‰. On the contrary, soil δ18O composition from both land uses fall between -5‰ and 25‰.