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‰.