3. Results and discussion
3.1. Concentrations and d15N of soil inorganic N
Concentrations of soil inorganic N (NH4
+ and NO3
)
increased from 24.8 to 308.5 mg N kg1 for NH4
+ and from
89.2 to 200.7 mg N kg1 for NO3
by compost or urea
application at 20 DAT, but thereafter decreased rapidly
such that values were as low as those of control at 60 DAT,
presumably due to plant N uptake, microbial N immobilization,
and N loss (Table 1). The d15N of soil inorganic N
seemed to reflect the integrative effects of N sources with
different 15N/14N ratios, and N transformations causing
isotopic fractionation against 15N (Choi and Ro, 2003).
The N source effect was apparent in C4 soils at 20 and 40
DAT, i.e., the d15N values of NH4
+– (27.2% and 25.4%,
respectively) and NO3
–N (17.0% and 16.7%, respectively)
were similar to those (29.6 for NH4
+ and 16.5% for NO3
)
of compost. In the other treatments, however, such effects
were not obvious due probably to a relatively low rate of
compost application (C2) or to a greater effect of N
fractionation associated with N transformation than N
source effect (U1 and U2).