N application significantly affected NH4+ N content in the 0–40 cm soil layer,
but no significant difference among treatments was observed in the 40–60 cm soil layer (Fig. 2).
After N fertilizer application, NH4+ N content in the 0–40 cm soil layer at the overwintering
stage increased as the basal N rate increased,
and also increased with an increase in the topdressing N rate at the booting stage;
then it declined rapidly and did not differ significantly from N0.
NH4+ N content did not exceed 10 mg kg−1,
which was significantly lower than the NO3− N content, perhaps because NH4+N transformed into NO3− N rapidly after the application of N fertilizer
Inorganic N accumulation in soil
Table 2 shows the amounts of accumulated soil inorganic N(NO3− N and NH4+ N) at jointing and maturity stages.
Our results showed that NH4+ N accumulation did not differ at jointing and maturity stages among soil layers perhaps due to the rapid nitrification