3.1 Effects of N fertilization on inorganic nitrogen content
The NH4+-N concentration was very low, at approximately 3 mg N kg-1, in the four treatments (Table S1). There were no significant differences in soil NH4+-N concentration between N fertilization treatments and N non-fertilization treatment in the first seven days (Fig. 1a). The soil NH4+-N concentration in the N225 treatment was significantly lower (P< 0.05) than that in the control treatment on the 9th and 12th day.
The NO3–N concentration in all treatments had a significantly upward trend with N addition (P < 0.05). The NO3–N concentration in the N150, N225, and N300 treatment treatments was 36.46, 38.22, and 41.65 mg N kg-1, respectively, approximately 1.4 to 1.6 times higher than that in the N0 control (Table S1). Soil NO3–N concentration showed significant increase in all treatments by 15 days of incubation (Fig. 1b). The NO3–N concentration in the nitrogen treatments was significantly (P < 0.05) higher than that in the control treatment, and the accumulation of NO3–N in the N225 treatment was higher than that in the N300 treatment.