3.1.1 Soil chemical properties
The soil pH gradually decreased with the increase of N addition concentration, and the pH value in HN treatment significantly decreased to 5.24 (P<0.05) (Table 1). TP content was significantly increased by 16.7% in the MN treatment. The AP content was significantly reduced after N addition. There were significant changes in NH4+-N and NO3--N content. The NH4+-N content increased with the concentration of N addition increasingly, and there was a significant increase in MN and HN treatments. The NO3--N content in the LN treatment was significantly increased and was significantly decreased in the HN treatment.